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Nonlinear mechanics of thermoreversibly associating dendrimer glasses: We model the mechanics of associating trivalent dendrimer network glasses
with a focus on their energy dissipation properties. Various combinations of
sticky bond (SB) strength and kinetics are employed. The toughness
(work-to-fracture) of these systems displays a surprising deformation-protocol
dependence; different association parameters optimize different properties. In
particular, "strong, slow" SBs optimize strength, while "weak, fast" SBs
optimize ductility via self-healing during deformation. We relate these
observations to breaking, reformation, and partner-switching of SBs during
deformation. These studies point the way to creating associating-polymer
network glasses with tailorable mechanical properties. | cond-mat_soft |
Quenches and crunchs: Does the system explore in aging the same part of
the configuration space explored in equilibrium ?: Numerical studies are providing novel information on the physical processes
associated to physical aging. The process of aging has been shown to consist in
a slow process of explorations of deeper and deeper minima of the system
potential energy surface. In this article we compare the properties of the
basins explored in equilibrium with those explored during the aging process
both for sudden temperature changes and for sudden density changes. We find
that the hypothesis that during the aging process the system explores the part
of the configuration space explored in equilibrium holds only for shallow
quenches or for the early aging dynamics. At longer times, systematic
deviations are observed. In the case of crunches, such deviations are much more
apparent. | cond-mat_soft |
Mechanisms and Rates of Nucleation of Amyloid Fibrils: The classical nucleation theory finds the rate of nucleation proportional to
the monomer concentration raised to the power, which is the `critical nucleaus
size', ${n_c}$. The implicit assumption, that amyloids nucleate in the same
way, has been recently challenged by an alternative two-step mechanism, when
the soluble monomers first form a metastable aggregate (micelle), and then
undergo conversion into the conformation rich in ${\beta}$-strands that are
able to form a stable growing nucleus for the protofilament. Here we put
together the elements of extensive knowledge about aggregation and nucleation
kinetics, using a specific case of A${\beta_{1\mathrm{-}42}}$ amyloidogenic
peptide for illustration, to find theoretical expressions for the effective
rate of amyloid nucleation. We find that at low monomer concentration in
solution, and also at low interaction energy between two peptide conformations
in the micelle, the nucleation occurs via the classical route. At higher
monomer concentration, and a range of other interaction parameters between
peptides, the two-step `aggregation-conversion' mechanism of nucleation takes
over. In this regime, the effective rate of the process can be interpreted as a
power of monomer concentration in a certain range of parameters, however, the
exponent is determined by a complicated interplay of interaction parameters and
is not related to the minimum size of the growing nucleus (which we find to be
${\sim}$ 7-8 for A${\beta_{1-42}}$). | cond-mat_soft |
Rotational motion of a droplet induced by interfacial tension: Spontaneous rotation of a droplet induced by the Marangoni flow is analyzed
in a two-dimensional system. The droplet with the small particle which supplies
a surfactant at the interface is considered. We calculated flow field around
the droplet using Stokes equation and found that advective nonlinearity breaks
symmetry for rotation. Theoretical calculation indicates that the droplet
spontaneously rotates when the radius of the droplet is an appropriate size.
The theoretical results were validated through comparison with the experiments. | cond-mat_soft |
Vortex formation in a slowly rotating Bose-Einstein condensate confined
in a harmonic-plus-gaussian laser trap: Motivated by the recent experiment at ENS [ V. Bretin, S. Stock, Y. Seurin
and, J. Dalibard, Phys. Rev. Lett. {\bf 92}, 050403 (2004)], we study a
rotating (non-)interacting atomic Bose-Einstein condensate confined in a
harmonic-plus-gaussian laser trap potential. By adjusting the amplitude of the
laser potential, one can make quadratic-plus-quartic potential,purely quartic
potential, and quartic-minus-quadratic potential. We show that an interacting
Bose-Einstein condensate confined in a harmonic-plus-gaussian laser trap breaks
the rotational symmetry of the Hamiltonian when rotational frequency is greater
than one-half of the lowest energy surface mode frequency. We also show that by
increasing the amplitude of the gaussian laser trap, a vortex appears in a
slowly rotating Bose-Einstein condensate. Moreover, one can also create a
vortex in non-interacting Bose-Einstein condensate confined in
harmonic-plus-gaussian laser potential. | cond-mat_soft |
Modeling liquid migration in active swollen gel spheres: Liquid migration in active soft solids is a very common phenomenon in Nature
at different scales: from cells to leaves. It can be caused by mechanical as
well as chemical actions. The work focuses on the migration of liquid provoked
by remodeling processes in an active impermeable gel sphere. Within this
context, we present a consistent mathematical theory capable to gain a deep
understanding of the phenomenon in both steady and transient conditions. | cond-mat_soft |
The elasto-/hydro-dynamics of quasicrystals with 12- and 18-fold
symmetries in some soft matters and mathematical solutions: The observation recently of 12-fold quasicrystals in polymers, nanoparticle
mixture and 12-fold and 18-fold quasicrystals in colloidal solutions are
important events for the study of quasicrystals. To describe the mechanical
behaviour we propose a new solid-liquid phase quasicrystal model for some soft
matters including polymers and colloids. The so-called new solid-liquid phase,
is a new phase model of anisotropic fluid, but different from liquid crystal
phase, here the structure presents quasiperiodic symmetry. Based on the model,
the elasticity, fluidity and viscosity of the material have been studied, the
relevant mathematical theory has also been proposed. Some mathematical
solutions of the theory are discussed. | cond-mat_soft |
Heirarchical and synergistic self-assembly in composites of model
Wormlike micellar-polymers and nanoparticles results in nanostructures with
diverse morphologies: Using Monte Carlo simulations, we investigate the self-assembly of model
nanoparticles inside a matrix of model equilibrium polymers (or matrix of
Wormlike micelles) as a function of the polymeric matrix density and the
excluded volume parameter between polymers and nanoparticles. In this paper, we
show morphological transitions in the system architecture via synergistic
self-assembly of nanoparticles and the equilibrium polymers. In a synergistic
self-assembly, the resulting morphology of the system is a result of the
interaction between both nanoparticles and the polymers, unlike the polymer
templating method. We report the morphological transition of nanoparticle
aggregates from percolating network-like structures to non-percolating clusters
as a result of the change in the excluded volume parameter between
nanoparticles and polymeric chains. In parallel with the change in the
self-assembled structures of nanoparticles, the matrix of equilibrium polymers
also shows a transition from a dispersed state to a percolating network-like
structure formed by the clusters of polymeric chains. We show that the shape
anisotropy of the nanoparticle clusters formed is governed by the polymeric
density resulting in rod-like, sheet-like or other anisotropic nanoclusters. It
is also shown that the pore shape and the pore size of the porous network of
nanoparticles can be changed by changing the minimum approaching distance
between nanoparticles and polymers. We provide a theoretical understanding of
why various nanostructures with very different morphologies are obtained. | cond-mat_soft |
Single-Trajectory Characterization of Active Swimmers in a Flow: We develop a maximum likelihood method to infer relevant physical properties
of elongated active particles. Using individual trajectories of advected
swimmers as input, we are able to accurately determine their rotational
diffusion coefficients and an effective measure of their aspect ratio, also
providing reliable estimators for the uncertainties of such quantities. We
validate our theoretical construction using numerically generated active
trajectories upon no-flow, simple shear, and Poiseuille flow, with excellent
results. Being designed to rely on single-particle data, our method eases
applications in experimental conditions where swimmers exhibit a strong
morphological diversity. We briefly discuss some of such ongoing experimental
applications, specifically, in the characterization of swimming E.coli in a
flow. | cond-mat_soft |
Efficient equilibration of confined and free-standing films of highly
entangled polymer melts: Equilibration of polymer melts containing highly entangled long polymer
chains in confinement or with free surfaces is a challenge for computer
simulations. We approach this problem by first studying polymer melts based on
the soft-sphere coarse-grained model confined between two walls with periodic
boundary conditions in two directions parallel to the walls. Then we apply
backmapping to reinsert the microscopic details of the underlying bead-spring
model. Tuning the strength of the wall potential, the monomer density of
confined polymer melts in equilibrium is kept at the bulk density even near the
walls. In a weak confining regime, we observe the same conformational
properties of chains as in the bulk melt showing that our confined polymer
melts have reached their equilibrated state. Our methodology provides an
efficient way of equilibrating large polymer films with different thicknesses
and is not confined to a specific underlying microscopic model. Switching off
the wall potential in the direction perpendicular to the walls, enables to
study free-standing highly entangled polymer films or polymer films with one
supporting substrate. | cond-mat_soft |
Comment on "Layering transition in confined molecular thin films:
Nucleation and growth": When fluid is confined between two molecularly smooth surfaces to a few
molecular diameters, it shows a large enhancement of its viscosity. From
experiments it seems clear that the fluid is squeezed out layer by layer. A
simple solution of the Stokes equation for quasi-two-dimensional confined flow,
with the assmption of layer-by-layer flow is found. The results presented here
correct those in Phys. Rev. B, 50, 5590 (1994), and show that both the
kinematic viscosity of the confined fluid and the coefficient of surface drag
can be obtained from the time dependence of the area squeezed out. Fitting our
solution to the available experimental data gives the value of viscosity which
is ~7 orders of magnitude higher than that in the bulk. | cond-mat_soft |
Potential of Mean Force between a Spherical Particle Suspended in a
Nematic Liquid Crystal and a Substrate: We consider a system where a spherical particle is suspended in a nematic
liquid crystal confined between two walls. We calculate the liquid-crystal
mediated potential of mean force between the sphere and a substrate by means of
Monte Carlo simulations. Three methods are used: a traditional Monte Carlo
approach, umbrella sampling, and a novel technique that combines canonical
expanded ensemble simulations with a recently proposed density of states
formalism. The latter method offers clear advantages in that it ensures good
sampling of phase space without prior knowledge of the energy landscape of the
system. The resulting potential of mean force, computed as a function of the
normal distance between the sphere and a surface, suggests that the sphere is
attracted to the surface, even in the absence of attractive molecular
interactions. | cond-mat_soft |
Conformational Properties of an Adsorbed Charged Polymer: The behavior of a strongly charged polymer adsorbed on an oppositely charged
surface of low-dielectric constant is formulated by the functional integral
method. By separating the translational, conformational, and fluctuational
degrees of freedom, the scaling behaviors for both the height of the polymer
and the thickness of the diffusion layer are determined. Unlike the results
predicted by scaling theory, we identified the continuous crossover from the
weak compression to the compression regime. All teh analytical results are
found to be consistent with Monte-Carlo simulations. Finally, an alternative
(operational) definition of a charged polymer adsorption is proposed. | cond-mat_soft |
Reversibility and hysteresis of the sharp yielding transition of a
colloidal glass under oscillatory shear: The mechanical response of glasses remains challenging to understand. Recent
results indicate that the oscillatory rheology of soft glasses is accompanied
by a sharp non-equilibrium transition in the microscopic dynamics. Here, we use
simultaneous x-ray scattering and rheology to investigate the reversibility and
hysteresis of the sharp sharp symmetry change from anisotropic solid to
isotropic liquid dynamics observed in the oscillatory shear of colloidal
glasses [D. V. Denisov, M. T. Dang, B. Struth, A. Zaccone, and P. Schall, Sci.
Rep. 5, 14359 (2015)]. We use strain sweeps with increasing and decreasing
strain amplitude to show that, in analogy to equilibrium transitions, this
sharp symmetry change is reversible and exhibits systematic frequency-dependent
hysteresis. Using the non-affine response formalism of amorphous solids, we
show that these hysteresis effects arise from frequency-dependent non-affine
structural cage rearrangements at large strain. These results consolidate the
first-order like nature of the oscillatory shear transition and quantify
related hysteresis effects both via measurements and theoretical modelling. | cond-mat_soft |
Colloids in Two-Dimensional Active Nematics: Conformal Cogs and
Controllable Spontaneous Rotation: A major challenge in the study of active systems is to harness their
non-equilibrium dynamics into useful work. We address this by showing how to
design colloids with controllable spontaneous propulsion or rotation when
immersed in active nematics. This is illustrated for discs with tilted
anchoring and chiral cogs, for which we determine the nematic director through
conformal mappings. Our analysis identifies two regimes of behaviour for chiral
cogs: orientation-dependent handedness and persistent active rotation. Finally,
we provide design principles for active nematic colloids to achieve desired
rotational dynamics. | cond-mat_soft |
Dispersing Nanoparticles in a Polymer Film via Solvent Evaporation: Large scale molecular dynamics simulations are used to study the dispersion
of nanoparticles (NPs) in a polymer film during solvent evaporation. As the
solvent evaporates, a dense polymer-rich skin layer forms at the liquid/vapor
interface, which is either NP rich or poor depending on the strength of the
NP/polymer interaction. When the NPs are strongly wet by the polymer, the NPs
accumulate at the interface and form layers. However when the NPs are only
partially wet by the polymer, most NPs are uniformly distributed in the bulk of
the polymer film with the dense skin layer serving as a barrier to prevent the
NPs from moving to the interface. Our results point to a possible route to
employ less favorable NP/polymer interactions and fast solvent evaporation to
uniformly disperse NPs in a polymer film, contrary to the common belief that
strong NP/polymer attractions are needed to make NPs well dispersed in polymer
nanocomposites. | cond-mat_soft |
Mesoscopic theory for inhomogeneous mixtures: Mesoscopic density functional theory for inhomogeneous mixtures of sperical
particles is developed in terms of mesoscopic volume fractions by a systematic
coarse-graining procedure starting form microscopic theory. Approximate
expressions for the correlation functions and for the grand potential are
obtained for weak ordering on mesoscopic length scales. Stability analysis of
the disordered phase is performed in mean-field approximation (MF) and beyond.
MF shows existence of either a spinodal or a $\lambda$-surface on the
volume-fractions - temperature phase diagram. Separation into homogeneous
phases or formation of inhomogeneous distribution of particles occurs on the
low-temperature side of the former or the latter surface respectively,
depending on both the interaction potentials and the size ratios between
particles of different species. Beyond MF the spinodal surface is shifted, and
the instability at the $\lambda$-surface is suppressed by fluctuations. We
interpret the $\lambda$-surface as a borderline between homogeneous and
inhomogeneous (containing clusters or other aggregates) structure of the
disordered phase. For two-component systems explicit expressions for the MF
spinodal and $\lambda$-surfaces are derived. Examples of interaction potentials
of simple form are analyzed in some detail, in order to identify conditions
leading to inhomogeneous structures. | cond-mat_soft |
Laminar Flow and Backrind Formation in Molding of Viscoelastic Silicone
Rubber: When a thermoset polymer is cured at elevated temperature in a closed mold,
thermal expansion can produce flaws in the finished product. Those flaws occur
when rising internal pressure pushes the mold open and cured polymer flows out
through gaps at the parting lines. Known as backrind, such defects are
particularly common in compression molding, where the increasing pressure of a
trapped, incompressible polymer can overwhelm the clamping pressure on the mold
and expel polymer from the mold pocket. If that ejected material has already
cured, it leaves behind structural damage and consequently a flaw in the
finished product.
Backrind usually appears as a ragged seam line near the gap where cured
polymer exited the mold. Its appearance is typically irregular and fragmented,
suggesting no particular pattern or uniformity to the process that produced it.
In such cases, the cured polymer acts predominantly as a viscoelastic solid as
it is driven toward and through the parting line. The backrind's ragged
character results from tearing and fragmentation of that solid.
It is possible, however, for the cured polymer to act predominantly as a
viscoelastic liquid as it flows toward and through the parting line. Since the
Reynolds number is low, the flow is laminar and the backrind bears witness to
that laminar flow. More specifically, the backrind's observed shaped
corresponds to isochronous contours in the laminar flow toward the parting
line, contours that can be predicted using computational fluid dynamics. | cond-mat_soft |
Phase transitions of a tethered membrane model on a torus with intrinsic
curvature: A tethered surface model is investigated by using the canonical Monte Carlo
simulation technique on a torus with an intrinsic curvature. We find that the
model undergoes a first-order phase transition between the smooth phase and the
crumpled one. | cond-mat_soft |
DNA versus RNA -- which shows higher electronic conduction?: In this study, we compare the charge transport properties of multiple (double
stranded) dsRNA sequences with corresponding dsDNA sequences. Recent studies
have presented a contradictory picture of relative charge transport
efficiencies in A-form DNA:RNA hybrids and dsDNA. Using a multiscale modelling
framework, we compute conductance of dsDNA and dsRNA using Landauer formalism
in coherent limit and Marcus-Hush theory in the incoherent limit. We find that
dsDNA conducts better than dsRNA in both the charge transport regimes. Our
analysis shows that the structural differences in the twist angle and slide of
dsDNA and dsRNA are the main reasons behind the higher conductance of dsDNA in
the incoherent hopping regime. In the coherent limit however, for the same base
pair length, the conductance of dsRNA is higher than that of dsDNA for the
morphologies where dsRNA has smaller end-to-end length relative to that of
dsDNA. | cond-mat_soft |
Simple production of cellulose nanofibril microcapsules and the rheology
of their suspensions: Microcapsules are commonly used in applications ranging from therapeutics to
personal care products due to their ability to deliver encapsulated species
through their porous shells. Here, we demonstrate a simple and scalable
approach to fabricate microcapsules with porous shells by interfacial
complexation of cellulose nanofibrils and oleylamine, and investigate the
rheological properties of suspensions of the resulting microcapsules. The
suspensions of neat capsules are viscous liquids whose viscosity increases with
volume fraction according to a modified Kreiger-Dougherty relation with a
maximum packing fraction of 0.73 and an intrinsic viscosity of 4. When
polyacrylic acid (PAA) is added to the internal phase of the microcapsule,
however, the suspensions become elastic and display yield stresses with
power-law dependencies on capsule volume fraction and PAA concentration. The
elasticity appears to originate from associative interactions between
microcapsules induced by PAA that resides within the microcapsule shells. These
results demonstrate that it is possible to tune the rheological properties of
microcapsule suspensions by changing only the composition of the internal
phase, thereby providing a novel method to tailor complex fluid rheology. | cond-mat_soft |
1-D Cluster Array at the Three Phase Contact Line in Diluted Colloids
Subjected to A.C. Electric Fields: Colloidal particles provide an efficient mean of building multiple scale
structured materials from colloidal dispersions. In this Brief Report, we
account for experimental evidence on the formation of a colloidal cluster array
at a three-phase contact line. We study the influence of low frequency external
alternating electric fields on a diluted colloidal dispersion opened to the
air. We focus on the cluster formation and their evolution in the meniscus by
measuring characteristic times and lengths. We observe that the clusters are
separated by a well-defined length and that, in our experimental conditions,
they survive between five a fifteen minutes. These new results could be of
technological relevance in building tailored colloidal structures in
non-patterned substrates. | cond-mat_soft |
To grow or to shrink: A tale of two rubber balloons: Two identical rubber balloons are partially inflated with air (to different
extent) and connected by a hose with a valve. It is found that depending on
balloon volumes, when the valve is opened the air will flow either from the
larger (fuller) balloon to the smaller (emptier) balloon, or from the smaller
balloon to the larger one. The phenomenon is explained in terms of the
non-ideal rubber elasticity of balloons. The full phase diagram for the air
flow dynamics is constructed. | cond-mat_soft |
On the modulational stability of Gross-Pittaevskii type equations in 1+1
dimensions: The modulational stability of the nonlinear Schr{\"o}dinger (NLS) equation is
examined in the cases with linear and quadratic external potential. This study
is motivated by recent experimental studies in the context of matter waves in
Bose-Einstein condensates. The linear case can be examined by means of the
Tappert transformation and can be mapped to the NLS in the appropriate
(constant acceleration) frame. The quadratic case can be examined by using a
lens-type transformation that converts it into a regular NLS with an additional
linear growth term. | cond-mat_soft |
First-order phase transitions in lattice bilayers of Janus-like
particles: Monte Carlo simulations: The first-order phase transitions in the lattice model of Janus-like
particles confined in slit-like pores are studied. We assume a cubic lattice
with molecules that can freely change their orientation on a lattice site.
Moreover, the molecules can interact with the pore walls with
orientation-dependent forces. The performed calculations are limited to the
cases of bilayers. Our emphasis is on the competition between the fluid-wall
and fluid-fluid interactions. The oriented structures formed in the systems in
which the fluid-wall interactions acting contrary to the fluid-fluid
interactions differ from those appearing in the systems with neutral walls or
with walls attracting the repulsive parts of fluid molecules. | cond-mat_soft |
Out-of-equilibrium interactions and collective locomotion of colloidal
spheres with squirming of nematoelastic multipoles: Many living and artificial systems show a similar emergent behavior and
collective motions on different scales, starting from swarms of bacteria to
synthetic active particles, herds of mammals and crowds of people. What all
these systems often have in common is that new collective properties like
flocking emerge from interactions between individual self-propelled or
externally driven units. Such systems are naturally out-of-equilibrium and
propel at the expense of consumed energy. Mimicking nature by making
self-propelled or externally driven particles and studying their individual and
collective motility may allow for deeper understanding of physical
underpinnings behind the collective motion of large groups of interacting
objects or beings. Here, using a soft matter system of colloids immersed into a
liquid crystal, we show that resulting so-called nematoelastic multipoles can
be set into a bidirectional locomotion by external periodically oscillating
electric fields. Out-of-equilibrium elastic interactions between such colloids
lead to collective flock-like behaviors, which emerge from time-varying
elasticity-mediated interactions between externally driven propelling
particles. The repulsive elastic interactions in the equilibrium state can be
turned into attractive interactions in the out-of-equilibrium state under
applied electric fields. We probe this behavior at different number densities
of colloidal particles and show that particles in a dense dispersion
collectively select the same direction of a coherent motion due to elastic
interactions between near neighbors. In our experimentally implemented design,
their motion is highly ordered and without clustering or jamming often present
in other colloidal transport systems, which is promising for technological and
fundamental-science applications, like nano-cargo transport, out-of-equilibrium
assembly and microrobotics. | cond-mat_soft |
Channel Flow of Smectic Films: The hydrodynamics of smectic films at an air-water interface is discussed,
with particular focus on the viscous response of the film under flow normal to
the layers. The corrections to the response functions of the smectic phase,
arising from the coupling between the flow and the smectic order parameter, are
calculated. The results for the effective viscosity are illustrated by
analysing smectic film flow in a channel geometry. Two limiting cases of the
flow, namely, motion dominated by dislocation-induced shear-softening and
dislocation-free motion dominated by the permeation mode of mass transfer, are
studied. The effect of drag from a finite depth liquid subphase is considered.
The results are compared to those for hexatic and liquid films. | cond-mat_soft |
Chiral oily streaks in a smectic-A liquid crystal: The liquid crystal octylcyanobiphenyl (8CB) was doped with the chiral agent
CB15 and spin-coated onto a substrate treated for planar alignment of the
director, resulting in a film of thickness several hundred nm in the smectic-A
phase. In both doped and undoped samples, the competing boundary conditions -
planar alignment at the substrate and vertical alignment at the free surface -
cause the liquid crystal to break into a series of flattened hemicylinders to
satisfy the boundary conditions. When viewed under an optical microscope with
crossed polarizers, this structure results in a series of dark and light
stripes ("oily streaks") of period ~ 1 $\mu$m. In the absence of chiral dopant
the stripes run perpendicular to the substrate's easy axis. However, when doped
with chiral CB15 at concentrations up to c = 4 wt-%, the stripe orientation
rotates by a temperature-dependent angle $\phi$ with respect to the c = 0
stripe orientation, where $\phi$ increases monotonically with c. $\phi$ is
largest just below the nematic -- smectic-A transition temperature TNA and
decreases with decreasing temperature. As the temperature is lowered, $\phi$
relaxes to a steady-state orientation close to zero within ~1$^\circ$ C of TNA.
We suggest that the rotation phenomenon is a manifestation of the surface
electroclinic effect: The rotation is due to the weak smectic order parameter
and resulting large director tilt susceptibility with respect to the smectic
layer normal near TNA, in conjunction with an effective surface electric field
due to polar interactions between the liquid crystal and substrate. | cond-mat_soft |
Can pulling cause right- to left-handed structural transitions in
negatively supercoiled DNA double-helix?: The folding angle distribution of stretched and negatively supercoiled DNA
double-helix is investigated based on a theoretical model we proposed earlier
[H. Zhou et al., Phys. Rev. Lett. 82, 4560 (1999)]. It is shown that pulling
can transit a negatively supercoiled DNA double-helix from the right-handed
B-form to a left-handed configuration which resembles DNA Z-form in some
important respects. The energetics of this possible transition is calculated
and the comparison with recent experimental observations are qualitatively
discussed. | cond-mat_soft |
Anisotropic imbibition on surfaces patterned with polygonal posts: We present and interpret lattice Boltzmann simulations of thick films
spreading on surfaces patterned with polygonal posts. We show that the
mechanism of pinning and depinning differs with the direction of advance, and
demonstrate that this leads to anisotropic spreading within a certain range of
material contact angles. | cond-mat_soft |
Numerical study of the spherically-symmetric Gross-Pitaevskii equation
in two space dimensions: We present a numerical study of the time-dependent and time-independent
Gross-Pitaevskii (GP) equation in two space dimensions, which describes the
Bose-Einstein condensate of trapped bosons at ultralow temperature with both
attractive and repulsive interatomic interactions. Both time-dependent and
time-independent GP equations are used to study the stationary problems. In
addition the time-dependent approach is used to study some evolution problems
of the condensate. Specifically, we study the evolution problem where the trap
energy is suddenly changed in a stable preformed condensate. In this case the
system oscillates with increasing amplitude and does not remain limited between
two stable configurations. Good convergence is obtained in all cases studied. | cond-mat_soft |
Skinny emulsions take on granular matter: Our understanding of the structural features of foams and emulsions has
advanced significantly over the last 20 years. However, with a search for
"super-stable" liquid dispersions, foam and emulsion science employs
increasingly complex formulations which create solid-like visco-elastic layers
at the bubble/drop surfaces. These lead to elastic, adhesive and frictional
forces between bubbles/drops, impacting strongly how they pack and deform
against each other, asking for an adaptation of the currently available
structural description. The possibility to modify systematically the
interfacial properties makes these dispersions ideal systems for the
exploration of soft granular materials with complex interactions.
We present here a first systematic analysis of the structural features of
such a system using a model silicone emulsion containing millimetre-sized
polyethylene glycol drops (PEG). Solid-like drop surfaces are obtained by
polymeric cross-linking reactions at the PEG-silicone interface. Using a novel
droplet-micromanipulator, we highlight the presence of elastic, adhesive and
frictional interactions between two drops. We then provide for the first time a
full tomographic analysis of the structural features of these emulsions. An
in-depth analysis of the angle of repose, local volume fraction distributions,
pair correlation functions and the drop deformations for different skin
formulations allow us to put in evidence the striking difference with
"ordinary" emulsions having fluid-like drop surfaces. While strong analogies
with frictional hard-sphere systems can be drawn, these systems display a set
of unique features due to the high deformability of the drops which await
systematic exploration. | cond-mat_soft |
Temperature distribution in a gas-solid fixed bed probed by rapid
magnetic resonance imaging: Controlling the temperature distribution inside catalytic fixed bed reactors
is crucial for yield optimization and process stability. Yet, in situ
temperature measurements with spatial and temporal resolution are still
challenging. In this work, we perform temperature measurements in a cylindrical
fixed bed reactor by combining the capabilities of real-time magnetic resonance
imaging (MRI) with the temperature-dependent proton resonance frequency (PRF)
shift of water. Three-dimensional (3D) temperature maps are acquired while
heating the bed from room temperature to 60~$^{\circ}$C using hot air. The
obtained results show a clear temperature gradient along the axial and radial
dimensions and agree with optical temperature probe measurements with an
average error of $\pm$ 1.5~$^{\circ}$C. We believe that the MR thermometry
methodology presented here opens new perspectives for the fundamental study of
mass and heat transfer in gas-solid fixed beds and in the future might be
extended to the study of reactive gas-solid systems. | cond-mat_soft |
Statistical properties of a granular gas fluidized by turbulent air
wakes: We perform experiments with a granular system that consists of a collection
of identical hollow spheres (ping-pong balls). Particles rest on a horizontal
metallic grid and are confined within a circular region. Fluidization is
achieved by means of a turbulent air current coming from below. Air flow is
adjusted so that the balls do not elevate over the grid, as an approach to 2D
dynamics. With a high-speed camera, we take images of the system. From these
images we can infer horizontal particle positions and velocities by means of
particle-tracking algorithms. With the obtained data we analyze: a) the
systematic measurement error in the determination of positions and velocities
from our digital images; b) the degree of homogeneity achieved in our
experiments (which depends on possible deviations of the grid from the
horizontal and on the homogeneity of turbulent air wakes). Interestingly, we
have observed evidences of crystallization at high enough densities. | cond-mat_soft |
Transport of Particles in Liquid Crystals: Colloidal particles in a liquid crystal (LC) behave very differently from
their counterparts in isotropic fluids. Elastic nature of the orientational
order and surface anchoring of the director cause long-range anisotropic
interactions and lead to the phenomenon of levitation. The LC environment
enables new mechanisms of particle transport that are reviewed in this work.
Among them the motion of particles caused by gradients of the director, and
effects in the electric field: backflow powered by director reorientations,
dielectrophoresis in LC with varying dielectric permittivity and LC-enabled
nonlinear electrophoresis with velocity that depends on the square of the
applied electric field and can be directed differently from the field
direction. | cond-mat_soft |
Packing of elastic wires in spherical cavities: We investigate the morphologies and maximum packing density of thin wires
packed into spherical cavities. Using simulations and experiments, we find that
ordered as well as disordered structures emerge, depending on the amount of
internal torsion. We find that the highest packing densities are achieved in
low torsion packings for large systems, but in high torsion packings for small
systems. An analysis of both situations is given in terms of energetics and
comparison is made to analytical models of DNA packing in viral capsids. | cond-mat_soft |
Surface creasing of soft elastic continua as a Kosterlitz-Thouless
transition: Harnessing a model from composite materials science, we show how point-like
cusped surface features arise as quasi-particle excitations, termed "ghost
fibers", on the surface of a homogeneous soft elastic material. These
deformations appear above a critical compressive strain at which ghost fiber
dipoles unbind, analogous to vortices in the Kosterlitz-Thouless transition.
Finite-length creases can be described in the same framework. Our predictions
for crease surface profiles and onset strain agree with previous experiments
and simulations, and further experimental tests are proposed. | cond-mat_soft |
Realization of Bose-Einstein condensates in lower dimensions: Bose-Einstein condensates of sodium atoms have been prepared in optical and
magnetic traps in which the energy-level spacing in one or two dimensions
exceeds the interaction energy between atoms, realizing condensates of lower
dimensionality. The cross-over into two-dimensional and one-dimensional
condensates was observed by a change in aspect ratio and saturation of the
release energy when the number of trapped atoms was reduced. | cond-mat_soft |
Pattern formation in two-dimensional hard-core/soft-shell systems with
variable soft shell profiles: Hard-core/soft shell (HCSS) particles have been shown to self-assemble into a
remarkably rich variety of structures under compression due to the simple
interplay between the hard-core and soft-shoulder length scales in their
interactions. Most studies in this area model the soft shell interaction as a
square shoulder potential. Although appealing from a theoretical point of view,
the potential is physically unrealistic because there is no repulsive force in
the soft shell regime, unlike in experimental HCSS systems. To make the model
more realistic, here we consider HCSS particles with a range soft shell
potential profiles beyond the standard square shoulder form and study the model
using both minimum energy calculations and Monte Carlo simulations. We find
that by tuning density and the soft shell profile, HCSS particles in the thin
shell regime (i.e., shell to core ratio $r_1/r_0 \leq \sqrt{3}$) can form a
large range of structures, including hexagons, chains, squares, rhomboids and
two distinct zig-zag structures. Furthermore, by tuning the density and
$r_1/r_0$, we find that HCSS particles with experimentally realistic linear
ramp soft shoulder repulsions can form honeycombs and quasicrystals with
10-fold and 12-fold symmetry. Our study therefore suggests the exciting
possibility of fabricating these exotic 2D structures experimentally through
colloidal self-assembly. | cond-mat_soft |
Template Dissolution Interfacial Patterning of Single Colloids for
Nanoelectrochemistry and Nanosensing: Deterministic positioning and assembly of colloidal nanoparticles (NPs) onto
substrates is a core requirement and a promising alternative to top down
lithography to create functional nanostructures and nanodevices with intriguing
optical, electrical, and catalytic features. Capillary-assisted particle
assembly (CAPA) has emerged as an attractive technique to this end, as it
allows controlled and selective assembly of a wide variety of NPs onto
predefined topographical templates using capillary forces. One critical issue
with CAPA, however, lies in its final printing step, where high printing yields
are possible only with the use of an adhesive polymer film. To address this
problem, we have developed a template dissolution interfacial patterning (TDIP)
technique to assemble and print single colloidal AuNP arrays onto various
dielectric and conductive substrates in the absence of any adhesion layer, with
printing yields higher than 98%. The TDIP approach grants direct access to the
interface between the AuNP and the target surface, enabling the use of
colloidal AuNPs as building blocks for practical applications. The versatile
applicability of TDIP is demonstrated by the creation of direct electrical
junctions for electro- and photoelectrochemistry and nanoparticle-on-mirror
geometries for single particle molecular sensing. | cond-mat_soft |
Dynamic Elastic Moduli in Magnetic Gels: Normal Modes and Linear
Response: In the perspective of developing smart hybrid materials with customized
features, ferrogels and magnetorheological elastomers allow a synergy of
elasticity and magnetism. The interplay between elastic and magnetic properties
gives rise to a unique reversible control of the material behavior by applying
an external magnetic field. Albeit few works have been performed on the
time-dependent properties so far, understanding the dynamic behavior is the key
to model many practical situations, e.g. applications as vibration absorbers.
Here we present a way to calculate the frequency-dependent elastic moduli based
on the decomposition of the linear response to an external stress in normal
modes. We use a minimal three-dimensional dipole-spring model to theoretically
describe the magnetic and elastic interactions on the mesoscopic level.
Specifically, the magnetic particles carry permanent magnetic dipole moments
and are spatially arranged in a prescribed way, before they are linked by
elastic springs. An external magnetic field aligns the magnetic moments. On the
one hand, we study regular lattice-like particle arrangements to compare with
previous results in the literature. On the other hand, we calculate the dynamic
elastic moduli for irregular, more realistic particle distributions. Our
approach measures the tunability of the linear dynamic response as a function
of the particle arrangement, the system orientation with respect to the
external magnetic field, as well as the magnitude of the magnetic interaction
between the particles. The strength of the present approach is that it
explicitly connects the relaxational modes of the system with the rheological
properties as well as with the internal rearrangement of the particles in the
sample, providing new insight into the dynamics of these remarkable materials. | cond-mat_soft |
Dilute polymer solutions under shear flow: comprehensive qualitative
analysis using a bead-spring chain model with a FENE-Fraenkel spring: Although the non-equilibrium behaviour of polymer solutions is generally well
understood, particularly in extensional flow, there remain several unanswered
questions for dilute solutions in simple shear flow, and full quantitative
agreement with experiments has not been achieved. For example, experimental
viscosity data exhibit qualitative differences in shear-thinning exponents,
shear rate for onset of shear-thinning and high-shear Newtonian plateaus
depending on polymer semiflexibility, contour length and solvent quality. While
polymer models are able to incorporate all of these effects through various
spring force laws, bending potentials, excluded volume (EV) potentials, and
hydrodynamic interaction (HI), the inclusion of each piece of physics has not
been systematically matched to experimentally observed behaviour. Furthermore,
attempts to develop multiscale models (in the sense of representing an
arbitrarily small or large polymer chain) which can make quantitative
predictions are hindered by the lack of ability to fully match the results of
bead-rod models, often used to represent a polymer chain at the Kuhn step
level, with bead-spring models, which take into account the entropic
elasticity.In light of these difficulties, this work aims to develop a general
model based on the so-called FENE-Fraenkel spring, originally formulated by
Larson and coworkers [J. Chem. Phys. 124 (2006), 10.1063/1.2161210], which can
span the range from rigid rod to traditional entropic spring, as well as
include a bending potential, EV and HI. As we show, this model can reproduce,
and smoothly move between, a wide range of previously observed polymer solution
rheology in shear flow. | cond-mat_soft |
The effect of an exterior electric field on the instability of
dielectric plates: We investigate the theoretical nonlinear response, Hessian stability, and
possible wrinkling behaviour of a voltage-activated dielectric plate immersed
in a tank filled with silicone oil. Fixed rigid electrodes are placed on the
top and bottom of the tank, and an electric field is generated by a potential
difference between the electrodes. We solve the associated incremental boundary
value problem of superimposed, inhomogeneous small-amplitude wrinkles,
signalling the onset of instability. We decouple the resulting bifurcation
equation into symmetric and antisymmetric modes. For a neo-Hookean dielectric
plate, we show that a potential difference between the electrodes can induce a
thinning of the plate and thus an increase of its planar area, similar to the
scenarios encountered when there is no silicone oil. However, we also find
that, depending on the material and geometric parameters, an increasing applied
voltage can also lead to a thickening of the plate, and thus a shrinking of its
area. In that scenario, Hessian instability and wrinkling bifurcation may then
occur spontaneously once some critical voltages are reached. | cond-mat_soft |
A sheet on deformable sphere: "wrinklogami" patterns suppress
curvature-induced delamination: The adhesion of a stiff film onto a curved substrate often generates elastic
stresses in the film that eventually give rise to its delamination. Here we
predict that delamination of very thin films can be dramatically suppressed
through tiny, smooth deformations of the substrate, dubbed here "wrinklogami",
that barely affect the macroscale topography. This "pro-lamination" effect
reflects a surprising capability of smooth wrinkles to suppress compression in
elastic films even when spherical or other doubly-curved topography is imposed,
in a similar fashion to origami folds that enable construction of curved
structures from an unstretchable paper. We show that the emergence of a
wrinklogami pattern signals a nontrivial isometry of the sheet to its planar,
undeformed state, in the doubly asymptotic limit of small thickness and weak
tensile load exerted by the adhesive substrate. We explain how such an
"asymptotic isometry" concept broadens the standard usage of isometries for
describing the response of elastic sheets to geomertric constraints and
mechanical loads. | cond-mat_soft |
Temperature of maximum density and excess properties of short-chain
alcohol aqueous solutions: A simplified model simulation study: We perform an extensive computational study of binary mixtures of water and
short-chain alcohols resorting to two-scale potential models to account for the
singularities of hydrogen bonded liquids. Water molecules are represented by a
well studied core softened potential which is known to qualitatively account
for a large number of water's characteristic anomalies. Along the same lines,
alcohol molecules are idealized by dimers in which the hydroxyl groups interact
with each other and with water with a core softened potential as well.
Interactions involving non-polar groups are all deemed purely repulsive. We
find that the qualitative behavior of excess properties (excess volume,
enthalpy and constant pressure heat capacity) agrees with that found
experimentally for alcohols such as t-butanol in water. Moreover, we observe
that our simple solute under certain conditions acts as an "structure-maker",
in the sense that the temperature of maximum density of the bulk water model
increases as the solute is added, i.e. the anomalous behavior of the solvent is
enhanced by the solute. | cond-mat_soft |
Local structural features elucidate crystallization of complex
structures: Complex crystal structures are composed of multiple local environments, and
how this type of order emerges spontaneously during crystal growth has yet to
be fully understood. We study crystal growth across various structures and
along different crystallization pathways, using self-assembly simulations of
identical particles that interact via multi-well isotropic pair potentials. We
apply an unsupervised machine learning method to features from
bond-orientational order metrics to identify different local motifs present
during a given structure's crystallization process. In this manner, we
distinguish different crystallographic sites in highly complex structures.
Tailoring this newly developed order parameter to structures of varying
complexity and coordination number, we study the emergence of local order along
a multi-step crystal growth pathway -- from a low-density fluid to a
high-density, supercooled amorphous liquid droplet and to a bulk crystal. We
find a consistent under-coordination of the liquid relative to the average
coordination number in the bulk crystal. We use our order parameter to analyze
the geometrically frustrated growth of a Frank--Kasper phase and discover how
structural defects compete with the formation of crystallographic sites that
are higher-coordinated than the liquid environments. The here-presented method
for classifying order on a particle-by-particle level have broad applicability
to future studies of structural self-assembly and crystal growth, and they can
aid in the design of building blocks and for targeting pathways of formation of
novel soft-matter structures. | cond-mat_soft |
Bubble Statistics and Dynamics in Double-Stranded DNA: The dynamical properties of double-stranded DNA are studied in the framework
of the Peyrard-Bishop-Dauxois model using Langevin dynamics. Our simulations
are analyzed in terms of two probability functions describing coherently
localized separations ("bubbles") of the double strand. We find that the
resulting bubble distributions are more sharply peaked at the active sites than
found in thermodynamically obtained distributions. Our analysis ascribes this
to the fact that the bubble life-times significantly afects the distribution
function. We find that certain base-pair sequences promote long-lived bubbles
and we argue that this is due to a length scale competition between the
nonlinearity and disorder present in the system. | cond-mat_soft |
Thermalization of plastic flow versus stationarity of thermomechanical
equilibrium in SGR theory: We discuss issues related to thermalization of plastic flow in the context of
soft glassy rheology (SGR) theory. An apparent problem with the theory in its
current form is that the stationarity of thermomechanical equilibrium obtained
by requiring that its flow rule satisfy detailed balance in the absence of
applied deformation requires plastic flow to be athermal. This prevents proper
application of SGR to small-molecule and polymer glasses where plastic flow is
often well-thermalized. Clearly, one would like to have a SGR-like theory of
thermalized plastic flow that satisfies stationarity. We discuss reasons why
such a theory could prove very useful and clarify obstacles that must be
overcome in order to develop it. | cond-mat_soft |
Nucleation of stable cylinders from a metastable lamellar phase in a
diblock copolymer melt: The nucleation of a droplet of stable cylinder phase from a metastable
lamellar phase is examined within the single-mode approximation to the
Brazovskii model for diblock copolymer melts. By employing a variational ansatz
for the droplet interfacial profile, an analytic expression for the interfacial
free-energy of an interface of arbitrary orientation between cylinders and
lamellae is found. The interfacial free-energy is anisotropic, and is lower
when the cylinder axis is perpendicular to the interface than when the
cylinders lie along the interface. Consequently, the droplet shape computed via
the Wulff construction is lens-like, being flattened along the axis of the
cylinders. The size of the critical droplet and the nucleation barrier are
determined within classical nucleation theory. Near the lamellar/cylinder phase
boundary, where classical nucleation theory is applicable, critical droplets of
size 30--400 cylinders across with aspect ratios of 4--10 and nucleation
barriers of 30--40 k_B T are typically found. The general trend is to larger
critical droplets, higher aspect ratios and smaller nucleation barriers as the
mean-field critical point is approached. | cond-mat_soft |
The Origin of Persistent Shear Stress in Supercooled Liquids: We show that the long time tail of the shear stress autocorrelation, whose
growth at large supercooling is responsible for the apparent divergence of the
shear viscosity, is a direct result of a residual shear stress in the
structures associated with the local potential minima. We argue that the
essential mechanical transition experienced by a liquid on cooling occurs at a
temperature well above the glass transition temperature and corresponds to the
crossover from the high temperature liquid to the viscous liquid, the latter
characterised by stress relaxation dominated by the residual stress. Following
on from this observation, as the density is decreased, the local potential
minima become unable to sustain any persistent stress (and, hence, support a
glass transition), in a manner that can be explicitly connected to the
interactions between atoms. The reported crossover implies an associated change
in the mechanism of dissipation in liquids and, hence, raises the prospect of a
coherent microscopic treatment of nonlinear rheology and the relationship
between self diffusion and viscosity in supercooled liquids. | cond-mat_soft |
Active Viscoelastic Matter: from Bacterial Drag Reduction to Turbulent
Solids: A paradigm for internally driven matter is the active nematic liquid crystal,
whereby the equations of a conventional nematic are supplemented by a minimal
active stress that violates time reversal symmetry. In practice, active fluids
may have not only liquid crystalline but also viscoelastic polymer degrees of
freedom. Here we explore the resulting interplay by coupling an active nematic
to a minimal model of polymer rheology. We find that adding polymer can greatly
increase the complexity of spontaneous flow, but can also have calming effects,
thereby increasing the net throughput of spontaneous flow along a pipe (a
'drag-reduction' effect). Remarkably, active turbulence can also arise after
switching on activity in a sufficiently soft elastomeric solid. | cond-mat_soft |
Swimming statistics of cargo-loaded single bacteria: Burgeoning interest in the area of bacteria-powered micro robotic systems
prompted us to study the dynamics of cargo transport by single bacteria. In
this paper, we have studied the swimming behaviour of oil-droplets attached as
a cargo to the cell bodies of single bacteria. The oil-droplet loaded bacteria
exhibit super-diffusive motion which is characterized by high degree of
directional persistence. Interestingly, bacteria could navigate even when
loaded with oil-droplets as large as 8 microns with an effective increase in
rotational drag by more than 2 orders when compared to free bacteria. Further,
the directional persistence of oil-droplet loaded bacteria was independent of
the cargo size. | cond-mat_soft |
Liquid-Gas phase transition in Bose-Einstein Condensates with time
evolution: We study the effects of a repulsive three-body interaction on a system of
trapped ultra-cold atoms in Bose-Einstein condensed state. The stationary
solutions of the corresponding $s-$wave non-linear Schr\"{o}dinger equation
suggest a scenario of first-order liquid-gas phase transition in the condensed
state up to a critical strength of the effective three-body force. The time
evolution of the condensate with feeding process and three-body recombination
losses has a new characteristic pattern. Also, the decay time of the dense
(liquid) phase is longer than expected due to strong oscillations of the
mean-square-radius. | cond-mat_soft |
An adaptive hierarchical domain decomposition method for parallel
contact dynamics simulations of granular materials: A fully parallel version of the contact dynamics (CD) method is presented in
this paper. For large enough systems, 100% efficiency has been demonstrated for
up to 256 processors using a hierarchical domain decomposition with dynamic
load balancing. The iterative scheme to calculate the contact forces is left
domain-wise sequential, with data exchange after each iteration step, which
ensures its stability. The number of additional iterations required for
convergence by the partially parallel updates at the domain boundaries becomes
negligible with increasing number of particles, which allows for an effective
parallelization. Compared to the sequential implementation, we found no
influence of the parallelization on simulation results. | cond-mat_soft |
Novel self-assembled morphologies from isotropic interactions: We present results from particle simulations with isotropic medium range
interactions in two dimensions. At low temperature novel types of aggregated
structures appear. We show that these structures can be explained by
spontaneous symmetry breaking in analytic solutions to an adaptation of the
spherical spin model. We predict the critical particle number where the
symmetry breaking occurs and show that the resulting phase diagram agrees well
with results from particle simulations. | cond-mat_soft |
Interaction between Nearly Hard Colloidal Spheres at an Oil-Water
Interface: We show that the interaction potential between sterically stabilized, nearly
hard-sphere [poly(methylmethacrylate)-poly(lauryl methacrylate) (PMMA-PLMA)]
colloids at a water-oil interface has a negligible unscreened-dipole
contribution, suggesting that models previously developed for charged particles
at liquid interfaces are not necessarily applicable to sterically stabilized
particles. Interparticle potentials, $U(r)$, are extracted from radial
distribution functions [$g(r)$, measured by fluorescence microscopy] via
Ornstein-Zernike inversion and via a reverse Monte Carlo scheme. The results
are then validated by particle tracking in a blinking optical trap. Using a
Bayesian model comparison, we find that our PMMA-PLMA data is better described
by a screened monopole only rather than a functional form having a screened
monopole plus an unscreened dipole term. We postulate that the long range
repulsion we observe arises mainly through interactions between neutral holes
on a charged interface, i.e., the charge of the liquid interface cannot, in
general, be ignored. In agreement with this interpretation, we find that the
interaction can be tuned by varying salt concentration in the aqueous phase.
Inspired by recent theoretical work on point charges at dielectric interfaces,
which we explain is relevant here, we show that a screened $\frac{1}{r^2}$ term
can also be used to fit our data. Finally, we present measurements for
poly(methyl methacrylate)-poly(12-hydroxystearic acid) (PMMA-PHSA) particles at
a water-oil interface. These suggest that, for PMMA-PHSA particles, there is an
additional contribution to the interaction potential. This is in line with our
optical-tweezer measurements for PMMA-PHSA colloids in bulk oil, which indicate
that they are slightly charged. | cond-mat_soft |
Complex Memory Formation in Frictional Granular Media: Using numerical simulations it is shown that a jammed, random pack of soft
frictional grains can store an arbitrary waveform that is applied as a small
time-dependent shear while the system is slowly compressed. When the system is
decompressed at a later time, an approximation of the input waveform is
recalled in time-reversed order as shear stresses on the system boundaries.
This effect depends on friction between the grains, and is independent of some
aspects of the friction model. This type of memory could potentially be
observable in other types of random media that form internal contacts when
compressed. | cond-mat_soft |
Smectic-A and Smectic-C Phases and Phase Transitions in Liquid
Crystal-Aerosil Gels: High-resolution X-ray scattering studies of the nonpolar thermotropic liquid
crystal 4-n-pentylphenylthiol-4'-n-octyloxybenzoate (\bar{8}S5) in aerosil gel
nano-networks reveal that the aerosil-induced disorder significantly alters
both the nematic to Smectic-A and Smectic-A to Smectic-C phase transitions. The
limiting Smectic-A correlation length follows a power-law dependence on the
aerosil density in quantitative agreement with the limiting lengths measured
previously in other Smectic-A liquid crystal gels. The Smectic-A to Smectic-C
liquid crystalline phase transition is altered fundamentally by the presence of
the aerosil gel. The onset of the Smectic-C phase remains relatively sharp but
there is an extended coexistence region where Smectic-A and Smectic-C domains
can exist. | cond-mat_soft |
Experimental verification of Arcsine laws in mesoscopic non-equilibrium
and active systems: A large number of processes in the mesoscopic world occur out of equilibrium,
where the time course of a system evolution becomes immensely important since
it is driven principally by dissipative effects. Non-equilibrium steady states
(NESS) represent a crucial category in such systems, where relaxation
timescales are comparable to the operational timescales. In this study, we
employ a model NESS stochastic system which comprises of a colloidal
microparticle, optically trapped in a viscous fluid, externally driven by a
temporally correlated noise, and show that time-integrated observables such as
the entropic current, the work done on the system or the work dissipated by it,
follow the three Levy arcsine laws [1], in the large time limit. We discover
that cumulative distributions converge faster to arcsine distributions when it
is near equilibrium and the rate of entropy production is small, because in
that case the entropic current has weaker temporal autocorrelation. We study
this phenomenon changing the strength of the added noise or by perturbing our
system with a flow field produced by a microbubble at close proximity to the
trapped particle. We confirm our experimental findings with theoretical
simulations of the systems. Our work provides an interesting insight into the
NESS statistics of the meso-regime, where stochastic fluctuations play a
pivotal role. | cond-mat_soft |
Sensitivity of the stress response function to packing preparation: A granular assembly composed of a collection of identical grains may pack
under different microscopic configurations with microscopic features that are
sensitive to the preparation history. A given configuration may also change in
response to external actions such as compression, shearing etc. We show using a
mechanical response function method developed experimentally and numerically,
that the macroscopic stress profiles are strongly dependent on these
preparation procedures. These results were obtained for both two and three
dimensions. The method reveals that, under a given preparation history, the
macroscopic symmetries of the granular material is affected and in most cases
significant departures from isotropy should be observed. This suggests a new
path toward a non-intrusive test of granular material constitutive properties. | cond-mat_soft |
Bending and Twisting Elasticity: a Revised Marko-Siggia Model on DNA
Chirality: A revised Marko-Siggia elastic model for DNA double helix [Macromolecules 27,
981 (1994)] is proposed, which includes the WLC bending energy and a new chiral
twisting energy term. It is predicted that the mean helical repeat length (HRL)
for short DNA rings increases with the decreasing of chain length; while for
very long chains, their mean HRL is the same, independent of both the chain
length and whether the ends are closed, it is longer than the value for
rectilinear DNAs. Our results are in good agreement with experiments. | cond-mat_soft |
Dynamical properties of densely packed confined hard-sphere fluids: Numerical solutions of the mode-coupling theory (MCT) equations for a
hard-sphere fluid confined between two parallel hard walls are elaborated. The
governing equations feature multiple parallel relaxation channels which
significantly complicate their numerical integration. We investigate the
intermediate scattering functions and the susceptibility spectra close to
structural arrest and compare to an asymptotic analysis of the MCT equations.
We corroborate that the data converge in the $\beta$-scaling regime to two
asymptotic power laws, viz. the critical decay and the von Schweidler law. The
numerical results reveal a non-monotonic dependence of the power-law exponents
on the slab width and a non-trivial kink in the low-frequency susceptibility
spectra. We also find qualitative agreement of these theoretical results to
event-driven molecular-dynamics simulations of polydisperse hard-sphere system.
In particular, the non-trivial dependence of the dynamical properties on the
slab width is well reproduced. | cond-mat_soft |
The effect of chain polydispersity on the elasticity of disordered
polymer networks: Due to their unique structural and mechanical properties,
randomly-crosslinked polymer networks play an important role in many different
fields, ranging from cellular biology to industrial processes. In order to
elucidate how these properties are controlled by the physical details of the
network (\textit{e.g.} chain-length and end-to-end distributions), we generate
disordered phantom networks with different crosslinker concentrations $C$ and
initial density $\rho_{\rm init}$ and evaluate their elastic properties. We
find that the shear modulus computed at the same strand concentration for
networks with the same $C$, which determines the number of chains and the
chain-length distribution, depends strongly on the preparation protocol of the
network, here controlled by $\rho_{\rm init}$. We rationalise this dependence
by employing a generic stress-strain relation for polymer networks that does
not rely on the specific form of the polymer end-to-end distance distribution.
We find that the shear modulus of the networks is a non-monotonic function of
the density of elastically-active strands, and that this behaviour has a purely
entropic origin. Our results show that if short chains are abundant, as it is
always the case for randomly-crosslinked polymer networks, the knowledge of the
exact chain conformation distribution is essential for predicting correctly the
elastic properties. Finally, we apply our theoretical approach to published
experimental data, qualitatively confirming our interpretations. | cond-mat_soft |
Mechanics and force transmission in soft composites of rods in elastic
gels: We report detailed theoretical investigations of the micro-mechanics and bulk
elastic properties of composites consisting of randomly distributed stiff
fibers embedded in an elastic matrix in two and three dimensions. Recent
experiments published in Physical Review Letters [102, 188303 (2009)] have
suggested that the inclusion of stiff microtubules in a softer, nearly
incompressible biopolymer matrix can lead to emergent compressibility. This can
be understood in terms of the enhancement of the compressibility of the
composite relative to its shear compliance as a result of the addition of stiff
rod-like inclusions. We show that the Poisson's ratio $\nu$ of such a composite
evolves with increasing rod density towards a particular value, or {\em fixed
point}, independent of the material properties of the matrix, so long as it has
a finite initial compressibility. This fixed point is $\nu=1/4$ in three
dimensions and $\nu=1/3$ in two dimensions. Our results suggest an important
role for stiff filaments such as microtubules and stress fibers in cell
mechanics. At the same time, our work has a wider elasticity context, with
potential applications to composite elastic media with a wide separation of
scales in stiffness of its constituents such as carbon nanotube-polymer
composites, which have been shown to have highly tunable mechanics. | cond-mat_soft |
Dynamic correlations in Brownian many-body systems: For classical Brownian systems driven out of equilibrium we derive
inhomogeneous two-time correlation functions from functional differentiation of
the one-body density and current with respect to external fields. In order to
allow for appropriate freedom upon building the derivatives, we formally
supplement the Smoluchowski dynamics by a source term, which vanishes at the
physical solution. These techniques are applied to obtain a complete set of
dynamic Ornstein-Zernike equations, which serve for the development of
approximation schemes. The rules of functional calculus lead naturally to
non-Markovian equations of motion for the two-time correlators. Memory
functions are identified as functional derivatives of a unique space- and
time-nonlocal dissipation power functional. | cond-mat_soft |
Localization transition of random copolymers at interfaces: We consider adsorption of random copolymer chains onto an interface within
the model of Garel et al. Europhysics Letters 8, 9 (1989). By using the replica
method the adsorption of the copolymer at the interface is mapped onto the
problem of finding the ground state of a quantum mechanical Hamiltonian. To
study this ground state we introduce a novel variational principle for the
Green's function, which generalizes the well-known Rayleigh-Ritz method of
Quantum Mechanics to nonstationary states. Minimization with an appropriate
trial Green's function enables us to find the phase diagram for the
localization-delocalization transition for an ideal random copolymer at the
interface. | cond-mat_soft |
Determining the Anchoring Strength of a Capillary Using Topological
Defects: We consider a smectic-A* in a capillary with surface anchoring that favors
parallel alignment. If the bulk phase of the smectic is the standard
twist-grain-boundary phase of chiral smectics, then there will be a critical
radius below which the smectic will not have any topological defects. Above
this radius a single screw dislocation in the center of the capillary will be
favored. Along with surface anchoring, a magnetic field will also suppress the
formation of a screw dislocation. In this note, we calculate the critical field
at which a defect is energetically preferred as a function of the surface
anchoring strength and the capillary radius. Experiments at a few different
radii could thus determine the anchoring strength. | cond-mat_soft |
Shapes enhancing the propulsion of multiflagellated helical
microswimmers: In this paper we are interested in optimizing the shape of multi-flagellated
helical microswimmers. Mimicking the propagation of helical waves along the
flagella, they self-propel by rotating their tails. The swimmer's dynamics is
computed using the Boundary Element Method, implemented in the open source
Matlab library $Gypsilab$. We exploit a Bayesian optimization algorithm to
maximize the swimmer's speeds through their shape optimization. Our results
show that the optimal tail shapes are helices with large wavelength, such that
the shape periodicity is disregarded. Moreover, the best propulsion speed is
achieved for elongated heads when the swimmer has one or two flagella.
Surprisingly, a round head is obtained when more flagella are considered. Our
results indicate that the position and number of flagella modify the propulsion
pattern and play a significant role in the optimal design of the head. It
appears that Bayesian optimization is a promising method for performance
improvement in microswimming. | cond-mat_soft |
Surface relaxation of lyotropic lamellar phases: We study the relaxation modes of an interface between a lyotropic lamellar
phase and a gas or a simple liquid. The response is found to be qualitatively
different from those of both simple liquids and single-component smectic-A
liquid crystals. At low rates it is governed by a non-inertial, diffusive mode
whose decay rate increases quadratically with wavenumber, $|\omega|=Aq^2$. The
coefficient $A$ depends on the restoring forces of surface tension,
compressibility and bending, while the dissipation is dominated by the
so-called slip mechanism, i.e, relative motion of the two components of the
phase parallel to the lamellae. This surface mode has a large penetration depth
which, for sterically stabilised phases, is of order $(dq^2)^{-1}$, where $d$
is the microscopic lamellar spacing. | cond-mat_soft |
Thermal fluctuations of an interface near a contact line: The effect of thermal fluctuations near a contact line of a liquid interface
partially wetting an impenetrable substrate is studied analytically and
numerically. Promoting both the interface profile and the contact line position
to random variables, we explore the equilibrium properties of the corresponding
fluctuating contact line problem based on an interfacial Hamiltonian involving
a "contact" binding potential. To facilitate an analytical treatment we
consider the case of a one-dimensional interface. The effective boundary
condition at the contact line is determined by a dimensionless parameter that
encodes the relative importance of thermal energy and substrate energy at the
microscopic scale. We find that this parameter controls the transition from a
partially wetting to a pseudo-partial wetting state, the latter being
characterized by a thin prewetting film of fixed thickness. In the partial
wetting regime, instead, the profile typically approaches the substrate via an
exponentially thinning prewetting film. We show that, independently of the
physics at the microscopic scale, Young's angle is recovered sufficiently far
from the substrate. The fluctuations of the interface and of the contact line
give rise to an effective disjoining pressure, exponentially decreasing with
height. Fluctuations therefore provide a regularization of the singular contact
forces occurring in the corresponding deterministic problem. | cond-mat_soft |
Dancing disclinations in confined active nematics: The spontaneous emergence of collective flows is a generic property of active
fluids and often leads to chaotic flow patterns characterised by swirls, jets,
and topological disclinations in their orientation field. However, the ability
to achieve structured flows and ordered disclinations is of particular
importance in the design and control of active systems. By confining an active
nematic fluid within a channel, we find a regular motion of disclinations, in
conjunction with a well defined and dynamic vortex lattice. As pairs of moving
disclinations travel through the channel, they continually exchange partners
producing a dynamic ordered state, reminiscent of Ceilidh dancing. We
anticipate that this biomimetic ability to self-assemble organised topological
disclinations and dynamically structured flow fields in engineered geometries
will pave the road towards establishing new active topological microfluidic
devices. | cond-mat_soft |
Neural Network Model for Structure Factor of Polymer Systems: As an important physical quantity to understand the internal structure of
polymer chains, the structure factor is being studied both in theory and
experiment. Theoretically, the structure factor of Gaussian chains have been
solved analytically, but for wormlike chains, numerical approaches are often
used, such as Monte Carlo (MC) simulations, solving modified diffusion equation
(MDE), etc. In those works, the structure factor needs to be calculated
differently for different regions of the wave vector and chain rigidity, and
some calculation processes are resource consuming. In this work, by training a
deep neural network (NN), we obtained an efficient model to calculate the
structure factor of polymer chains, without considering different regions of
wavenumber and chain rigidity. Furthermore, based on the trained neural network
model, we predicted the contour and Kuhn length of some polymer chains by using
scattering experimental data, and we found our model can get pretty reasonable
predictions. This work provides a method to obtain structure factor for polymer
chains, which is as good as previous, and with a more computationally
efficient. Also, it provides a potential way for the experimental researchers
to measure the contour and Kuhn length of polymer chains. | cond-mat_soft |
A molecular dynamics study of chemical gelation in a patchy particle
model: We report event-driven molecular dynamics simulations of the irreversible
gelation of hard ellipsoids of revolution containing several associating
groups, characterizing how the cluster size distribution evolves as a function
of the extent of reaction, both below and above the gel point. We find that in
a very large interval of values of the extent of reaction, parameter-free
mean-field predictions are extremely accurate, providing evidence that in this
model the Ginzburg zone near the gel point, where non-mean field effects are
important, is very limited. We also find that the Flory's hypothesis for the
post-gelation regime properly describes the connectivity of the clusters even
if the long-time limit of the extent of reaction does not reach the fully
reacted state. This study shows that irreversibly aggregating asymmetric
hard-core patchy particles may provide a close realization of the mean-field
model, for which available theoretical predictions may help control the
structure and the connectivity of the gel state. Besides chemical gels, the
model is relevant to network-forming soft materials like systems with
bioselective interactions, functionalized molecules and patchy colloids. | cond-mat_soft |
Curvature Dependence of Hydrophobic Hydration Dynamics: We investigate the curvature-dependence of water dynamics in the vicinity of
hydrophobic spherical solutes using molecular dynamics simulations. For both,
the lateral and perpendicular diffusivity as well as for H-bond kinetics of
water in the first hydration shell, we find a non-monotonic solute-size
dependence, exhibiting extrema close to the well-known structural crossover
length scale for hydrophobic hydration. Additionally, we find an apparently
anomalous diffusion for water moving parallel to the surface of small solutes,
which, however, can be explained by topology effects. The intimate connection
between solute curvature, water structure and dynamics has implications for our
understanding of hydration dynamics at heterogeneous biomolecular surfaces. | cond-mat_soft |
Phase diagram of heteronuclear Janus dumbbells: Using Aggregation-Volume-Bias Monte Carlo simulations along with Successive
Umbrella Sampling and Histogram Re-weighting, we study the phase diagram of a
system of dumbbells formed by two touching spheres having variable sizes, as
well as different interaction properties. The first sphere ($h$) interacts with
all other spheres belonging to different dumbbells with a hard-sphere
potential. The second sphere ($s$) interacts via a square-well interaction with
other $s$ spheres belonging to different dumbbells and with a hard-sphere
potential with all remaining $h$ spheres. We focus on the region where the $s$
sphere is larger than the $h$ sphere, as measured by a parameter $1\le
\alpha\le 2 $ controlling the relative size of the two spheres.
As $\alpha \to 2$ a simple fluid of square-well spheres is recovered, whereas
$\alpha \to 1$ corresponds to the Janus dumbbell limit, where the $h$ and $s$
spheres have equal sizes. Many phase diagrams falling into three classes are
observed, depending on the value of $\alpha$. The $1.8 \le \alpha \le 2$ is
dominated by a gas-liquid phase separation very similar to that of a pure
square-well fluid with varied critical temperature and density. When $1.3 \le
\alpha \le 1.8$ we find a progressive destabilization of the gas-liquid phase
diagram by the onset of self-assembled structures, that eventually lead to a
metastability of the gas-liquid transition below $\alpha=1.2$. | cond-mat_soft |
Local Cooperativity Mechanism in the DNA Melting Transition: We propose a new statistical mechanics model for the melting transition of
DNA. Base pairing and stacking are treated as separate degrees of freedom, and
the interplay between pairing and stacking is described by a set of local rules
which mimic the geometrical constraints in the real molecule. This microscopic
mechanism intrinsically accounts for the cooperativity related to the free
energy penalty of bubble nucleation. The model describes both the unpairing and
unstacking parts of the spectroscopically determined experimental melting
curves. Furthermore, the model explains the observed temperature dependence of
the effective thermodynamic parameters used in models of the nearest neighbor
(NN) type. We compute the partition function for the model through the transfer
matrix formalism, which we also generalize to include non local chain entropy
terms. This part introduces a new parametrization of the Yeramian-like transfer
matrix approach to the Poland-Scheraga description of DNA melting. The model is
exactly solvable in the homogeneous thermodynamic limit, and we calculate all
observables without use of the grand partition function. As is well known,
models of this class have a first order or continuous phase transition at the
temperature of complete strand separation depending on the value of the
exponent of the bubble entropy. | cond-mat_soft |
Heterogeneous Solvent Dissipation Coupled with Particle Rearrangement in
Shear Thinning Non-Brownian Suspensions: Dense non-Brownian suspensions exhibit significant shear thinning, although a
comprehensive understanding of the full scope of this phenomenon remains
elusive. This study numerically reveals intimate heterogenous coupled dynamics
between many-body particle motions and solvent hydrodynamics in shear-thinning
non-Brownian suspensions. We demonstrate the spatially correlated viscous
dissipation and particle motions; they share the same characteristic length,
which decreases with increasing shear rate. We further show that, at lower
shear rates, significant particle density changes are induced against the
incompressibility of the solvent, suggesting the cooperative creation and
annihilation of gaps and flow channels. We discuss that hydrodynamic
interactions may substantially restrict particle rearrangements even in highly
dense suspensions, influencing the quantitative aspects of macroscopic
rheology. | cond-mat_soft |
Role of the glassy dynamics and thermal mixing in the dynamic nuclear
polarization and relaxation mechanisms of pyruvic acid: The temperature dependence of $^1$H and $^{13}$C nuclear spin-lattice
relaxation rate $1/T_1$ has been studied in the 1.6 K - 4.2 K temperature range
in pure pyruvic acid and in pyruvic acid containing trityl radicals at a
concentration of 15 mM. The temperature dependence of $1/T_1$ is found to
follow a quadratic power law for both nuclei in the two samples. Remarkably the
same temperature dependence is displayed also by the electron spin-lattice
relaxation rate $1/T_{1e}$ in the sample containing radicals. These results are
explained by considering the effect of the structural dynamics on the
relaxation rates in pyruvic acid. Dynamic nuclear polarization experiments show
that below 4 K the $^{13}$C build up rate scales with $1/T_{\text{1e}}$, in
analogy to $^{13}$C $1/T_1$ and consistently with a thermal mixing scenario
where all the electrons are collectively involved in the dynamic nuclear
polarization process and the nuclear spin reservoir is in good thermal contact
with the electron spin system. | cond-mat_soft |
Wall slip and flow of concentrated hard-sphere colloidal suspensions: We present a comprehensive study of the slip and flow of concentrated
colloidal suspensions using cone-plate rheometry and simultaneous confocal
imaging. In the colloidal glass regime, for smooth, non-stick walls, the solid
nature of the suspension causes a transition in the rheology from
Herschel-Bulkley (HB) bulk flow behavior at large stress to a Bingham-like slip
behavior at low stress, which is suppressed for sufficient colloid-wall
attraction or colloid-scale wall roughness. Visualization shows how the
slip-shear transition depends on gap size and the boundary conditions at both
walls and that partial slip persist well above the yield stress. A
phenomenological model, incorporating the Bingham slip law and HB bulk flow,
fully accounts for the behavior. Microscopically, the Bingham law is related to
a thin (sub-colloidal) lubrication layer at the wall, giving rise to a
characteristic dependence of slip parameters on particle size and
concentration. We relate this to the suspension's osmotic pressure and yield
stress and also analyze the influence of van der Waals interaction. For the
largest concentrations, we observe non-uniform flow around the yield stress, in
line with recent work on bulk shear-banding of concentrated pastes. We also
describe residual slip in concentrated liquid suspensions, where the vanishing
yield stress causes coexistence of (weak) slip and bulk shear flow for all
measured rates. | cond-mat_soft |
Charged dendrimers revisited: Effective charge and surface potential of
dendritic polyglycerol sulfate: We investigate key electrostatic features of charged dendrimers at hand of
the biomedically important dendritic polyglycerol sulfate (dPGS) macromolecule
using multi-scale computer simulations and Zetasizer experiments. In our
simulation study, we first develop an effective mesoscale Hamiltonian specific
to dPGS based on input from all-atom, explicit-water simulations of dPGS of low
generation. Employing this in coarse-grained, implicit-solvent/explicit-salt
Langevin dynamics simulations, we then study dPGS structural and electrostatic
properties up to the sixth generation. By systematically mapping then the
calculated electrostatic potential onto the Debye-H\"uckel form -- that serves
as a basic defining equation for the effective charge -- we determine
well-defined effective net charges and corresponding radii, surface charge
densities, and surface potentials of dPGS. The latter are found to be up to one
order of magnitude smaller than the bare values and consistent with previously
derived theories on charge renormalization and weak saturation for high
dendrimer generations (charges). Finally, we find that the surface potential of
the dendrimers estimated from the simulations compare very well with our new
electrophoretic experiments. | cond-mat_soft |
Versatile coating platform for metal oxide nanoparticles: applications
to materials and biological science: In this feature article, we provide an overview of our research on
statistical copolymers as a coating material for metal oxide nanoparticles and
surfaces. These copolymers contain functional groups enabling non-covalent
binding to oxide surfaces and poly(ethylene glycol) (PEG) polymers for
colloidal stability and stealthiness. The functional groups are organic
derivatives of phosphorous acid compounds R-H$_2$PO$_3$, also known as
phosphonic acids that have been screened for their strong affinity to metals
and their ability to build multidentate binding. Herein we develop a
polymer-based coating platform that shares features with the techniques of
self-assembled monolayers (SAM) and Layer-by-Layer (L-b-L) deposition. The
milestones of this endeavor are the synthesis of PEG-based copolymers
containing multiple phosphonic acid groups, the implementation of simple
protocols combining versatility with high particle production yields and the
experimental demonstration of the colloidal stability of the coated particles.
As a demonstration, coating studies are conducted on cerium (CeO$_2$), iron
($\gamma$-Fe$_2$O$_3$), aluminum (Al$_2$O$_3$) and titanium (TiO$_2$) oxides of
different sizes and morphologies. We finally discuss applications in the domain
of nanomaterials and nanomedicine. We evaluate the beneficial effects of
coating on redispersible nanopowders, contrast agents for In Vitro/Vivo assays
and stimuli-responsive particles. | cond-mat_soft |
Intra-Globular Structures in Multiblock Copolymer Chains from a Monte
Carlo Simulation: Multiblock copolymer chains in implicit nonselective solvents are studied by
Monte Carlo method which employs a parallel tempering algorithm. Chains
consisting of 120 $A$ and 120 $B$ monomers, arranged in three distinct
microarchitectures: $(10-10)_{12}$, $(6-6)_{20}$, and $(3-3)_{40}$, collapse to
globular states upon cooling, as expected. By varying both the reduced
temperature $T^*$ and compatibility between monomers $\omega$, numerous
intra-globular structures are obtained: diclusters (handshake, spiral, torus
with a core, etc.), triclusters, and $n$-clusters with $n>3$ (lamellar and
other), which are reminiscent of the block copolymer nanophases for spherically
confined geometries. Phase diagrams for various chains in the $(T^*,
\omega)$-space are mapped. The structure factor $S(k)$, for a selected
microarchitecture and $\omega$, is calculated. Since $S(k)$ can be measured in
scattering experiments, it can be used to relate simulation results to an
experiment. Self-assembly in those systems is interpreted in term of
competition between minimization of the interfacial area separating different
types of monomers and minimization of contacts between chain and solvent.
Finally, the relevance of this model to the protein folding is addressed. | cond-mat_soft |
Surprising simplicity in the modeling of dynamic granular intrusion: Granular intrusions, such as dynamic impact or wheel locomotion, are complex
multiphase phenomena where the grains exhibit solid-like and fluid-like
characteristics together with an ejected gas-like phase. Despite decades of
modeling efforts, a unified description of the physics in such intrusions is as
yet unknown. Here we show that a continuum model based on the simple notions of
frictional flow and tension-free separation describes complex granular
intrusions near free surfaces. This model captures dynamics in a variety of
experiments including wheel locomotion, plate intrusions, and running legged
robots. The model reveals that three effects (a static contribution and two
dynamic ones) primarily give rise to intrusion forces in such scenarios.
Identification of these effects enables the development of a further
reduced-order technique (Dynamic Resistive Force Theory) for rapid modeling of
granular locomotion of arbitrarily shaped intruders. The continuum-motivated
strategy we propose for identifying physical mechanisms and corresponding
reduced-order relations has potential use for a variety of other materials. | cond-mat_soft |
Micelle formation, gelation and phase separation of amphiphilic
multiblock copolymers: The phase behaviour of amphiphilic multiblock copolymers with a large number
of blocks in semidilute solutions is studied by lattice Monte Carlo
simulations. The influence on the resulting structures of the concentration,
the solvent quality and the ratio of hydrophobic to hydrophilic monomers in the
chains has been assessed explicitely. Several distinct regimes are put in
evidence. For poorly substituted (mainly hydrophilic) copolymers formation of
micelles is observed, either isolated or connected by the hydrophilic moieties,
depending on concentration and chain length. For more highly substituted chains
larger tubular hydrophobic structures appear which, at higher concentration,
join to form extended hydrophobic cores. For both substitution ratios gelation
is observed, but with a very different gel network structure. For the poorly
substituted chains the gel consists of micelles cross-linked by hydrophilic
blocks whereas for the highly substituted copolymers the extended hydrophobic
cores form the gelling network. The interplay between gelation and phase
separation clearly appears in the phase diagram. In particular, for poorly
substituted copolymers and in a narrow concentration range, we observe a
sol-gel transition followed by an inverse gel-sol transition when increasing
the interaction energy. The simulation results are discussed in the context of
the experimentally observed phase properties of methylcellulose, a
hydrophobically substituted polysaccharide. | cond-mat_soft |
Force spectroscopy of polymer desorption: Theory and Molecular Dynamics
simulation: Forced detachment of a single polymer chain, strongly-adsorbed on a solid
substrate, is investigated by two complementary methods: a coarse-grained
analytical dynamical model, based on the Onsager stochastic equation, and
Molecular Dynamics (MD) simulations with Langevin thermostat. The suggested
approach makes it possible to go beyond the limitations of the conventional
Bell-Evans model. We observe a series of characteristic force spikes when the
pulling force is measured against the cantilever displacement during detachment
at constant velocity $v_c$ (displacement control mode) and find that the
average magnitude of this force increases as $v_c$ grows. The probability
distributions of the pulling force and the end-monomer distance from the
surface at the moment of final detachment are investigated for different
adsorption energy $\epsilon$ and pulling velocity $v_c$. Our extensive
MD-simulations validate and support the main theoretical findings. Moreover,
the simulation reveals a novel behavior: for a strong-friction and massive
cantilever the force spikes pattern is smeared out at large $v_c$. As a
challenging task for experimental bio-polymers sequencing in future we suggest
the fabrication of stiff, super-light, nanometer-sized AFM probe. | cond-mat_soft |
Fundamental measure theory for the electric double layer: implications
for blue-energy harvesting and water desalination: Capacitive mixing (CAPMIX) and capacitive deionization (CDI) are promising
candidates for harvesting clean, renewable energy and for the energy efficient
production of potable water, respectively. Both CAPMIX and CDI involve
water-immersed porous carbon (supercapacitors) electrodes at voltages of the
order of hundreds of millivolts, such that counter-ionic packing is important
for the electric double layer (EDL) which forms near the surface of these
porous materials. Thus, we propose a density functional theory (DFT) to model
the EDL, where the White-Bear mark II fundamental measure theory functional is
combined with a mean-field Coulombic and a mean spherical approximation-type
correction to describe the interplay between dense packing and electrostatics,
in good agreement with molecular dynamics simulations. We discuss the
concentration-dependent potential rise due to changes in the chemical potential
in capacitors in the context of an over-ideal theoretical description and its
impact on energy harvesting and water desalination. Compared to less elaborate
mean-field models our DFT calculations reveal a higher work output for
blue-energy cycles and a higher energy demand for desalination cycles. | cond-mat_soft |
Deformation and motion of giant unilamellar vesicles loaded with gold
nanoparticles driven by induced charge electro-osmotic flow: A vesicle is a spherical structure composed of a phospholipid bilayer that is
used as a container for chemicals, both it in vivo and it in vitro systems. In
both cases, the vesicles can be passively moved using external molecular motors
or flows. The active motion of the vesicles can potentially expand their
applications in microfluid devices. In this study, we created giant unilamellar
vesicles (GUVs) that loads dodecanethiol-functionalized gold nanoparticles
(AuNPs) using natural swelling method. An external alternating current (AC)
electric field was applied to the sample to drive the system. A flow was
confirmed with dense optical flow method around GUVs, even in the absence of
AuNPs. The quadratic dependence of the flow on applied elecric fields confirms
that the flow is due to induced charge electro-osmotic (ICEO) mechanism.
Furthermore, the GUVs containing AuNPs moved and deformed significantly under
external AC electric fields compared with those without AuNPs. We also
confirmed that the translational speed of GUVs was positively correlated with
the volume ratio of AuNPs. These experimental results suggest that the motion
and deformation of GUVs were cause by ICEO flow, which was unbalanced owing to
the presence of localized AuNPs on the membrane. | cond-mat_soft |
Some comments on the fracture of viscoelastic solids: Crack propagation in viscoelastic solids like rubber is of great practical
importance. Shrimali and Lopez-Pamies have proposed a new interesting approach
for the crack propagation in viscoelastic solids. We give comments on the
validity of the theory and point out some effects not included in the theory. | cond-mat_soft |
Withdrawing a solid from a bath: how much liquid is coated?: A solid withdrawn from a liquid bath entrains a film. In this review, after
recalling the predictions and results for pure Newtonian liquids coated on
simple solids, we analyze the deviations to this ideal case exploring
successively three potential sources of complexity: the liquid-air interface,
the bulk rheological properties of the liquid and the mechanical or chemical
properties of the solid. For these different complexities, we show that
significant effects on the film thickness are observed experimentally and we
summarize the theoretical analysis presented in the literature, which attempt
to rationalize these measurements. | cond-mat_soft |
Lift force on an asymmetrical obstacle immersed in a dilute granular
flow: This paper investigates the lift force exerted on an elliptical obstacle
immersed in a granular flow through analytical calculations and computer
simulations. The results are shown as a function of the obstacle size,
orientation with respect to the flow direction (tilt angle), the restitution
coefficient and ellipse eccentricity. The theoretical argument, based on the
force exerted on the obstacle due to inelastic, frictionless collisions of a
very dilute flow, captures the qualitative features of the lift, but fails to
reproduce the data quantitatively. The reason behind this disagreement is that
the dilute flow assumption on which this argument is built breaks down as a
granular shock wave forms in front of the obstacle. More specifically, the
shock wave change the grains impact velocity at the obstacle, decreasing the
overall net lift obtained from a very dilute flow. | cond-mat_soft |
Anisotropic Characteristic Lengths of Colloidal Monolayers Near a
Water-Air Interface: Near-interface colloidal monolayers have often been used as model systems for
research on hydrodynamics in biophysics and microfluidic systems. Using optical
microscopy and multiparticle tracking techniques, the correlated diffusion of
particles is experimentally measured in colloidal monolayers near a water-air
interface. It is found that the characteristic lengths X1 and X2 of such a
colloidal monolayer are anisotropic in these two perpendicular directions. The
former (X1)is equal to the Saffman length of the monolayer and reflects the
continuous nature of the system in the longitudinal direction. The latter
(X2)is a function of both the Saffman length and the radius of the colloids and
reflects the discrete nature of the system in the transverse direction. This
discovery demonstrates that the hydrodynamics intrinsically follow different
rules in these two directions in this system. | cond-mat_soft |
Physics of suction cups in air and in water: We present experimental results for the dependency of the pull-off time
(failure time) on the pull-off force for suction cups in the air and in water.
The results are analyzed using a theory we have developed for the contact
between suction cups and randomly rough surfaces. The theory predicts the
dependency of the pull-off time (failure time) on the pull-off force, and is
tested with measurements performed on suction cups made from a soft polyvinyl
chloride (PVC). As substrates we used sandblasted poly(methyl methacrylate)
(PMMA). The theory is in good agreement with the experiments in air, except for
surfaces with the root-mean-square (rms) roughness below $\approx 1 \ {\rm \mu
m}$, where we observed lifetimes much longer than predicted by the theory. We
show that this is due to out-diffusion of plasticizer from the soft PVC, which
block the critical constrictions along the air flow channels. In water some
deviation between theory and experiments is observed which may be due to
capillary forces. We discuss the role of cavitation for the failure time of
suction cups in water. | cond-mat_soft |
Global perspectives on the energy landscapes of liquids, supercooled
liquids, and glassy systems: The potential energy landscape ensemble: In principle, all of the dynamical complexities of many-body systems are
encapsulated in the potential energy landscapes on which the atoms move - an
observation that suggests that the essentials of the dynamics ought to be
determined by the geometry of those landscapes. But what are the principal
geometric features that control the long-time dynamics? We suggest that the key
lies not in the local minima and saddles of the landscape, but in a more global
property of the surface: its accessible pathways. In order to make this notion
more precise we introduce two ideas: (1) a switch to a new ensemble that
removes the concept of potential barriers from the problem, and (2) a way of
finding optimum pathways within this new ensemble. The potential energy
landscape ensemble, which we describe in the current paper, regards the maximum
accessible potential energy, rather than the temperature, as a control
variable. We show here that while this approach is thermodynamically equivalent
to the canonical ensemble, it not only sidesteps the idea of barriers, it
allows us to be quantitative about the connectivity of a landscape. We
illustrate these ideas with calculations on a simple atomic liquid and on the
Kob-Andersen model of a glass-forming liquid, showing, in the process, that the
landscape of the Kob-Anderson model appears to have a connectivity transition
at the landscape energy associated with its mode-coupling transition. We turn
to the problem of finding the most efficient pathways through potential energy
landscapes in our companion paper. | cond-mat_soft |
Temperature as an external field for colloid-polymer mixtures :
"quenching" by heating and "melting" by cooling: We investigate the response to temperature of a well-known colloid-polymer
mixture. At room temperature, the critical value of the second virial
coefficient of the effective interaction for the Asakura-Oosawa model predicts
the onset of gelation with remarkable accuracy. Upon cooling the system, the
effective attractions between colloids induced by polymer depletion are
reduced, because the polymer radius of gyration is decreases as the
theta-temperature is approached. Paradoxically, this raises the effective
temperature, leading to "melting" of colloidal gels. We find the Asakura-Oosawa
model of effective colloid interactions with a simple description of the
polymer temperature response provides a quantitative description of the
fluid-gel transition. Further we present evidence for enhancement of
crystallisation rates near the metastable critical point. | cond-mat_soft |
Helix formation in linear achiral dendronized polymers. A computer
simulation study: We present a molecular simulation study of the structure of linear
dendronized polymers. We use excluded volume interactions in the context of a
generic coarse-grained molecular model whose geometrical parameters are tuned
to represent a poly(para-phenylene) backbone with benzyl ether, Frechet type
dendrons. We apply Monte Carlo sampling in order to investigate the formation
of packing-induced chiral structures along the polymer backbone of these
chemically non-chiral systems. We find that helical structures can be formed,
usually with defects consisting of domains with reversed helical handedness.
Clear signs of helical arrangements of the dendrons begin to appear for
dendritic generation g=4, while for g=5 these arrangements dominate and perfect
helices can even be observed as equilibrium structures obtained from certain
types of starting configurations. | cond-mat_soft |
Parametric excitation of wrinkles in elastic sheets on elastic and
viscoelastic substrates: Thin elastic sheets supported on compliant media form wrinkles under lateral
compression. Since the lateral pressure is coupled to the sheet's deformation,
varying it periodically in time creates a parametric excitation. We study the
resulting parametric resonance of wrinkling modes in sheets supported on
semi-infinite elastic or viscoelastic media, at pressures smaller than the
critical pressure of static wrinkling. We find distinctive behaviors as a
function of excitation amplitude and frequency, including (a) a different
dependence of the dynamic wrinkle wavelength on sheet thickness compared to the
static wavelength; and (b) a discontinuous decrease of the wrinkle wavelength
upon increasing excitation frequency at sufficiently large pressures. In the
case of a viscoelastic substrate, resonant wrinkling requires crossing a
threshold of excitation amplitude. The frequencies for observing these
phenomena in relevant experimental systems are of the order of a kilohertz and
above. We discuss experimental implications of the results. | cond-mat_soft |
Superfast collective motion of magnetic particles: It is well-known that magnetic forces can induce a formation of densely
packed strings of magnetic particles or even sheafs of several strings
(spindles). Here we show that in a sufficiently strong magnetic field, more
complex aggregates of particles, translating with a much faster speed than
would be for a single particle or even a spindle, can be assembled at the
water-air interface. Such a superfast flotilla is composed of many distant
strings or spindles, playing a role of its vessels, and moves, practically, as
a whole. We provide theoretical results to interpret the effect of a collective
motion of such magnetic vessels. Our theory shows that, in contrast to an
isolated chain or spindle, which velocity grows logarithmically with the number
of magnetic particles, the speed of the interface flotilla becomes much higher,
being proportional to the square root of their number. These results may guide
the design of magnetic systems for extremely fast controlled delivery. | cond-mat_soft |
Saffman-Taylor instability in a non-Brownian suspension: finger
selection and destabilization: We study the Saffman-Taylor instability in a non-Brownian suspension by
injection of air. We find that flow structuration in the Hele-Shaw cell can be
described by an effective viscosity depending on the volume fraction. When this
viscosity is used to define the control parameter of the instability, the
classical finger selection for Newtonian fluids is recovered. However, this
picture breaks down when the cell thickness is decreased below approximatively
10 grain sizes. The discrete nature of the grains plays also a determinant role
in the the early destabilization of the fingers observed. The grains produce a
perturbation at the interface proportional to the grain size and can thus be
considered as a "controlled noise". The finite amplitude instability mechanism
proposed earlier by Bensimon et al. allows to link this perturbation to the
actual values of the destabilization threshold. | cond-mat_soft |
Interfacial Reactions: Mixed Order Kinetics and Segregation Effects: We study A-B reaction kinetics at a fixed interface separating A and B bulks.
Initially, the number of reactions ${\cal R}_t \sim t n_A^\infty n_B^\infty$ is
2nd order in the far-field densities $n_A^\infty,n_B^\infty$. First order
kinetics, governed by diffusion from the dilute bulk, onset at long times:
${\cal R}_t\approx x_t n_A^\infty$ where $x_t\sim t^{1/z}$ is the rms molecular
displacement. Below a critical dimension, $d<d_c=z-1$, mean field theory is
invalid: a new regime appears, ${\cal R}_t\sim x_t^{d+1} n_A^\infty
n_B^\infty$, and long time A-B segregation (similar to bulk $A+B\gt\emptyset$)
leads to anomalous decay of interfacial densities. Numerical simulations for
$z=2$ support the theory. | cond-mat_soft |
Wave mixing of optical pulses and Bose-Einstein condensates: We investigate theoretically the four-wave mixing of optical and matter waves
resulting from the scattering of a short light pulse off an atomic
Bose-Einstein condensate, as recently demonstrated by D. Schneble {\em et al.}
[ Science {\bf 300}, 475 (2003)]. We show that atomic ``pair production'' from
the condensate results in the generation of both forward- and
backward-propagating matter waves. These waves are characterized by different
phase-matching conditions, resulting in different angular distributions and
temporal evolutions. | cond-mat_soft |
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