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Electric-Field Induced Phase Transitions in Capillary Electrophoretic
Systems: The movement of the particles in a capillary electrophoretic system under
electroosmotic flow was modeled using Monte Carlo simulation with Metropolis
algorithm. Two different cases, with repulsive and attractive interactions
between molecules were taken into consideration. The simulation was done using
a spin-like system where the interactions between the nearest and second
closest neighbors were considered in two separate steps of the modeling study.
A total of 20 different cases with different rate of interactions for both
repulsive and attractive interactions were modeled. The movement of the
particles through the capillary is defined as current. At a low interaction
level between molecules, a regular electroosmotic flow is obtained, on the
other hand, with increasing interactions between molecules the current shows a
phase transition behavior. The results also show that a modular electroosmotic
flow can be obtained for separations by tuning the ratio between molecular
interactions and electric field strength. | cond-mat_soft |
Conformational Collapse of Surface-Bound Helical Filaments: Chiral polymers are ubiquitous in nature and in the cellular context they are
often found in association with membranes. Here we show that surface bound
polymers with an intrinsic twist and anisotropic bending stiffness can exhibit
a sharp continuous phase transition between states with very different
effective persistence lengths as the binding affinity is increased. Above a
critical value for the binding strength, determined solely by the torsional
modulus and intrinsic twist rate, the filament can exist in a zero twist,
surface bound state with a homogeneous stiffness. Below the critical binding
strength, twist walls proliferate and function as weak or floppy joints that
sharply reduce the effective persistence length that is measurable on long
lengthscales. The existence of such dramatically different conformational
states has implications for both biopolymer function {\it in vivo} and for
experimental observations of such filaments {\it in vitro}. | cond-mat_soft |
Tuned, driven, and active soft matter: One characteristic feature of soft matter systems is their strong response to
external stimuli. As a consequence they are comparatively easily driven out of
their ground state and out of equilibrium, which leads to many of their
fascinating properties. Here, we review illustrative examples. This review is
structured by an increasing distance from the equilibrium ground state. On each
level, examples of increasing degree of complexity are considered. In detail,
we first consider systems that are quasi-statically tuned or switched to a new
state by applying external fields. These are common liquid crystals, liquid
crystalline elastomers, or ferrogels and magnetic elastomers. Next, we
concentrate on systems steadily driven from outside e.g. by an imposed flow
field. In our case, we review the reaction of nematic liquid crystals, of
bulk-filling periodically modulated structures such as block copolymers, and of
localized vesicular objects to an imposed shear flow. Finally, we focus on
systems that are "active" and "self-driven". Here our range spans from
idealized self-propelled point particles, via sterically interacting particles
like granular hoppers, via microswimmers such as self-phoretically driven
artificial Janus particles or biological microorganisms, via deformable
self-propelled particles like droplets, up to the collective behavior of
insects, fish, and birds. As we emphasize, similarities emerge in the features
and behavior of systems that at first glance may not necessarily appear
related. We thus hope that our overview will further stimulate the search for
basic unifying principles underlying the physics of these soft materials out of
their equilibrium ground state. | cond-mat_soft |
Bogoliubov excitation spectrum in anharmonic traps: We study the linearized Bogoliubov excitation spectrum of infinitely long
anharmonically trapped Bose-Einstein condensates, with the aim of overcoming
inhomogeneous broadening. We compare the Bogoliubov spectrum of a harmonic trap
with that of a theoretical flat-bottom trap and find a dramatic reduction in
the inhomogeneous broadening of the lineshape of Bogoliubov excitations. While
the Bragg excitation spectrum for a condensate in a harmonic trap supports a
number of radial modes, the flat trap is found to significantly support just
one mode. We also study the excitation spectrum of realistic anharmonic traps
with potentials of finite power dependence on the radial coordinate. We observe
a correlation between the number of radial modes and the number of bound states
in the effective potential of the quasi-particles. Finally we compare a full
numerical Gross-Pitaevskii simulation of a finite-length condensate to our
model of infinite, linearized Gross-Pitaevskii excitations. We conclude that
our model captures the essential physics. | cond-mat_soft |
Two step melting of the Weeks-Chandler-Anderson system in two dimensions: We present a detailed numerical simulation study of a two dimensional system
of particles interacting via the Weeks-Chandler-Anderson potential, the
repulsive part of the Lennard-Jones potential. With reduction of density, the
system shows a two-step melting: a continuous melting from solid to hexatic
phase, followed by a a first order melting of hexatic to liquid. The
solid-hexatic melting is consistent with the
Kosterlitz-Thouless-Halperin-Nelson-Young (KTHNY) scenario and shows
dislocation unbinding. The first order melting of hexatic to fluid phase, on
the other hand, is dominated by formation of string of defects at the
hexatic-fluid interfaces. | cond-mat_soft |
Pinning Dislocations in Colloidal Crystals with Active Particles that
Seek Stacking Faults: There is growing interest in functional, adaptive devices built from
colloidal subunits of micron size or smaller. A colloidal material with dynamic
mechanical properties could facilitate such microrobotic machines. Here we
study via computer simulation how active interstitial particles in small
quantities can be used to modify the bulk mechanical properties of a colloidal
crystal. Passive interstitial particles are known to pin dislocations in
metals, thereby increasing resistance to plastic deformation. We extend this
tactic by employing anisotropic active interstitials that travel
super-diffusively and bind strongly to stacking faults associated with partial
dislocations. We find that: 1) interstitials that are effective at reducing
plasticity compromise between strong binding to stacking faults and high
mobility in the crystal bulk. 2) Reorientation of active interstitials in the
crystal depends upon rotational transitions between high-symmetry crystal
directions. 3) The addition of certain active interstitial shapes at
concentrations as low as $60$ per million host particles ($0.006\%$) can create
a shear threshold for dislocation migration. | cond-mat_soft |
Microscopic theory for hyperuniformity in two-dimensional chiral active
fluid: Some nonequilibrium systems exhibit anomalous suppression of the large-scale
density fluctuations, so-called hyperuniformity. Recently, hyperuniformity was
found numerically in a simple model of chiral active fluids [Q.-L. Lei et al.,
Sci. Adv. 5, eaau7423 (2019)]. We revisit this phenomenon and put forward a
microscopic theory to explain it. An effective fluctuating hydrodynamic
equation is derived for a simple particle model of chiral active matter. We
show that the linear analysis of the obtained hydrodynamic equation captures
hyperuniformity. Our theory yields hyperuniformity characterized by the same
exponents as the numerical observation, but the agreement with the numerical
data is qualitative. We also argue that the hydrodynamic equation for the
effective particle representation, in which each rotating trajectory is
regarded as an effective particle, has the same form as the macroscopic
description of the random organization model with the center of mass
conservation. | cond-mat_soft |
How should the contact angle of a noncircular wetting boundary be
described?: For over 200 years, wettability has made significant contributions to
understanding the properties of objects, advancing technological progress.
Theoretical model of the contact angle (CA) for evaluating wettability has
constantly been modified to address relevant emerging issues. However, these
existing models disregard the difference in the CA along the contact line and
use a single-point CA to evaluate the entire contact line. From this
perspective, there is no reasonable explanation for noncircular wetting. Here,
we reveal that noncircular wetting boundaries result from property differences
in the surfaces along the boundary, and utilize friction as a comprehensive
factor reflecting local wettability. Average CA is proposed to evaluate the
contact line instead of the single-point CA, making the Cassie method and
Wenzel method obsolete, which will take an average property of the whole
surface as a weight coefficient of the single-point CA, ignoring the
subordination between physical properties and roughness in systematics. | cond-mat_soft |
Modeling of Branched Thickening Polymers under Poiseuille Flow Gives
Clues as to how to Increase a Solvent Viscosity: The viscosity enhancement of a solvent produced by the addition of thickening
branched polymers is predicted as a function of polymer concentration, branch
length and persistence length, and strength of the covalent bonding
interactions. Non equilibrium, stationary state Poiseuille numerical
simulations are performed using the dissipative particle dynamics model to
obtain the viscosity of the fluid. It is found that the clustering of the
polymers into aggregates raises the viscosity and that it is more strongly
affected by the strength of the bonding interactions. General scaling
relationships are found for the viscosity as a function of the variables
studied, which are expected to be useful for the design and synthesis of new
viscosifying polymers. It is argued that our results can be applied to aqueous
thickeners, of importance for colloidal fluids such as paints and coatings, and
also for nonpolar fluids such as supercritical CO2, which is a promising non
hydraulic fracking fluid also useful in enhanced oil recovery. | cond-mat_soft |
Defect Structures in the Growth Kinetics of the Swift-Hohenberg Model: The growth of striped order resulting from a quench of the two-dimensional
Swift-Hohenberg model is studied in the regime of a small control parameter and
quenches to zero temperature. We introduce an algorithm for finding and
identifying the disordering defects (dislocations, disclinations and grain
boundaries) at a given time. We can track their trajectories separately. We
find that the coarsening of the defects and lowering of the effective free
energy in the system are governed by a growth law $L(t)\approx t^{x}$ with an
exponent x near 1/3. We obtain scaling for the correlations of the nematic
order parameter with the same growth law. The scaling for the order parameter
structure factor is governed, as found by others, by a growth law with an
exponent smaller than x and near to 1/4. By comparing two systems with
different sizes, we clarify the finite size effect. We find that the system has
a very low density of disclinations compared to that for dislocations and
fraction of points in grain boundaries. We also measure the speed distributions
of the defects at different times and find that they all have power-law tails
and the average speed decreases as a power law. | cond-mat_soft |
Directed Paths in a Wedge: Directed paths have been used extensively in the scientific literature as a
model of a linear polymer. Such paths models in particular the conformational
entropy of a linear polymer and the effects it has on the free energy. These
directed models are simplified versions of the self-avoiding walk, but they do
nevertheless give insight into the phase behaviour of a polymer, and also serve
as a tool to study the effects of conformational degrees of freedom in the
behaviour of a linear polymer. In this paper we examine a directed path model
of a linear polymer in a confining geometry (a wedge). The main focus of our
attention is $c_n$, the number of directed lattice paths of length $n$ steps
which takes steps in the North-East and South-East directions and which is
confined to the wedge $Y=\pm X/p$, where $p$ is an integer. In this paper we
examine the case $p=2$ in detail, and we determine the generating function
using the iterated kernel method. We also examine the asymtotics of $c_n$. In
particular, we show that $$ c_n = [0.67874...]\times 2^{n-1}(1+(-1)^n) +
O((4/3^{3/4})^{n+o(n)}) + o((4/3^{3/4})^n) $$ where we can determine the
constant $0.67874...$ to arbitrary accuracy with little effort. | cond-mat_soft |
Propagation of a Thermo-mechanical Perturbation on a Lipid Membrane: The propagation of sound waves on lipid monolayers supported on water has
been studied during the melting transition. Since changes in volume, area, and
compressibility in lipid membranes have biological relevance, the observed
sound propagation is of paramount importance. However, it is unknown what would
occur on a lipid bilayer, which is a more approximate model of a cell membrane.
With the aim to answer this relevant question, we built an experimental setup
to assemble long artificial lipid membranes. We found that if these membranes
are heated in order to force local melting, a thermo-mechanical perturbation
propagates a long distance. Our findings may support the existence of solitary
waves, postulated to explain the propagation of isentropic signals together
with the action potential in neurons. | cond-mat_soft |
High-speed confined granular flows down inclines revisited: Recent numerical work has shown that high-speed confined granular flows down
inclines exhibit a rich variety of flow patterns, including dense
unidirectional flows, flows with longitudinal vortices and supported flows
characterized by a dense core surrounded by a dilute hot granular gas (Brodu et
al, JFM 2015). Here, we revisit the results obtained by Brodu et al. (JFM,
2015) and present new features characterizing these flows. In particular, we
provide vertical and transverse profiles for the packing fraction, velocity and
granular temperature.We also characterize carefully the transition between the
different flow regimes and show that the packing fraction and the vorticity can
be successfully used to describe these transitions. Additionally, we emphasize
that the effective friction at the basal and side walls can be described by a
unique function of a dimensionless number which is the analog of a Froude
number: $Fr=V/\sqrt{gH\cos \theta}$ where $V$ is the particle velocity at the
walls, $\theta$ is the inclination angle and $H$ the particle holdup (defined
as the depth-integrated particle volume fraction). This universal function
bears some similarities with the $\mu(I)$ rheological curve derived for dense
granular flows. | cond-mat_soft |
Decoupled algorithm for the multicomponent potential theory of
adsorption of gas mixtures: In this paper, we present a new implementation of the Multicomponent
Potential Theory of Adsorption model. The proposed interpretation establishes a
clear cut between parameters that depends on the adsorbent from those depending
on the adsorbate, which leads to a better understanding of the parameters
signification. The interdependence between pure isotherms is eliminated, which
mean that each component can be individually finely adjusted. This new approach
was tested against 14 datasets for a total of 510 experimental mixture
adsorption data of CH4, CO2, N2, H2, O2, H2S, C2H6, C3H6 and C3H8 on activated
carbons, MOF and zeolites. A slight improvement of 4.67% on excess adsorption
predictions was found, leading to an overall average error of 6.97% for total
excess adsorption and 15.30% for combined mixtures and components excess
adsorption predictions. | cond-mat_soft |
Soft elasticity in biaxial smectic and smectic-C elastomers: Ideal (monodomain) smectic-$A$ elastomers crosslinked in the smectic-$A$
phase are simply uniaxial rubbers, provided deformations are small. From these
materials smectic-$C$ elastomers are produced by a cooling through the
smectic-$A$ to smectic-$C$ phase transition. At least in principle, biaxial
smectic elastomers could also be produced via cooling from the smectic-$A$ to a
biaxial smectic phase. These phase transitions, respectively from $D_{\infty
h}$ to $C_{2h}$ and from $D_{\infty h}$ to $D_{2h}$ symmetry, spontaneously
break the rotational symmetry in the smectic planes. We study the above
transitions and the elasticity of the smectic-$C$ and biaxial phases in three
different but related models: Landau-like phenomenological models as functions
of the Cauchy--Saint-Laurent strain tensor for both the biaxial and the
smectic-$C$ phases and a detailed model, including contributions from the
elastic network, smectic layer compression, and smectic-$C$ tilt for the
smectic-$C$ phase as a function of both strain and the $c$-director. We show
that the emergent phases exhibit soft elasticity characterized by the vanishing
of certain elastic moduli. We analyze in some detail the role of spontaneous
symmetry breaking as the origin of soft elasticity and we discuss different
manifestations of softness like the absence of restoring forces under certain
shears and extensional strains. | cond-mat_soft |
A Fragile-Strong Fluid Crossover and Universal Relaxation Times in a
Confined Hard Disc Fluid: We show that a system of hard discs confined to a narrow channel exhibits a
fragile-strong fluid crossover located at the maximum of the isobaric heat
capacity and that the relaxation times for different channel widths fall onto a
single master curve when rescaled by the relaxation times and temperatures of
the crossover. Calculations of the configurational entropy and the inherent
structure equation of state find that the crossover is related to properties of
the jamming landscape for the model but that the Adams-Gibbs relation does not
predict the relaxation behavior. We also show that a facilitated dynamics
description of the system, where kinetically excited regions are identified
with local packing arrangements of the discs, successfully describes the
fragile-strong crossover. | cond-mat_soft |
Anomalous temperature dependence of surface tension and capillary waves
at liquid gallium: The temperature dependence of surface tension \gamma(T) at liquid gallium is
studied theoretically and experimentally using light scattering from capillary
waves. The theoretical model based on the Gibbs thermodynamics relates the
temperature derivative of \gamma to the surface excess entropy -\Delta S.
Although capillary waves contribute to the surface entropy with a positive sign
the effect of dipole layer on \Delta S is negative. Experimental data collected
at a free Ga surface in the temperature range from 30 to 160 C show that the
temperature derivative of the tension changes sign near 100 C. | cond-mat_soft |
Possible realization of Josephson charge qubits in two coupled
Bose-Einstein condensates: We demonstrate that two coupled Bose-Einstein condensates (BEC) at zero
temperature can be used to realize a qubit which is the counterpart of
Josephson charge qubits. The two BEC are weakly coupled and confined in an
asymmetric double-well trap. When the "charging energy" of the system is much
larger than the Josephson energy and the system is biased near a degeneracy
point, the two BEC represent a qubit with two states differing only by one
atom. The realization of the BEC qubits in realistic BEC experiments is briefly
discussed. | cond-mat_soft |
Quantifying nematic order in evaporation-driven self-assembly of
Halloysite nanotubes: Nematic islands and critical aspect ratio: Halloysite nanotubes (HNTs) are naturally occurring clay minerals found in
Earth's crust that typically exist in the form of high aspect-ratio
nanometers-long rods. Here, we investigate the evaporation-driven self-assembly
process of HNTs and show that a highly polydisperse collection of HNTs
self-sort into a spatially inhomogeneous structure, displaying a systematic
variation in the resulting nematic order. Through detailed quantification using
nematic order parameter $S$ and nematic correlation functions, we show the
existence of well-defined isotropic-nematic transitions in the emerging
structures. We also show that the onset of these transitions gives rise to the
formation of nematic islands - phase coexisting ordered nematic domains
surrounded by isotropic phase - which grow in size with $S$. Detailed image
analysis indicates a strong correlation between local $S$ and the local aspect
ratio, $L/D$, with nematic order possible only for rods with $L/D \ge 6.5 \pm
1$. Finally, we conclude that observed phenomena directly result from aspect
ratio-based sorting in our system. Altogether, our results provide a unique
method of tuning the local microscopic structure in self-assembled HNTs using
$L/D$ as an external parameter. | cond-mat_soft |
Effects of three-body interactions on the structure and thermodynamics
of liquid krypton: Large-scale molecular dynamics simulations are performed to predict the
structural and thermodynamic properties of liquid krypton using a potential
energy function based on the two-body potential of Aziz and Slaman plus the
triple-dipole Axilrod-Teller (AT) potential. By varying the strength of the AT
potential we study the influence of three-body contribution beyond the
triple-dipole dispersion. It is seen that the AT potential gives an overall
good description of liquid Kr, though other contributions such as higher order
three-body dispersion and exchange terms cannot be ignored. | cond-mat_soft |
Guiding the self-assembly of colloidal diamond: The assembly of colloidal cubic diamond is a challenging process since the
shape and interaction parameters and the thermodynamic conditions where this
structure is stable are elusive. The simultaneous use of shape-anisotropic
particles and strong directional interactions has proven to be a successful
path to exclusively nucleate this structure. Here, using molecular dynamics
simulations, we explore in detail the conditions where nucleation of cubic
diamond from tetrahedral building blocks is favored. In particular, we focus on
the effect of depletion and DNA-mediated interactions to form and stabilize
this cubic diamond crystal. We find that a particular balance between the
strength and range of the depletion interactions enhances the self-assembly of
stable cubic diamond, leading to a narrow region where this structure is
nucleated. Moreover, we determine that stronger short-range depletion
attractions may arrest the system leading to the formation of percolating
diamond networks or fully disordered gel structures. Accordingly, the internal
arrangements of these structures exhibit a distinct variation in terms of
fractal dimension and the presence of six-membered rings that increasingly
acquire internal strain as the arrest gets more pronounced. With these results
we provide a clear route for the self-assembly of cubic colloidal diamond,
towards the realization of crystals with superior photonic properties. | cond-mat_soft |
Theory and simulation of macromolecular crowding effects on protein
folding stability and kinetics: We investigate the effect of macromolecular crowding on protein folding,
using purely repulsive crowding particles and a self-organizing polymer model
of protein folding. We find that the thermodynamics of folding for typical
alpha-, beta- and alpha/beta-proteins are well described by an adaptation of
the scaled particle theory (SPT). In this approach, the native state,
transition state, and the unfolded protein are treated as effective hard
spheres with radii approximately independent of the size and concentration of
the crowders. The same model predicts the effect of crowding on the folding
barrier and therefore refolding rates with no adjustable parameters. A simple
extension of the SPT model, assuming additivity, can also describe the behavior
of mixtures of crowding particles. | cond-mat_soft |
On the mechanism of the highly viscous flow: The asymmetry model for the highly viscous flow postulates thermally
activated jumps from a practically undistorted ground state to strongly
distorted, but stable structures, with a pronounced Eshelby backstress from the
distorted surroundings. The viscosity is ascribed to those stable distorted
structures which do not jump back, but relax by the relaxation of the
surrounding viscoelastic matrix. It is shown that this mechanism implies a
description in terms of the shear compliance, with a viscosity which can be
calculated from the cutoff of the retardation spectrum. Consistency requires
that this cutoff lies close to the Maxwell time. The improved asymmetry model
compares well with experiment. | cond-mat_soft |
Strength and energy consumption of inherently anisotropic rocks at
failure: Using a discrete-element approach and a bonding interaction law, we model and
test crushable inherently anisotropic structures reminiscent of the layering
found in sedimentary and metamorphic rocks. By systematically modifying the
level of inherent anisotropy, we characterize the evolution of the failure
strength of circular rock samples discretized using a modified Voronoi
tesselation under diametral point loading at different orientations relative to
the sample's layers. We characterize the failure strength, which can
dramatically increase as the loading becomes orthogonal to the rock layers. We
also describe the evolution of the macroscopic failure modes as a function of
the loading orientation and the energy consumption at fissuring. Our simulation
strategy let us conclude that the length of bonds between Voronoi cells
controls the energy being consumed in fissuring the rock sample, although
failure modes and strength are considerably changing. We end up this work
showing that the microstructure is largely afected by the level of inherent
anisotropy and loading orientation. | cond-mat_soft |
Spontaneous Patterning of Confined Granular Rods: Vertically vibrated rod-shaped granular materials confined to quasi-2D
containers self organize into distinct patterns. We find, consistent with
theory and simulation, a density dependent isotropic-nematic transition. Along
the walls, rods interact sterically to form a wetting layer. For high rod
densities, complex patterns emerge as a result of competition between bulk and
boundary alignment. A continuum elastic energy accounting for nematic
distortion and local wall anchoring reproduces the structures seen
experimentally. | cond-mat_soft |
Phase Behaviour of Binary Hard-Sphere Mixtures: Free Volume Theory
Including Reservoir Hard-Core Interactions: Comprehensive calculations were performed to predict the phase behaviour of
large spherical colloids mixed with small spherical colloids that act as
depletant. To this end, the free volume theory (FVT) of Lekkerkerker et al.
[Europhys. Lett. 20 (1992) 559] is used as a basis and is extended to
explicitly include the hard-sphere character of colloidal depletants into the
expression for the free volume fraction. Taking the excluded volume of the
depletants into account in both the system and the reservoir provides a
relation between the depletant concentration in the reservoir and in the system
that accurately matches with computer simulation results of Dijkstra et al.
[Phys. Rev. E 59 (1999) 5744]. Moreover, the phase diagrams for highly
asymmetric mixtures with size ratios q . 0:2 obtained by using this new
approach corroborates simulation results significantly better than earlier FVT
applications to binary hard-sphere mixtures. The phase diagram of a binary
hard-sphere mixture with a size ratio of q = 0:4, where a binary interstitial
solid solution is formed at high densities, is investigated using a numerical
free volume approach. At this size ratio, the obtained phase diagram is
qualitatively different from previous FVT approaches for hard-sphere and
penetrable depletants, but again compares well with simulation predictions. | cond-mat_soft |
Timescales of emulsion formation caused by anisotropic particles: Particle stabilized emulsions have received an enormous interest in the
recent past, but our understanding of the dynamics of emulsion formation is
still limited. For simple spherical particles, the time dependent growth of
fluid domains is dominated by the formation of droplets, particle adsorption
and coalescence of droplets (Ostwald ripening), which eventually can be almost
fully blocked due to the presence of the particles. Ellipsoidal particles are
known to be more efficient stabilizers of fluid interfaces than spherical
particles and their anisotropic shape and the related additional rotational
degrees of freedom have an impact on the dynamics of emulsion formation. In
this paper, we investigate this point by means of simple model systems
consisting of a single ellipsoidal particle or a particle ensemble at a flat
interface as well as a particle ensemble at a spherical interface. By applying
combined multicomponent lattice Boltzmann and molecular dynamics simulations we
demonstrate that the anisotropic shape of ellipsoidal particles causes two
additional timescales to be of relevance in the dynamics of emulsion formation:
a relatively short timescale can be attributed to the adsorption of single
particles and the involved rotation of particles towards the interface. As soon
as the interface is jammed, however, capillary interactions between the
particles cause a local reordering on very long timescales leading to a
continuous change in the interface configuration and increase of interfacial
area. This effect can be utilized to counteract the thermodynamic instability
of particle stabilized emulsions and thus offers the possibility to produce
emulsions with exceptional stability. | cond-mat_soft |
Three-body critical Casimir forces: Within mean-field theory we calculate universal scaling functions associated
with critical Casimir forces for a system consisting of three parallel
cylindrical colloids immersed in a near-critical binary liquid mixture. For
several geometrical arrangements and boundary conditions at the surfaces of the
colloids we study the force between two colloidal particles along their
center-to-center axis, analyzing the influence of the presence of a third
particle on that force. Upon changing temperature or the relative positions of
the particles we observe interesting features such as a change of sign of this
force caused by the presence of the third particle. We determine the three-body
component of the forces acting on one of the colloids by subtracting the
pairwise forces from the total force. The three-body contribution to the total
critical Casimir force turns out to be more pronounced for small
surface-to-surface distances between the colloids as well as for temperatures
close to criticality. Moreover we compare our results with similar ones for
other physical systems such as three atoms interacting via van der Waals
forces. | cond-mat_soft |
The Origin of Tilted Phase Generation in Systems of Ellipsoidal
Molecules with Dipolar Interactions: We report Monte-Carlo simulation studies of some systems consisting of polar
rod-like molecules interacting via a pair potential that exhibit liquid crystal
phases, attributed with tilt angles of large magnitude. For theoretical
understanding of the microscopic origin of the tilted phases, different systems
consisting of prolate ellipsoidal molecules of three different lengths,
embedded with two symmetrically placed anti-parallel terminal dipoles are
considered. We find that the presence of a stable tilted phase crucially
depends on the molecular elongation which effectively makes dipolar separation
longer. We observe that in case of mesogens with transverse dipoles the tilt in
the layered smectic phase gradually increases from zero to a large magnitude as
we increase the molecular length. However tilt remains weak with molecular
elongation for systems with longitudinal dipoles which shows a small tilt at
shorter lengths. This is the first work determining the combined contribution
of dipolar separation and orientations in generating biaxial liquid crystal
phases with large tilt angles. | cond-mat_soft |
Kinetically driven ordered phase formation in binary colloidal crystals: The aggregation of binary colloids of same size and balanced charges is
studied by Brownian dynamics simulations for dilute suspensions. It is shown
that, under appropriate conditions, the formation of colloidal crystals is
dominated by kinetic effects leading to the growth of well-ordered crystallites
of the sodium-chloride (NaCl) bulk phase. These crystallites form with very
high probability even when the cesium-chloride (CsCl) phase is more stable
thermodynamically. Global optimization searches show that this result is not
related to the most favorable structures of small clusters, that are either
amorphous or of CsCl structure. The formation of the NaCl phase is related to
the specific kinetics of the crystallization process, which takes place by a
two-step mechanism. In this mechanism, dense fluid aggregates form at first and
then crystallization follows. It is shown that the type of short-range order in
these dense fluid aggregates determines which phase is finally formed in the
crystallites. The role of hydrodynamic effects in the aggregation process is
analyzed by Stochastic Rotation Dynamics - Molecular Dynamics simulations,
finding that these effects do not play a major role in the formation of the
crystallites. | cond-mat_soft |
Avalanches, loading and finite size effects in 2D amorphous plasticity:
results from a finite element model: Crystalline plasticity is strongly interlinked with dislocation mechanics and
nowadays is relatively well understood. Concepts and physical models of plastic
deformation in amorphous materials on the other hand - where the concept of
linear lattice defects is not applicable - still are lagging behind. We
introduce an eigenstrain-based finite element lattice model for simulations of
shear band formation and strain avalanches. Our model allows us to study the
influence of surfaces and finite size effects on the statistics of avalanches.
We find that even with relatively complex loading conditions and open boundary
conditions, critical exponents describing avalanche statistics are unchanged,
which validates the use of simpler scalar lattice-based models to study these
phenomena. | cond-mat_soft |
Three length scales colloidal gels: the clusters of clusters versus the
interpenetrating clusters approach: Typically, in quiescent conditions, attractive colloids at low volume
fractions form fractal gels structured into two length scales: the colloidal
and the fractal cluster scales. However when flow interfere with gelation
colloidal fractal gels may display three distinct length scales [Dag\`es, et
al., Soft Matter 18, 6645 (2022)]. Following those recent experimental
investigations, we derive two models that account for the structure and the
rheological properties of such atypical colloidal gels. The gel elasticity is
inferred from scaling arguments and the structure is translated into scattering
intensities following the global scattering functions approach proposed by
Beaucage and typically measured in small angle X-ray scattering (SAXS). In both
models, we consider that the colloids condensate into fractal clusters. In the
clusters of clusters model, the clusters form superagregates which then build
the gel network. In the interpenetrating clusters model, the clusters
interpenetrate one-another to form the gel network. Those two models are then
utilised to analyse rheo-SAXS experiments carried out on carbon black gels
formed through flow cessation. The results of the analysis vouch for the
clusters of clusters model with a densification of the structures as the gel
characteristic length scales increase. | cond-mat_soft |
Albe 1998 - La grande Motte 2009 : quelles avancées en diffusion de
neutrons aux petits angles en 10 ans ?: The importance of neutron scattering techniques for the characterization of
samples in soft condensed matter has been demonstrated all along the present
book. The fine understanding of the physical properties is closely linked to
progress in the field of instrumentation. This chapter describes the advances
over the last decade in technical domains, such as neutron detection,
electronics and sample environment. The news software for data reduction and
analysis are also discussed before to conclude with the ILL and LLB projects
for new instruments. | cond-mat_soft |
Effect of Energy Polydispersity on the Nature of Lennard-Jones Liquids: In the companion paper [T. S. Ingebrigtsen and H. Tanaka, J. Phys. Chem. B
119, 11052 (2015)] the effect of size polydispersity on the nature of
Lennard-Jones (LJ) liquids, which represent most molecular liquids without
hydrogen bonds, was studied. More specifically, it was shown that even highly
size polydisperse LJ liquids are Roskilde-simple (RS) liquids. RS liquids are
liquids with strong correlation between constant volume equilibrium
fluctuations of virial and potential energy and are simpler than other types of
liquids. Moreover, it was shown that size polydisperse LJ liquids have
isomorphs to a good approximation. Isomorphs are curves in the phase diagram of
RS liquids along which structure, dynamics, and some thermodynamic quantities
are invariant in dimensionless (reduced) units. In this paper, we study the
effect of energy polydispersity on the nature of LJ liquids. We show that
energy polydisperse LJ liquids are RS liquids. However, a tendency of particle
segregation which increases with the degree of polydispersity leads to a loss
of strong virial-potential energy correlation, but is mitigated with increasing
temperature and/or density. Isomorphs are a good approximation also for energy
polydisperse LJ liquids, although particle-resolved quantities display a
somewhat poorer scaling compared to the mean quantities along the isomorph. | cond-mat_soft |
Unified Theory of Inertial Granular Flows and Non-Brownian Suspensions: Rheological properties of dense flows of hard particles are singular as one
approaches the jamming threshold where flow ceases, both for aerial granular
flows dominated by inertia, and for over-damped suspensions. Concomitantly, the
lengthscale characterizing velocity correlations appears to diverge at jamming.
Here we introduce a theoretical framework that proposes a tentative, but
potentially complete scaling description of stationary flows. Our analysis,
which focuses on frictionless particles, applies {\it both} to suspensions and
inertial flows of hard particles. We compare our predictions with the empirical
literature, as well as with novel numerical data. Overall we find a very good
agreement between theory and observations, except for frictional inertial flows
whose scaling properties clearly differ from frictionless systems. For
over-damped flows, more observations are needed to decide if friction is a
relevant perturbation or not. Our analysis makes several new predictions on
microscopic dynamical quantities that should be accessible experimentally. | cond-mat_soft |
Numerical studies of aerofractures in porous media / Estudios numericos
de aerofractures en medios porosos: During the hydraulically induced compaction of a granular layer fracture
patterns arise. In numerical simulations we study how these patterns depend on
the gas properties as well as on the properties of the porous medium. In
particular the relation between the speed of fracture propagation and injection
pressure is here studied in detail. | cond-mat_soft |
Bose-Einstein condensates near a microfabricated surface: Magnetically and optically confined Bose-Einstein condensates were studied
near a microfabricated surface. Condensate fragmentation observed in
microfabricated magnetic traps was not observed in optical dipole traps at the
same location. The measured condensate lifetime was $\geq 20$ s and independent
of the atom-surface separation under both magnetic and optical confinement.
Radio-frequency spin-flip transitions driven by technical noise were directly
observed for optically confined condensates and could limit the condensate
lifetime in microfabricated magnetic traps. | cond-mat_soft |
Entangled quantum tunneling of two-component Bose-Einstein condensates: We examine the quantum tunneling process in Bose condensates of two
interacting species trapped in a double well configuration. We discover the
condition under which particles of different species can tunnel as pairs
through the potential barrier between two wells in opposition directions. This
novel form of tunneling is due to the interspecies interaction that eliminates
the self- trapping effect. The correlated motion of tunneling atoms leads to
the generation of quantum entanglement between two macroscopically coherent
systems. | cond-mat_soft |
Simulation of fluid-solid coexistence in finite volumes: A method to
study the properties of wall-attached crystalline nuclei: The Asakura-Oosawa model for colloid-polymer mixtures is studied by Monte
Carlo simulations at densities inside the two-phase coexistence region of fluid
and solid. Choosing a geometry where the system is confined between two flat
walls, and a wall-colloid potential that leads to incomplete wetting of the
crystal at the wall, conditions can be created where a single nanoscopic
wall-attached crystalline cluster coexists with fluid in the remainder of the
simulation box. Following related ideas that have been useful to study
heterogeneous nucleation of liquid droplets at the vapor-liquid coexistence, we
estimate the contact angles from observations of the crystalline clusters in
thermal equilibrium. We find fair agreement with a prediction based on Young's
equation, using estimates of interface and wall tension from the study of flat
surfaces. It is shown that the pressure versus density curve of the finite
system exhibits a loop, but the pressure maximum signifies the "droplet
evaporation-condensation" transition and thus has nothing in common with a van
der Waals-like loop. Preparing systems where the packing fraction is deep
inside the two-phase coexistence region, the system spontaneously forms a "slab
state", with two wall-attached crystalline domains separated by (flat)
interfaces from liquid in full equilibrium with the crystal in between;
analysis of such states allows a precise estimation of the bulk equilibrium
properties at phase coexistence. | cond-mat_soft |
Density functional theory for Baxter's sticky hard spheres in
confinement: It has recently been shown that a free energy for Baxter's sticky hard sphere
fluid is uniquely defined within the framework of fundamental measure theory
(FMT) for the inhomogeneous hard sphere fluid, provided that it obeys
scaled-particle theory and the Percus-Yevick (PY) result for the direct
correlation function [Hansen-Goos and Wettlaufer, J. Chem. Phys. {\bf 134},
014506 (2011)]. Here, combining weighted densities from common versions of FMT
with a new vectorial weighted density, we derive a regularization of the
divergences of the associated strongly confined limit. Moreover, the simple
free energy that emerges is exact in the zero-dimensional limit, leaves the
underlying equation of state unaffected, and yields a direct correlation
function distinct from the PY expression. Comparison with simulation data for
both the bulk pair correlation function and the density profiles in confinement
shows that the new theory is significantly more accurate than the PY-based
results. Finally, the resulting free energy is applicable to a glass of
adhesive hard spheres. | cond-mat_soft |
Mixing by Unstirring: Hyperuniform Dispersion of Interacting Particles
upon Chaotic Advection: We show how to achieve both fast and hyperuniform dispersions of particles in
viscous fluids. To do so, we first extend the concept of critical random
organization to chaotic drives. We show how palindromic sequences of chaotic
advection cause microscopic particles to effectively interact at long range
thereby inhibiting critical self-organization. Based on this understanding we
go around this limitation and design sequences of stirring and unstirring which
simultaneously optimize the speed of particle spreading and the homogeneity of
the resulting dispersions. | cond-mat_soft |
A basic swimmer at low Reynolds number: Swimming and pumping at low Reynolds numbers are subject to the "Scallop
theorem", which states that there will be no net fluid flow for time reversible
motions. Living organisms such as bacteria and cells are subject to this
constraint, and so are existing and future artificial "nano-bots" or
microfluidic pumps. We study a very simple mechanism to induce fluid pumping,
based on the forced motion of three colloidal beads through a cycle that breaks
time-reversal symmetry. Optical tweezers are used to vary the inter-bead
distance. This model is inspired by a strut-based theoretical swimmer proposed
by Najafi and Golestanian [Phys.Rev. E, 69, 062901, 2004], but in this work the
relative softness of the optical trapping potential introduces a new control
parameter. We show that this system is able to generate flow in a controlled
fashion, characterizing the model experimentally and numerically. | cond-mat_soft |
Dielectric microscopy with submillimeter resolution: In analogy with optical near-field scanning methods, we use tapered
dielectric waveguides as probes for a millimeter wave vector network analyzer.
By scanning thin samples between two such probes we are able to map the
spatially varying dielectric properties of materials with sub-wavelength
resolution; using a 150 GHz probe in transmision mode we see spatial resolution
of around 500 microns. We have applied this method to a variety of highly
heterogeneous materials. Here we show dielectric maps of granite and oil shale. | cond-mat_soft |
The many origins of charge inversion in electrolyte solutions: effects
of discrete interfacial charges: We show that charge inversion, i.e. interfacial charges attracting
counterions in excess of their own nominal charge, is a general effect that
takes place in most charged systems next to aqueous solutions with multivalent
ions and identify three different electrostatic origins for this effect 1)
counterion-counterion correlations, 2) correlations between counterions and
interfacial charges and 3) complexation. We briefly describe the first two
regimes and provide a detailed characterization of the complexation regime from
united atom molecular dynamics simulation of a phospholipid domain in contact
with an aqueous solution. We examine the expected conditions where each regime
should apply and describe a representative experimental example to illustrate
each case. We point out that our results provide a characterization of ionic
distributions irrespectively of whether charge inversion takes place and show
that processes such as proton release and transfer are also linked to ionic
correlations. We conclude with a discussion of further experimental and
theoretical implications. | cond-mat_soft |
Network physics of attractive colloidal gels: Resilience, Rigidity, and
Phase Diagram: Attractive colloidal gels exhibit solid-like behavior at vanishingly small
fractions of solids, owing to ramified space-spanning networks that form due to
particle-particle interactions. These networks give the gel its rigidity, and
as the attraction between the particles grows, so does the elasticity of the
colloidal network formed. The emergence of this rigidity can be described
through a mean field approach; nonetheless, fundamental understanding of how
rigidity varies in gels of different attraction strengths is lacking. Moreover,
recovering an accurate gelation phase diagram based on the system's variables
have been an extremely challenging task. Understanding the nature of these
fractal clusters, and how rigidity emerges from their connections is key to
controlling and designing gels with desirable properties. Here, we employ
well-established concepts of network science to interrogate and characterize
the network of colloidal gels. We construct a particle-level network, having
all the spatial coordinates of colloids with different attraction levels, and
also identify polydisperse rigid fractal clusters using a Gaussian Mixture
Model, to form a coarse-grained cluster network that distinctly shows main
physical features of the colloidal gels. A simple mass-spring model then is
used to recover quantitatively the elasticity of colloidal gels from these
cluster networks. Interrogating the resilience of these gel networks show that
the elasticity of a gel (a dynamic property) is directly correlated to its
cluster network's resilience (a static measure). Finally, we use the resilience
investigations to devise [and experimentally validate] a fully resolved phase
diagram for colloidal gelation, with a clear solid-liquid phase boundary using
a single volume fraction of particles well beyond this phase boundary. | cond-mat_soft |
Connecting shear localization with the long-range correlated polarized
stress fields in granular materials: One long-lasting puzzle in amorphous solids is shear localization, where
local plastic deformation involves cooperative particle rearrangements in small
regions of a few inter-particle distances, self-organizing into shear bands and
eventually leading to the material failure. Understanding the connection
between the structure and dynamics of amorphous solids is essential in physics,
material sciences, geotechnical and civil engineering, and geophysics. Here we
show a deep connection between shear localization and the intrinsic structures
of internal stresses in an isotropically jammed granular material subject to
shear. Specifically, we find strong (anti)correlations between the micro shear
bands and two polarized stress fields along two directions of maximal shear. By
exploring the tensorial characteristics and the rotational symmetry of force
network, we reveal that such profound connection is a result of symmetry
breaking by shear. Finally, we provide the solid experimental evidence of
long-range correlated inherent shear stress in an isotropically jammed granular
system. | cond-mat_soft |
Local rheological measurements in the granular flow around an intruder: The rheological properties of granular matter within a two-dimensional flow
around a moving disk is investigated experimentally. Using a combination of
photoelastic and standard tessellation techniques, the strain and stress
tensors are estimated at the grain scale in the time-averaged flow field around
a large disk pulled at constant velocity in an assembly of smaller disks. On
the one hand, one observes inhomogeneous shear rate and strongly localized
shear stress and pressure fields. On the other hand, a significant dilation
rate, which has the same magnitude as the shear strain rate, is reported.
Significant deviations are observed with local rheology that justify the need
of searching for a non-local rheology. | cond-mat_soft |
Statistical Theory of Initiation of Explosives by Impact: When a given weight dropped onto an explosive charge, explosion or not is
probabilistic for certain impact energy and the frequency of explosion is
always increase with increasing impact energy. Based on experimental results
and recently theoretical work, we propose that the hot spot formation is
attributed to the activated molecules decomposition and the number of molecules
initiation is proportional to the impact energy but not the dropped weight
heating as the previous hot spot theory. A theoretical model based on two
states model has been put forward for this phenomena. It is shown that the
activated molecules to form a hot spot determine the probabilistic nature of
initiation by impact. It is shown a good agreement tested with Hexogen (RDX)
experimental impact data. | cond-mat_soft |
Extreme thermodynamics with polymer gel tori: harnessing thermodynamic
instabilities to induce large-scale deformations: When a swollen, thermoresponsive polymer gel is heated in a solvent bath, it
expels solvent and deswells. When this heating is slow, deswelling proceeds
homogeneously, as observed in a toroid-shaped gel that changes volume whilst
maintaining its toroidal shape. By contrast, if the gel is heated quickly, an
impermeable layer of collapsed polymer forms and traps solvent within the gel,
arresting the volume change. The ensuing evolution of the gel then happens at
fixed volume, leading to phase-separation and the development of inhomogeneous
stress that deforms the toroidal shape. We observe that this stress can cause
the torus to buckle out of the plane, via a mechanism analogous to the bending
of bimetallic strips upon heating. Our results demonstrate that thermodynamic
instabilities, i.e., phase transitions, can be used to actuate mechanical
deformation in an extreme thermodynamics of materials. | cond-mat_soft |
Field theoretical analysis of adsorption of polymer chains at surfaces:
Critical exponents and Scaling: The process of adsorption on a planar repulsive, "marginal" and attractive
wall of long-flexible polymer chains with excluded volume interactions is
investigated. The performed scaling analysis is based on formal analogy between
the polymer adsorption problem and the equivalent problem of critical phenomena
in the semi-infinite $|\phi|^4$ n-vector model (in the limit $n\to 0$) with a
planar boundary. The whole set of surface critical exponents characterizing the
process of adsorption of long-flexible polymer chains at the surface is
obtained. The polymer linear dimensions parallel and perpendicular to the
surface and the corresponding partition functions as well as the behavior of
monomer density profiles and the fraction of adsorbed monomers at the surface
and in the interior are studied on the basis of renormalization group field
theoretical approach directly in d=3 dimensions up to two-loop order for the
semi-infinite $|\phi|^4$ n-vector model. The obtained field- theoretical
results at fixed dimensions d=3 are in good agreement with recent Monte Carlo
calculations. Besides, we have performed the scaling analysis of
center-adsorbed star polymer chains with $f$ arms of the same length and we
have obtained the set of critical exponents for such system at fixed d=3
dimensions up to two-loop order. | cond-mat_soft |
Mixing and segregation rates in sheared granular materials: The size-segregation of granular materials, a process colloquially known as
the Brazil Nut Effect, has generally been thought to proceed faster the greater
the size difference of the particles. We experimentally investigate sheared
bidisperse granular materials as a function of the size ratio of the two
species, and find that the mixing rate at low confining pressure behaves as
expected from percolation-based arguments. However, we also observe an
anomalous effect for the re-segregation rates, wherein particles of both
dissimilar and similar sizes segregate more slowly than intermediate particle
size ratios. Combined with the fact that increasing the confining pressure
significantly suppresses both mixing and segregation rates of particles of
dissimilar size, we propose that the anomalous behavior may be attributed to a
species-dependent distribution of forces within the system. | cond-mat_soft |
Geometry-induced rectification for an active object: Study on a rectified current induced by active particles has received a great
attention due to its possible application to a microscopic motor in biological
environments. Insertion of an {\em asymmetric} passive object amid many active
particles has been regarded as an essential ingredient for generating such a
rectified motion. Here, we report that the reverse situation is also possible,
where the motion of an active object can be rectified by its geometric
asymmetry amid many passive particles. This may describe an unidirectional
motion of polar biological agents with asymmetric shape. We also find a weak
but less diffusive rectified motion in a {\em passive} mode without energy
pump-in. This "moving by dissipation" mechanism could be used as a design
principle for developing more reliable microscopic motors. | cond-mat_soft |
Myelin figures from microbial glycolipid biosurfactant amphiphiles: Myelin figures (MFs) -- cylindrical lyotropic liquid crystalline structures
consisting of concentric arrays of bilayers and aqueous media -- arise from the
hydration of the bulk lamellar phase of many common amphiphiles. Prior efforts
have concentrated on the formation, structure, and dynamics of myelin produced
by phosphatidylcholine (PC)-based amphiphiles. Here, we study the myelinization
of glycolipid microbial amphiphiles, commonly addressed as biosurfactants,
produced through the process of fermentation. The hydration characteristics
(and phase diagrams) of these biological amphiphiles are atypical (and thus
their capacity to form myelin) because unlike typical amphiphiles, their
molecular structure is characterized by two hydrophilic groups (sugar,
carboxylic acid) on both ends with a hydrophobic moiety in the middle. We
tested three different glycolipid molecules: C18:1 sophorolipids and
single-glucose C18:1 and C18:0 glucolipids, all in their nonacetylated acidic
form. Neither sophorolipids (too soluble) nor C18:0 glucolipids (too insoluble)
displayed myelin growth at room temperature (RT, 25 C). The glucolipid C18:1
(G-C18:1), on the other hand, showed dense myelin growth at RT below pH 7.0.
Examining their growth rates, we find that they display a linear L $\alpha$ t
(L, myelin length; t, time) growth rate, suggesting ballistic growth,
distinctly different from the L $\alpha$ t^(1/2) dependence, characterizing
diffusive growth such as what occurs in more conventional phospholipids. These
results offer some insight into lipidic mesophases arising from a previously
unexplored class of amphiphiles with potential applications in the field of
drug delivery. | cond-mat_soft |
Dissipative particle dynamics simulations of a single isolated polymer
chain in a dilute solution: In this study, we investigate the suitability of dissipative particle
dynamics (DPD) simulations to predict the dynamics of polymer chains in dilute
polymer solutions, where the chain is represented by a set of beads connected
by almost inextensible springs. In terms of behaviour, these springs closely
mimic rods that serve as representations of Kuhn steps. We find that the
predictions depend on the value of the repulsive parameter for bead-bead
pairwise interactions used in the DPD simulations ($a_{ij}$). For all systems,
the chain sizes and the relaxation time spectrum are analyzed. For $a_{ij} =
0$, theta solvent behaviour is obtained for the chain size, whereas the
dynamics at equilibrium agrees well with the predictions of the Zimm model. For
higher values of $a_{ij}$, the static properties of the chain show good solvent
behaviour. However, the scaling laws for the chain dynamics at equilibrium show
wide variations, with consistent results obtained only at an intermediate value
of $a_{ij} = 25$. At higher values of the repulsive parameter ($a_{ij} \geq
25$), our simulations are also able to predict the abrupt cut-off in the
relaxation spectrum, which has been observed earlier in experiments of dilute
solutions. The cut-off reached an extent that, for chain lengths of 10 Kuhn
steps, the spectrum consists of a single time scale. This agrees remarkably
well with earlier experiments and MD simulations. To verify further, we also
studied the chain dynamics in shear flow using DPD simulations. Specifically,
we analysed the variation of the chain stretch and end-over-end tumbling with
shear rates. Overall, the trends obtained from DPD simulations agree well with
those observed in earlier BD simulations. | cond-mat_soft |
Characterization of invariant patterns in a slowly rotated granular
tumbler: We report experimental results of the pattern developed by a mixture of two
types of grains in a triangular rotating tumbler operating in the avalanche
regime. At the centroid of the triangular tumbler an invariant zone appears
where the grains do not move relative to the tumbler. We characterize this
invariant zone by its normalized area, $A_i$, and its circularity index as a
function of the normalized filling area $A$. We find a critical filling area so
that only for $A>A_c$ invariant zones are obtained. These zones scale as
$A_i\sim (A-A_c)^2$ near $A_c$. We have obtained a maximum in the circularity
index for $A\approx 0.8$, for which the shape of the invariant zone is closer
to a circular one. The experimental results are reproduced by a simple model
which, based on the surface position, accounts for all the possible straight
lines within the triangle that satisfy the condition of constant $A$. We have
obtained an analytic expression for the contour of the invariant zone. | cond-mat_soft |
Formation of Non-uniform Double Helices for Elastic Rods under Torsion: The spontaneous formation of double helices for filaments under torsion is
common and significant. For example, the research on the supercoiling of DNA is
helpful for understanding the replication and transcription of DNA. Similar
double helices can appear in polymer chains, carbon nanotube yarns, cables,
telephone wires and so forth. We noticed that non-uniform double helices can be
produced due to the surface friction induced by the self-contact. Therefore an
ideal model was presented to investigate the formation of double helices for
elastic rods under torque. A general equilibrium condition which is valid for
both the smooth surface and the rough surface situations is derived by using
the variational method. Based on this, by adding further constraints, the
smooth and rough surface situations are investigated in detail respectively.
Additionally, the model showed that the specific process of how to twist and
slack the rod can determine the surface friction and hence influence the
configuration of the double helix formed by rods with rough surfaces. Based on
this principle, a method of manufacturing double helices with designed
configurations was proposed and demonstrated. Finally, experiments were
performed to verify the model and the results agreed well with the theory. | cond-mat_soft |
Depletion induced isotropic-isotropic phase separation in suspensions of
rod-like colloids: When non-adsorbing polymers are added to an isotropic suspension of rod-like
colloids, the colloids effectively attract each other via depletion forces. We
performed Monte Carlo simulations to study the phase diagram of such
rod-polymer mixture. The colloidal rods were modelled as hard spherocylinders;
the polymers were described as spheres of the same diameter as the rods. The
polymers may overlap with no energy cost, while overlap of polymers and rods is
forbidden.
Large amounts of depletant cause phase separation of the mixture. We
estimated the phase boundaries of isotropic-isotropic coexistence both, in the
bulk and in confinement. To determine the phase boundaries we applied the grand
canonical ensemble using successive umbrella sampling [J. Chem. Phys. 120,
10925 (2004)], and we performed a finite-size scaling analysis to estimate the
location of the critical point. The results are compared with predictions of
the free volume theory developed by Lekkerkerker and Stroobants [Nuovo Cimento
D 16, 949 (1994)]. We also give estimates for the interfacial tension between
the coexisting isotropic phases and analyse its power-law behaviour on approach
of the critical point. | cond-mat_soft |
Stoichiometry of Electrostatic Complexes Determined by Light Scattering: We report on the electrostatic complexation between oppositely charged
polymers and inorganic nanoparticles investigated by static and dynamical light
scattering. The nanoparticles put under scrutiny were citrate-coated
nanocrystals of cerium oxide (CeO2, nanoceria), of iron oxide (Fe2O3,
maghemite) and of europium-doped yttrium vanadate (Eu:YVO4) with sizes in the
10 nm range. For the polymers, we have used cationic-neutral diblock copolymers
(poly(trimethylammonium ethylacrylate)-b-poly(acrylamide), hereafter referred
to as PTEA-b-PAM) with different molecular weights. For the three colloidal
dispersions, we show that the electrostatic complexation gives rise to the
formation of stable nanoparticle clusters in the 100 nm range. The complexation
was monitored by systematic measurements of the scattering intensity versus X,
the mixing ratio between nanoparticles and polymers. For 5 nanoparticle/polymer
pairs, namely CeO2/PTEA5K-b-PAM30K, Fe2O3/PTEA5K-b-PAM30K,
Fe2O3/PTEA11K-b-PAM30K, Eu:YVO4/PTEA2K-b-PAM60K and Eu:YVO4/PTEA5K-b-PAM30K, we
found a unique behavior : the scattering intensity exhibits a sharp and
prominent peak in the intermediate X-range. To account for this behavior, we
have developed a model which assumes that regardless of X, the mixed aggregates
are formed at a fixed polymer-to-nanoparticle ratio. The agreement between the
results and the model is excellent on the 5 systems. Results at different
molecular weights suggest that the stoichiometry of the mixed aggregates is
controlled by the electrostatic interactions between the opposite charges. The
model allows to derive the molecular weight and the stoichiometry of the mixed
aggregates. | cond-mat_soft |
A nonlinear dynamical system approach for the yielding behaviour of a
viscoplastic fluid: A nonlinear dynamical system model that approximates a microscopic Gibbs
field model for the yielding of a viscoplastic material subjected to varying
external stress recently reported in [1] is presented. The predictions of the
model are in a fair agreement with the microscopic simulations and in a very
good agreement with the microstructural semi-empirical model reported in [2].
With only two internal parameters, the nonlinear dynamical system model
captures several key features of the solid-fluid transition observed in
experiments: the effect of the interactions between microscopic constituents on
the yield point, the abruptness of solid-fluid transition and the emergence of
a hysteresis of the micro-structural states upon increasing/decreasing external
forcing.The scaling behaviour of the magnitude of the hysteresis with the
degree of the steadiness of the flow is consistent with previous experimental
observations. | cond-mat_soft |
On the Determination of the Transition to Pure Reptation by Dielectric
Spectroscopy: Polymer melts show a characteristic molecular weight dependent relaxation
time that can be related to the unentangled and entangled regime. At high
molecular weights, influence of contour length fluctuations and con-straint
release cease and pure reptation prevails. With the broad frequency range
dielectric spectroscopy can fol-low polymer dynamics over a very broad
temperature range, with the additional advantage of recording the spec-tral
shape which contains information on reptation. Here we investigate the apparent
discrepancy in the molecular weight for the onset of pure reptation from the
molecular weight dependence and the spectral shape. We examined the popular
derivative method and compared it with a version that includes higher order
terms. Higher order terms lead to a more accurate peak shape and position than
those determined with the simpler version. This becomes important if
experimental spectra contain conductivity and polarization contributions.
Higher order terms require the introduction of an interpolating function to
analyze experimental spectra, which lets the Havriliak-Negami function appear
to be a more robust, yet reliable tool to determine the peak shapes. We reach
the conclusion that molecular weight dependence and spectral shape can be both
strongly affected by conductivity and polarization contributions. While this
leaves uncertainties on the accurate value of the transition molecular weight,
the peak shape points to the existence of reptation and contour length
fluctuations in polyisoprene with a molecular weight greater than 1000 kg/mol,
which would imply a ten times greater threshold molecular weight than expected
from previous estimates using the molecular weight dependence. | cond-mat_soft |
Fluid Mechanical and Electrical Fluctuation Forces in Colloids: Fluctuations in fluid velocity and fluctuations in electric fields may both
give rise to forces acting on small particles in colloidal suspensions. Such
forces in part determine the thermodynamic stability of the colloid. At the
classical statistical thermodynamic level, the fluid velocity and electric
field contributions to the forces are comparable in magnitude. When quantum
fluctuation effects are taken into account, the electric fluctuation induced
van der Waals forces dominate those induced by purely fluid mechanical motions.
The physical principles are applied in detail for the case of colloidal
particle attraction to the walls of the suspension container and more briefly
for the case of forces between colloidal particles. | cond-mat_soft |
Confined self-propulsion of an isotropic active colloid: To spontaneously break their intrinsic symmetry and self-propel at the micron
scale, isotropic active colloidal particles and droplets exploit the non-linear
convective transport of chemical solutes emitted/consumed at their surface by
the surface-driven fluid flows generated by these solutes. Significant progress
was recently made to understand the onset of self-propulsion and non-linear
dynamics. Yet, most models ignore a fundamental experimental feature, namely
the spatial confinement of the colloid, and its effect on propulsion. In this
work, the self-propulsion of an isotropic colloid inside a capillary tube is
investigated numerically. A flexible computational framework is proposed based
on a finite-volume approach on adaptative octree-grids and embedded boundary
methods. This method is able to account for complex geometric confinement, the
nonlinear coupling of chemical transport and flow fields, and the precise
resolution of the surface boundary conditions, that drive the system's
dynamics. Somewhat counter-intuitively, spatial confinement promotes the
colloid's spontaneous motion by reducing the minimum advection-to-diffusion
ratio or P\'clet number, Pe, required to self-propel; furthermore,
self-propulsion velocities are significantly modified as the
colloid-to-capillary size ratio $\kappa$ is increased, reaching a maximum at
fixed Pe for an optimal confinement $0<\kappa<1$. These properties stem from a
fundamental change in the dominant chemical transport mechanism with respect to
the unbounded problem : with diffusion now restricted in most directions by the
confining walls, the excess solute is predominantly convected away downstream
from the colloid, enhancing front-back concentration contrasts. These results
are confirmed quantitatively using conservation arguments and lubrication
analysis of the tightly-confined limit, $\kappa\rightarrow 1$. | cond-mat_soft |
Computational Modeling of an MRI Guided Drug Delivery System Based on
Magnetic Nanoparticle Aggregations for the Navigation of Paramagnetic
Nanocapsules: A computational method for magnetically guided drug delivery is presented and
the results are compared for the aggregation process of magnetic particles
within a fluid environment. The model is developed for the simulation of the
aggregation patterns of magnetic nanoparticles under the influence of MRI
magnetic coils. A novel approach for the calculation of the drag coefficient of
aggregates is presented. The comparison against experimental and numerical
results from the literature is showed that the proposed method predicts well
the aggregations in respect to their size and pattern dependance, on the
concentration and the strength of the magnetic field, as well as their velocity
when particles are driven through the fluid by magnetic gradients. | cond-mat_soft |
Experimental observations of non-equilibrium distributions and
transitions in a 2D granular gas: A large number (~10,000) of uniform stainless steel balls comprising less
than one layer coverage on a vertically shaken plate provides a rich system for
the study of excited granular media. Viewed from above, the horizontal motion
in the layer shows interesting collective behavior as a result of inelastic
particle-particle collisions. Clusters appear as localized fluctuations from
purely random density distributions, as demonstrated by increased particle
correlations. The clusters grow as the medium is "cooled" by reducing the rate
of energy input. Further reduction of the energy input leads to the nucleation
of a collapse: a close-packed crystal of particles at rest. High speed
photography allows for measurement of particle velocities between collisions.
The velocity distributions deviate strongly from a Maxwell distribution at low
accelerations, and show approximately exponential tails, possibly due to an
observed cross-correlation between density and velocity fluctuations. When the
layer is confined with a lid, the velocity distributions at higher
accelerations are non-Maxwellian and independent of the granular temperature. | cond-mat_soft |
Bulk viscosity of the Lennard-Jones system at the triple point by
dynamical Non Equilibrium Molecular Dynamics: Non-equilibrium Molecular Dynamics (NEMD) calculations of the bulk viscosity
of the triple point Lennard-Jones fluid are performed with the aim of
investigating the origin of the observed disagreement between Green-Kubo
estimates and previous NEMD data. We show that a careful application of the
Doll's perturbation field, the dynamical NEMD method, the instantaneous form of
the perturbation and the "subtraction technique" provides a NEMD estimate of
the bulk viscosity at zero field in full agreement with the value obtained by
the Green-Kubo formula. As previously reported for the shear viscosity, we find
that the bulk viscosity exhibits a large linear regime with the field intensity
which confirms the Lennard-Jones fluid as a genuine Newtonian fluid even at
triple point. | cond-mat_soft |
Variational approximation method for the long-range force transmission
in biopolymer gels: The variational principle of minimum free energy (MFEVP) has been widely used
in the study of soft matter statics. MFEVP can be used not only to derive
equilibrium equations (including both bulk equations and boundary conditions),
but also to develop direct variational methods (such as Ritz method) to find
approximate solutions to these equilibrium equations. In this work, we applied
these variational methods to study long-range force transmission in nonlinear
elastic biopolymer gels. We showed that the slow decay of cell-induced
displacements measured experimentally for fibroblast spheroids in
three-dimensional fibrin gels can be well explained by variational
approximations based on the three-chain model of biopolymer gels. | cond-mat_soft |
Direct measurement of the critical pore size in a model membrane: We study pore nucleation in a model membrane system, a freestanding polymer
film. Nucleated pores smaller than a critical size close, while pores larger
than the critical size grow. Holes of varying size were purposefully prepared
in liquid polymer films, and their evolution in time was monitored using
optical and atomic force microscopy to extract a critical radius. The critical
radius scales linearly with film thickness for a homopolymer film. The results
agree with a simple model which takes into account the energy cost due to
surface area at the edge of the pore. The energy cost at the edge of the pore
is experimentally varied by using a lamellar-forming diblock copolymer
membrane. The underlying molecular architecture causes increased frustration at
the pore edge resulting in an enhanced cost of pore formation. | cond-mat_soft |
Mixed equilibrium/nonequilibrium effects govern surface mobility in
polymer glasses: The temperature at which supercooled liquids turn into solid-like glasses
($T_g$) can change at the free surface, affecting the properties of
nanostructured glasses and their applications. However, inadequate experimental
resolution to determine the $T_g$ gradient and a longstanding debate over the
role of nonequilibrium effects have hindered fundamental understanding of this
phenomenon. Using spatially resolved $T_g$ measurements and molecular dynamics
simulations, we reveal a crossover from equilibrium behavior to a new regime of
near-surface nonequilibrium glass physics on cooling. This crossover causes the
form of the nonequilibrium $T_g$ gradient to change, highlighting the need to
include these physics for rational understanding of the properties of realistic
nanostructured glass-forming materials. They also potentially recast the
interpretation of decades of experimental data on nanoconfined glasses. | cond-mat_soft |
Self-Propelling Rotator Driven by Soluto-Capillary Marangoni Flows: The self-propelled, longstanding rotation of the polymer tubing containing
camphor continuing for dozens of hours is reported. The rotator is driven by
the solutocapillary Marangoni flows owing to the dissolution of camphor. The
phenomenological model of self-propulsion is suggested and verified. Scaling
laws describing the quasi-stationary self-propulsion are proposed and tested
experimentally. The change in the surface tension, arising from the dissolution
of camphor and driving the rotator is estimated as 0.3 mN/m. | cond-mat_soft |
Mechanical surface tension governs membrane thermal fluctuations: Motivated by the still ongoing debate about the various possible meanings of
the term surface tension of bilayer membranes, we present here a detailed
discussion that explains the differences between the "intrinsic",
"renormalized", and "mechanical" tensions. We use analytical considerations and
computer simulations to show that the membrane spectrum of thermal fluctuations
is governed by the mechanical and not the intrinsic tension. Our study
highlights the fact that the commonly used quadratic approximation of Helfrich
effective Hamiltonian is not rotationally invariant. We demonstrate that this
non-physical feature leads to a calculated mechanical tension that differs
dramatically from the correct mechanical tension. Specifically, our results
suggest that the mechanical and intrinsic tensions vanish simultaneously, which
contradicts recent theoretical predictions derived for the approximated
Hamiltonian. | cond-mat_soft |
Phase behavior of wormlike rods: By employing Molecular Dynamics computer simulations, the phase behavior of
systems of rodlike particles with varying degree of internal flexibility has
been traced from the perfectly rigid rod limit till very flexible particles,
and from the high density region till the isotropic phase. From the perfectly
rigid rod limit and enhancing the internal flexibility, the range of the
smectic A phase is squeezed out by the concomitant action of the scarcely
affected crystalline phase at higher density and the nematic phase at lower
density, until it disappears. These results confirm the supposition, drawn from
previous theoretical, simulational and experimental studies, that the smectic A
phase is destabilized by introducing and enhancing the degree of particle
internal flexibility. However, no significant changes in the order of
nematic--to--smectic A phase transition, which appears always first order, nor
in the value of the layer spacing, are observed upon varying the degree of
particle internal flexibility. Moreover, no evidence of a columnar phase, which
was tought of as a possible superseder of the smectic A phase in flexible rods,
has been obtained. | cond-mat_soft |
Liquid crystal phases with unusual structures and physical properties
formed by acute-angle bent-core molecules: Liquid crystals formed by acute-angle bent-core (ABC) molecules with a 1,7
naphthalene central core show an intriguing phase behavior with the nematic
phase accompanied by poorly understood additional phases. In this work, we
characterize the physical properties of an ABC material, such as birefringence,
dielectric permittivities, elastic constants, and surface alignment, and
present X-ray diffraction and transmission electron microscopy studies of their
ordering. The ABC molecular shape resembling the letter $\lambda$ yields a very
small splay elastic constant in the uniaxial nematic phase and results in the
formation of a tetragonal positionally ordered columnar phase consisting of
molecular columns with a uniform uniaxial director that can be bent but not
splayed. | cond-mat_soft |
Dynamics of Force Dipoles in Curved Biological Membranes: We construct a model to explore the hydrodynamic interactions of active
inclusions in curved biological membranes. The curved membrane is modelled as a
two dimensional layer of highly viscous fluid, surrounded by external solvents
of different viscosities. The active inclusions are modelled as point force
dipoles. The point dipole limit is taken along a geodesic of the curved
geometry, incorporating the change in orientation of the forces due to
curvature. We demonstrate this explicitly for the case of a spherical membrane,
leading to an analytic solution for the flow generated by a single inclusion.
We further show that the flow field features an additional defect of negative
index, arising from the membrane topology, which is not present in the planar
version of the model. We finally explore the hydrodynamic interactions of a
pair of inclusions in regimes of low and high curvature, as well as situations
where the external fluid outside the membrane is confined. Our study suggests
aggregation of dipoles in curved biological membranes of both low and high
curvatures, under strong confinement. However, very high curvatures tend to
destroy dipole aggregation, even under strong confinement. | cond-mat_soft |
A mechanistic model for the growth of cylindrical debris particles in
the presence of adhesion: The wear volume is known to keep increasing during frictional processes, and
Archard notably proposed a model to describe the probability of wear particle
formation upon asperity collision in a two-body contact configuration. While
this model is largely adopted in the investigations of wear, the presence of
wear debris trapped between the surfaces changes the system into a three-body
contact configuration already since the early stages of the process. In such a
configuration, a significant amount of wear is produced at the interface
between the trapped debris and the sliding bodies. Here, relying on analytical
models, we develop a framework that describes crack growth in a three-body
configuration at the particle-surface interface. We then show that crack growth
is favoured within the sliding surfaces, instead of within the debris particle,
and test such result by means of numerical simulations with a phase-field
approach to fracture. This leads to an increase in the wear volume and to
debris particle accretion, rather than its break down. The effects of adhesion,
coefficient of friction, and ratio of the applied global tangential and normal
forces are also investigated. | cond-mat_soft |
Packing frustration in dense confined fluids: Packing frustration for confined fluids, i.e., the incompatibility between
the preferred packing of the fluid particles and the packing constraints
imposed by the confining surfaces, is studied for a dense hard-sphere fluid
confined between planar hard surfaces at short separations. The detailed
mechanism for the frustration is investigated via an analysis of the
anisotropic pair distributions of the confined fluid, as obtained from integral
equation theory for inhomogeneous fluids at pair correlation level within the
anisotropic Percus-Yevick approximation. By examining the mean forces that
arise from interparticle collisions around the periphery of each particle in
the slit, we calculate the principal components of the mean force for the
density profile - each component being the sum of collisional forces on a
particle's hemisphere facing either surface. The variations of these components
with the slit width give rise to rather intricate changes in the layer
structure between the surfaces, but, as shown in this paper, the basis of these
variations can be easily understood qualitatively and often also
semi-quantitatively. It is found that the ordering of the fluid is in essence
governed locally by the packing constraints at each single solid-fluid
interface. A simple superposition of forces due to the presence of each surface
gives surprisingly good estimates of the density profiles, but there remain
nontrivial confinement effects that cannot be explained by superposition, most
notably the magnitude of the excess adsorption of particles in the slit
relative to bulk. | cond-mat_soft |
Theory of Chiral Modulations and Fluctuations in Smectic-A Liquid
Crystals Under an Electric Field: Chiral liquid crystals often exhibit periodic modulations in the molecular
director; in particular, thin films of the smectic-C* phase show a chiral
striped texture. Here, we investigate whether similar chiral modulations can
occur in the induced molecular tilt of the smectic-A phase under an applied
electric field. Using both continuum elastic theory and lattice simulations, we
find that the state of uniform induced tilt can become unstable when the system
approaches the smectic-A--smectic-C* transition, or when a high electric field
is applied. Beyond that instability point, the system develops chiral stripes
in the tilt, which induce corresponding ripples in the smectic layers. The
modulation persists up to an upper critical electric field and then disappears.
Furthermore, even in the uniform state, the system shows chiral fluctuations,
including both incipient chiral stripes and localized chiral vortices. We
compare these predictions with observed chiral modulations and fluctuations in
smectic-A liquid crystals. | cond-mat_soft |
Many-Body Force and Mobility Measurements in Colloidal Systems: We demonstrate a technique for simultaneously measuring each component of the
force vectors and mobility tensor of a small collection of colloidal particles
based on observing a set of particle trajectories. For a few-body system of
micron-sized polymer beads in oil separated by several particle radii, we find
that the mobility tensor is well-described by a pairwise Stokeslet model. This
stands in contrast to the electrostatic interactions, which were found to
deviate significantly from a pairwise model. The measurement technique
presented here should be simple to extend to systems of heterogeneous,
non-spherical particles arranged in arbitrary 3D geometries. | cond-mat_soft |
Incremental response of granular materials: DEM results: We systematically investigate the incremental response of various equilibrium
states of dense 2D model granular materials, along the biaxial compression path
(\sigma 11 < \sigma 22, \sigma 12 = 0). Stress increments are applied in
arbitrary directions in 3- dimensional stress space (\sigma 11, \sigma 22,
\sigma 12). In states with stable contact networks we compute the stiffness
matrix and the elastic moduli, and separate elastic and irreversible strains in
the range in which the latter are homogeneous functions of degree one of stress
increments. Without principal stress axis rotation, the response abides by
elastoplasticity with a Mohr-Coulomb criterion and a non-associated flow rule.
However a nonelastic shear strain is also observed for increments of \sigma 12,
and shear and in-plane responses couple. This behavior correlates to the
distribution of friction mobilization and sliding at contacts. | cond-mat_soft |
Stress Relaxation in Entangled Polymer Melts: We present an extensive set of simulation results for the stress relaxation
in equilibrium and step-strained bead-spring polymer melts. The data allow us
to explore the chain dynamics and the shear relaxation modulus, $G(t)$, into
the plateau regime for chains with $Z=40$ entanglements and into the terminal
relaxation regime for $Z=10$. Using the known (Rouse) mobility of unentangled
chains and the melt entanglement length determined via the primitive path
analysis of the microscopic topological state of our systems, we have performed
parameter -free tests of several different tube models. We find excellent
agreement for the Likhtman-McLeish theory using the double reptation
approximation for constraint release, if we remove the contribution of
high-frequency modes to contour length fluctuations of the primitive chain. | cond-mat_soft |
Liquid crystal-enabled electrophoresis of spheres in a nematic medium
with negative dielectric anisotropy: We describe electrophoresis of spherical dielectric particles in a uniformly
aligned nematic medium with a negative dielectric anisotropy. A spherical
particle that orients the liquid crystal (LC) perpendicularly to its surface
moves under the application of the uniform direct current (DC) or alternating
current (AC) electric field. The electric field causes no distortions of the LC
director far away from the sphere. Electrophoresis in the nematic LC shows two
types of nonlinearity in the velocity vs. field dependence. The velocity
component parallel to the applied electric field grows linearly with the field,
but when the field is high enough, it also shows a cubic dependence. The most
interesting is the second type of nonlinear electrophoresis that causes the
sphere to move perpendicularly to the applied field. This perpendicular
component of velocity is proportional to the square of the field. The effect
exists only in a LC and disappears when the material is melted into an
isotropic fluid. The quadratic effect is caused by the dipolar symmetry of
director distortions around the sphere and is classified as an LC-enabled
electrophoresis (LCEEP). The nonlinear electrophoretic mobility of particles in
LCEEP offers a rich variety of control parameters to design 3D trajectories of
particles for microfluidic and optofluidic applications. | cond-mat_soft |
Assessing long-range contributions to the charge asymmetry of ion
adsorption at the air-water interface: Anions generally associate more favorably with the air-water interface than
cations. In addition to solute size and polarizability, the intrinsic structure
of the unperturbed interface has been discussed as an important contributor to
this bias. Here we assess quantitatively the role that intrinsic charge
asymmetry of water's surface plays in ion adsorption, using computer
simulations to compare model solutes of various size and charge. In doing so,
we also evaluate the degree to which linear response theory for solvent
polarization is a reasonable approach for comparing the thermodynamics of bulk
and interfacial ion solvation. Consistent with previous works on bulk ion
solvation, we find that the average electrostatic potential at the center of a
neutral, sub-nanometer solute at the air-water interface depends sensitively on
its radius, and that this potential changes quite nonlinearly as the solute's
charge is introduced. The nonlinear response closely resembles that of the
bulk. As a result, the net nonlinearity of ion adsorption is weaker than in
bulk, but still substantial, comparable to the apparent magnitude of
macroscopically nonlocal contributions from the undisturbed interface. For the
simple-point-charge model of water we study, these results argue distinctly
against rationalizing ion adsorption in terms of surface potentials inherent to
molecular structure of the liquid's boundary. | cond-mat_soft |
Anisotropic Hydrodynamic Mean-Field Theory for Semiflexible Polymers
under Tension: We introduce an anisotropic mean-field approach for the dynamics of
semiflexible polymers under intermediate tension, the force range where a chain
is partially extended but not in the asymptotic regime of a nearly straight
contour. The theory is designed to exactly reproduce the lowest order
equilibrium averages of a stretched polymer, and treats the full complexity of
the problem: the resulting dynamics include the coupled effects of long-range
hydrodynamic interactions, backbone stiffness, and large-scale polymer contour
fluctuations. Validated by Brownian hydrodynamics simulations and comparison to
optical tweezer measurements on stretched DNA, the theory is highly accurate in
the intermediate tension regime over a broad dynamical range, without the need
for additional dynamic fitting parameters. | cond-mat_soft |
A Model for Double-Stranded Excitations of DNA: We calculate the spectrum of torsional vibrations of a double-stranded
structure that models the double helix of the DNA. We come to the conclusion
that within the framework of the model elementary excitations may display an
asymmetry as regards their winding and direction of the propagation, depending
on initial polarization. The asymmetry could have a bearing on processes that
take place in molecules of the DNA. | cond-mat_soft |
Critical Casimir interactions between Janus particles: Recently there is strong experimental and theoretical interest in studying
the self-assembly and the phase behavior of patchy and of Janus particles,
which form colloidal suspensions. Although in this quest a variety of effective
interactions have been proposed and used in order to achieve directed assembly,
the critical Casimir effect stands out as being particularly suitable in this
respect because it provides both attractive and repulsive interactions as well
as the potential of a sensitive temperature control of their strength.
Specifically, we have calculated the critical Casimir force between a single
Janus particle and a laterally homogeneous substrate as well as a substrate
with a chemical step. We have used the Derjaguin approximation and compared it
with results from full mean field theory. A modification of the Derjaguin
approximation turns out to be generally reliable. Based on this approach we
have derived the effective force and the effective potential between two Janus
cylinders as well as between two Janus spheres. | cond-mat_soft |
DNA uptake into nuclei: Numerical and analytical results: The dynamics of polymer translocation through a pore has been the subject of
recent theoretical and experimental works. We have considered theoretical
estimates and performed computer simulations to understand the mechanism of DNA
uptake into the cell nucleus, a phenomenon experimentally investigated by
attaching a small bead to the free end of the double helix and pulling this
bead with the help of an optical trap. The experiments show that the uptake is
monotonous and slows down when the remaining DNA segment becomes very short.
Numerical and analytical studies of the entropic repulsion between the DNA
filament and the membrane wall suggest a new interpretation of the experimental
observations. Our results indicate that the repulsion monotonically decreases
as the uptake progresses. Thus, the DNA is pulled in (i) either by a small
force of unknown origin, and then the slowing down can be interpreted only
statistically; (ii) or by a strong but slow ratchet mechanism, which would
naturally explain the observed monotonicity, but then the slowing down requires
additional explanations. Only further experiments can unambiguously distinguish
between these two mechanisms. | cond-mat_soft |
Dewetting of Thin Viscoelastic Polymer Films on Slippery Substrates: Dewetting of thin polystyrene films deposited onto silicone wafers at
temperatures close to the glass transition exhibits unusual dynamics and front
morphologies. Here, we present a new theoretical approach of these phenomena
taking into account both the viscoelastic properties of the film and the
non-zero velocity of the film at the interface with the substrate (due to
slippage). We then show how these two ingredients lead to : (a) A very
asymmetric shape of the rim as the film dewetts, (b) A decrease of the
dewetting velocity with time like $t^{-{1/2}}$ for times shorter than the
reptation time (for larger times, the dewetting velocity reaches a constant
value). Very recent experiments by Damman, Baudelet and Reiter [Phys. Rev.
Lett. {\bf 91}, 216101 (2003)] present, however, a much faster decrease of the
dewetting velocity. We then show how this striking result can be explained by
the presence of residual stresses in the film. | cond-mat_soft |
Quantifying thermally induced flowability of rennet cheese curds: Conversion of liquid milk to cheese curds is the first stage in cheese
manufacture. Changing the rigidity of cheese curds through heating and pH
control is an established method for preparing fresh curds, whereas a similar
method to prepare fully coagulated curds is largely unknown. This study
elucidated the effect of temperature variation on the viscoelastic moduli of
fully coagulated curds under different pH conditions. The results showed that
rennet curds treated at pH 4.8 exhibited drastic changes in the viscoelasticity
at 43 degrees C, above which the degree of fluidity exceeded the degree of
rigidity. The viscoelastic moduli exhibited exponential decay as a function of
temperature, which was independent of pH. | cond-mat_soft |
Suppression and enhancement of impurity scattering in a Bose-Einstein
condensate: Impurity atoms propagating at variable velocities through a trapped
Bose-Einstein condensate were produced using a stimulated Raman transition. The
redistribution of momentum by collisions between the impurity atoms and the
stationary condensate was observed in a time-of-flight analysis. The
collisional cross section was dramatically reduced when the velocity of the
impurities was reduced below the speed of sound of the condensate, in agreement
with the Landau criterion for superfluidity. For large numbers of impurity
atoms, we observed an enhancement of atomic collisions due to bosonic
stimulation. This enhancement is analogous to optical superradiance. | cond-mat_soft |
Colloidal suspensions of C-particles: Entanglement, percolation and
microrheology: We explore structural and dynamical behavior of concentrated colloidal
suspensions made up by C-shape particles using Brownian dynamics computer
simulations and theory. In particular, we focus on the entanglement process
between nearby particles for almost closed C-shapes with a small opening angle.
Depending on the opening angle and the particle concentration, there is a
percolation transition for the cluster of entangled particles which shows the
classical scaling characteristics. In a broad density range below the
percolation threshold, we find a stretched exponential function for the
dynamical decorrelation of the entanglement process. Finally, we study a set-up
typical in microrheology by dragging a single tagged particle with constant
speed through the suspension. We measure the cluster connected to and dragged
with this tagged particle. In agreement with a phenomenological theory, the
size of the dragged cluster depends on the dragging direction and increases
markedly with the dragging speed. | cond-mat_soft |
Influence of salt on membrane rigidity of neu-tral DOPC vesicles: Salt is a very common molecule in aqueous environments but the question of
whether the interactions of monovalent ions Na^+ and Cl^- ,with the neutral
heads of phospholipids are impactful enough to change the membrane rigidity is
still a mystery. To provide a resolution to this long simmering debate, we
investigated the dynamics of DOPC vesicles in the fluid phase with increasing
external salt concentration. At higher salt concentrations, we observe an
increase in bending rigidity from neutron spin echo spectroscopy (NSE) and an
increase in bilayer thickness from small-angle X-ray scattering (SAXS). We
compared different models to distinguish membrane undulations, lipid tail
motions and the translational diffusion of the vesicles. All the models
indicate an increase in bending rigidity by a factor of 1.3 to 3.6. We
demonstrate that even for t > 10 ns, and for Q > 0.07 1/{\AA} the observed NSE
relaxation spectra is clearly influenced by the translational diffusion of the
vesicles. For t < 5 ns, the lipid tail motions dominate the intermediate
dynamic structure factor. As the salt concentration increases this contribution
diminishes. We introduced a new time-dependent analysis for the bending
rigidity that highlights only a limited Zilman-Granek time window where the
rigidity is physically meaningful. | cond-mat_soft |
Soft Matrix: Extracting Inherent Length Scales in Sheared Amorphous
Solids: Amorphous solids yield upon crossing a strain threshold, after an initial
elastic response, when subjected to mechanical deformation. The yielding
process is characterized by local plastic events leading to non-affine
displacements, and their interactions. Despite the lack of long-range
structural order, these disordered materials exhibit long-range spatial
correlations in the non-affine displacement fields, which stems from the
underlying elasticity. Measuring a correlation length scale in deformed
amorphous solids, during the plastic process, is a non-trivial challenge, often
requiring an ad-hoc definition of localized regions. In this paper, we
introduce a novel computational approach called the "soft matrix" that enables
systematic analysis of mechanical response of local regions within a disordered
solid. In this method, we subject the amorphous solid to a quasistatic shear
and allow a local region of interest to relax freely while allowing for elastic
relaxation of the background. The dependence of the yield strain upon the size
of the probe region naturally reveals the existence of an intrinsic length
scale ($\zeta$) that governs the elasto-plastic properties, as observed in four
distinct model amorphous solids. This finding demonstrates the universality of
this characteristic length scale across a wide range of materials. We
investigate the dependence of this length scale on the material's preparation
history and find that $\zeta$ increases with better annealing. Furthermore, the
local mechanical properties measured within this framework provide more
accurate estimates compared to existing techniques. Our study paves the way for
a comprehensive understanding of amorphous solids and facilitates improved
characterization and design of these materials. | cond-mat_soft |
Compression stiffening in biological tissues: on the possibility of
classic elasticity origins: Compression stiffening, or an increase in shear modulus with increasing
compressive strain, has been observed in recent rheometry experiments on brain,
liver, and fat tissues. Here, we extend the known types of biomaterials
exhibiting this phenomenon to include agarose gel and fruit flesh. Further, we
show that two different results from classic elasticity theory can account for
the phenomenon of linear compression stiffening. One result is due to Barron
and Klein, extended here to the relevant geometry and pre-stresses; the other
is due to Birch. For incompressible materials, there are no adjustable
parameters in either theory. Which one applies to a given situation is a matter
of reference state, suggesting that the reference state is determined by the
tendency of the material to develop, or not develop, axial stress (in excess of
the applied pre-stress) when subjected to torsion at constant axial strain. Our
experiments and analysis also strengthen the notion that seemingly distinct
animal and plant tissues can have mechanically similar behavior under certain
conditions. | cond-mat_soft |
Measuring storage and loss moduli using optical tweezers: broadband
microrheology: We present an experimental procedure to perform broadband microrheological
measurements with optical tweezers. A generalised Langevin equation is adopted
to relate the time-dependent trajectory of a particle in an imposed flow to the
frequency-dependent moduli of the complex fluid. This procedure allows us to
measure the material linear viscoelastic properties across the widest frequency
range achievable with optical tweezers. | cond-mat_soft |
Simultaneous concentration and velocity maps in particle suspensions
under shear from rheo-ultrasonic imaging: We extend a previously developed ultrafast ultrasonic technique [Gallot et
al., Rev. Sci. Instrum. 84, 045107 (2013)] to concentration field measurements
in non-Brownian particle suspensions under shear. The technique provides access
to time-resolved concentration maps within the gap of a Taylor-Couette cell
simultaneously to local velocity measurements and standard rheological
characterization. Benchmark experiments in homogeneous particle suspensions are
used to calibrate the system. We then image heterogeneous concentration fields
that result from centrifugation effects, from the classical Taylor-Couette
instability and from sedimentation or shear-induced resuspension. | cond-mat_soft |
Lattice Boltzmann study of chemically-driven self-propelled droplets: We numerically study the behavior of self-propelled liquid droplets whose
motion is triggered by a Marangoni-like flow. This latter is generated by
variations of surfactant concentration which affect the droplet surface tension
promoting its motion. In the present paper a model for droplets with a third
amphiphilic component is adopted. The dynamics is described by Navier-Stokes
and convection-diffusion equations, solved by lattice Boltzmann method coupled
with finite-difference schemes. We focus on two cases. First the study of
self-propulsion of an isolated droplet is carried on and, then, the interaction
of two self-propelled droplets is investigated. In both cases, when the
surfactant migrates towards the interface, a quadrupolar vortex of the velocity
field forms inside the droplet and causes the motion. A weaker dipolar field
emerges instead when the surfactant is mainly diluted in the bulk. The dynamics
of two interacting droplets is more complex and strongly depends on their
reciprocal distance. If, in a head-on collision, droplets are close enough, the
velocity field initially attracts them until a motionless steady state is
achieved. If the droplets are vertically shifted, the hydrodynamic field leads
to an initial reciprocal attraction followed by a scattering along opposite
directions. This hydrodynamic interaction acts on a separation of some droplet
radii otherwise it becomes negligible and droplets motion is only driven by
Marangoni effect. Finally, if one of the droplets is passive, this latter is
generally advected by the fluid flow generated by the active one. | cond-mat_soft |
Molecular and all solid DFT studies of the magnetic and chemical bonding
properties within KM[Cr(CN)$_6$] (M = V, Ni) complexes: A study at both the molecular and extended solid level in the framework DFT
is carried out for KM[Cr(CN)$_6$] (M = V, Ni). From molecular calculations, the
exchange parameters J are obtained, pointing to the expected magnetic ground
states, i.e., antiferromagnetic for M = V with J = -296.5 cm$^{-1}$ and
ferromagnetic for M = Ni with J = +40.5 cm$^{-1}$. From solid state
computations the same ground states and J magnitudes are confirmed from energy
differences. Furthermore an analysis of the site projected density of states
and of the chemical bonding is developed in which the cyanide ion linkage is
analyzed addressing some isomerism aspects. | cond-mat_soft |
Yield stress and elastic modulus of suspensions of noncolloidal
particles in yield stress fluids: We study experimentally the behavior of isotropic suspensions of noncolloidal
particles in yield stress fluids. This problem has been poorly studied in the
literature, and only on specific materials. In this paper, we manage to develop
procedures and materials that allow us to focus on the purely mechanical
contribution of the particles to the yield stress fluid behavior, independently
of the physicochemical properties of the materials. This allows us to relate
the macroscopic properties of these suspensions to the mechanical properties of
the yield stress fluid and the particle volume fraction, and to provide results
applicable to any noncolloidal particle in any yield stress fluid. We find that
the elastic modulus-concentration relationship follows a Krieger-Dougherty law,
and we show that the yield stress-concentration relationship is related to the
elastic modulus-concentration relationship through a very simple law, in
agreement with results from a micromechanical analysis. | cond-mat_soft |
Impulsive correction to the elastic moduli obtained using the
stress-fluctuation formalism in systems with truncated pair potential: The truncation of a pair potential at a distance r_cut is well-known to imply
in general an impulsive correction to the pressure and other moments of the
first derivatives of the potential. That depending on r_cut the truncation may
also be of relevance to higher derivatives is shown theoretically for the Born
contributions to the elastic moduli obtained using the stress-fluctuation
formalism in d dimensions. Focusing on isotropic liquids for which the shear
modulus G must vanish by construction, the predicted corrections are tested
numerically for binary mixtures and polydisperse Lennard-Jones beads in,
respectively, d=3 and d=2 dimensions. | cond-mat_soft |
Orientational ordering of point dipoles on a sphere: Arrangement of interacting particles on a sphere is historically a well known
problem, however, ordering of particles with anisotropic interaction, such as
the dipole-dipole interaction, has remained unexplored. We solve the
orientational ordering of point dipoles on a sphere with fixed positional order
with numerical minimization of interaction energy and analyze stable
configurations depending on their symmetry and degree of ordering. We find that
a macrovortex is a generic ground state, with various discrete rotational
symmetries for different system sizes, while higher energy metastable states
are similar, but less ordered. We observe orientational phase transitions and
hysteresis in response to changing external field both for the fixed sphere
orientation with respect the field, as well as for a freely-rotating sphere.
For the case of a freely rotating sphere, we also observe changes of the
symmetry axis with increasing field strength. | cond-mat_soft |
First-order phase transition of the tethered membrane model on spherical
surfaces: We found that three types of tethered surface model undergo a first-order
phase transition between the smooth and the crumpled phase. The first and the
third are discrete models of Helfrich, Polyakov, and Kleinert, and the second
is that of Nambu and Goto. These are curvature models for biological membranes
including artificial vesicles. The results obtained in this paper indicate that
the first-order phase transition is universal in the sense that the order of
the transition is independent of discretization of the Hamiltonian for the
tethered surface model. | cond-mat_soft |
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