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Numerical solutions of thin film equations for polymer flows: We report on the numerical implementation of thin film equations that
describe the capillary-driven evolution of viscous films, in two-dimensional
configurations. After recalling the general forms and features of these
equations, we focus on two particular cases inspired by experiments: the
leveling of a step at the free surface of a polymer film, and the leveling of a
polymer droplet over an identical film. In each case, we first discuss the
long-term self-similar regime reached by the numerical solution before
comparing it to the experimental profile. The agreement between theory and
experiment is excellent, thus providing a versatile probe for nanorheology of
viscous liquids in thin film geometries. | cond-mat_soft |
An exact stochastic field method for the interacting Bose gas at thermal
equilibrium: We present a new exact method to numerically compute the thermodynamical
properties of an interacting Bose gas in the canonical ensemble. As in our
previous paper (Phys. Rev. A, 63 023606 (2001)), we write the density operator
$\rho$ as an average of Hartree dyadics $\ketbra{N:\phi_1}{N:\phi_2}$ and we
find stochastic evolution equations for the wave functions $\phi_{1,2}$ such
that the exact imaginary-time evolution of $\rho$ is recovered after average
over noise. In this way, the thermal equilibrium density operator can be
obtained for any temperature $T$. The method is then applied to study the
thermodynamical properties of a homogeneous one-dimensional $N$-boson system:
although Bose-Einstein condensation can not occur in the thermodynamical limit,
a macroscopic occupation of the lowest mode of a finite system is observed at
sufficiently low temperatures. If $k_B T \gg \mu$, the main effect of
interactions is to suppress density fluctuations and to reduce their
correlation length. Different effects such as a spatial antibunching of the
atoms are predicted for the opposite $k_B T\leq \mu$ regime. Our exact
stochastic calculations have been compared to existing approximate theories. | cond-mat_soft |
Probing the micromechanics of a multi-contact interface at the onset of
frictional sliding: Digital Image Correlation is used to study the micromechanics of a
multi-contact interface formed between a rough elastomer and a smooth glass
surface. The in-plane elastomer deformation is monitored during the incipient
sliding regime, i.e. the transition between static and sliding contact. As the
shear load is increased, an annular slip region, in coexistence with a central
stick region, is found to progressively invade the contact. From the
interfacial displacement field, the tangential stress field can be further
computed using a numerical inversion procedure. These local mechanical
measurements are found to be correctly captured by Cattaneo and Mindlin (CM)'s
model. However, close comparison reveals significant discrepancies in both the
displacements and stress fields that reflect the oversimplifying hypothesis
underlying CM's scenario. In particular, our optical measurements allow us to
exhibit an elasto-plastic like friction constitutive equation that differs from
the rigid-plastic behavior assumed in CM's model. This local constitutive law,
which involves a roughness-related length scale, is consistent with the model
of Bureau \textit{et al.} [Proc. R. Soc. London A \textbf{459}, 2787 (2003)]
derived for homogeneously loaded macroscopic multi-contact interfaces, thus
extending its validity to mesoscopic scales.measurements allow for the first
quantitative test of Cattaneo and Mindlin (CM) classical model of the incipient
sliding of a smooth interface. Small deviations are observed and interpreted as
a result of the finite compliance of the rough interface, a behavior which
contrasts with Amontons' law of friction assumed to be valid locally in CM's
model. We illustrate how these measurements actually provide a method for
probing the rheology of the rough interface, which we find to be of the
elasto-plastic type. | cond-mat_soft |
Cis-Trans Dynamical Asymmetry in Driven Polymer Translocation: During polymer translocation driven by e.g. voltage drop across a nanopore,
the segments in the cis-side is incessantly pulled into the pore, which are
then pushed out of it into the trans-side. This pulling and pushing polymer
segments are described in the continuum level by nonlinear transport processes
known, respectively, as fast and slow diffusions. By matching solutions of both
sides through the mass conservation across the pore, we provide a physical
basis for the cis and trans dynamical asymmetry, a feature repeatedly reported
in recent numerical simulations. We then predict how the total driving force is
dynamically allocated between cis (pulling) and trans (pushing) sides,
demonstrating that the trans-side event adds a finite-chain length effect to
the dynamical scaling, which may become substantial for weak force and/or high
pore friction cases. | cond-mat_soft |
Water affects morphogenesis of growing aquatic plant leaves: Lotus leaves floating on water usually experience short-wavelength edge
wrinkling that decays toward the center, while the leaves growing above water
normally morph into a global bending cone shape with long rippled waves near
the edge. Observations suggest that the underlying water (liquid substrate)
significantly affects the morphogenesis of leaves. To understand the
biophysical mechanism under such phenomena, we develop mathematical models that
can effectively account for inhomogeneous differential growth of floating and
free-standing leaves, to quantitatively predict formation and evolution of
their morphology. We find, both theoretically and experimentally, that the
short-wavelength buckled configuration is energetically favorable for growing
membranes lying on liquid, while the global buckling shape is more preferable
for suspended ones. Other influencing factors such as stem/vein, heterogeneity
and dimension are also investigated. Our results provide a fundamental insight
into a variety of plant morphogenesis affected by water foundation and suggest
that such surface instabilities can be harnessed for morphology control of
biomimetic deployable structures using substrate or edge actuation. | cond-mat_soft |
Molecular Origin of Limiting Shear Stress of Elastohydrodynamic
Lubrication Oil Film Studied by Molecular Dynamics: All-atom molecular dynamics simulations of an elastohydrodynamic lubrication
oil film are performed to study the effect of pressure. Fluid molecules of
n-hexane are confined between two solid plates under a constant normal force of
0.1--8.0 GPa. Traction simulations are performed by applying relative sliding
motion to the solid plates. A transition in the traction behavior is observed
around 0.5--2.0 GPa, which corresponds to the viscoelastic region to the
plastic--elastic region, which are experimentally observed. This phase
transition is related to the suppression of the fluctuation in molecular
motion. | cond-mat_soft |
Density profiles and surface tensions of polymers near colloidal
surfaces: The surface tension of interacting polymers in a good solvent is calculated
theoretically and by computer simulations for a planar wall geometry and for
the insertion of a single colloidal hard-sphere. This is achieved for the
planar wall and for the larger spheres by an adsorption method, and for smaller
spheres by a direct insertion technique. Results for the dilute and semi-dilute
regimes are compared to results for ideal polymers, the Asakura-Oosawa
penetrable-sphere model, and to integral equations, scaling and renormalization
group theories. The largest relative changes with density are found in the
dilute regime, so that theories based on non-interacting polymers rapidly break
down. A recently developed ``soft colloid'' approach to polymer-colloid
mixtures is shown to correctly describe the one-body insertion free-energy and
the related surface tension. | cond-mat_soft |
A computationally efficacious free-energy functional for studies of
inhomogeneous liquid water: We present an accurate equation of state for water based on a simple
microscopic Hamiltonian, with only four parameters that are well-constrained by
bulk experimental data. With one additional parameter for the range of
interaction, this model yields a computationally efficient free-energy
functional for inhomogeneous water which captures short-ranged correlations,
cavitation energies and, with suitable long-range corrections, the non-linear
dielectric response of water, making it an excellent candidate for studies of
mesoscale water and for use in ab initio solvation methods. | cond-mat_soft |
Cooperative Self-Propulsion of Active and Passive Rotors: Using minimal models for low Reynolds number passive and active rotors in a
fluid, we characterize the hydrodynamic interactions among rotors and the
resulting dynamics of a pair of interacting rotors. This allows us to treat in
a common framework passive or externally driven rotors, such as magnetic
colloids driven by a rotating magnetic field, and active or internally driven
rotors, such as sperm cells confined at boundaries. The hydrodynamic
interaction of passive rotors is known to contain an azimuthal component \sim
1/r^2 to dipolar order that can yield the recently discovered "cooperative
self-propulsion" of a pair of rotors of opposite vorticity. While this
interaction is identically zero for active rotors as a consequence of torque
balance, we show that a \sim 1/r^4 azimuthal component of the interaction
arises in active systems to octupolar order. Cooperative self-propulsion,
although weaker, can therefore also occur for pairs of active rotors. | cond-mat_soft |
A molecular dynamics simulation of polymer crystallization from oriented
amorphous state: Molecular process of crystallization from an oriented amorphous state was
reproduced by molecular dynamics simulation for a realistic polyethylene model.
Initial oriented amorphous state was obtained by uniaxial drawing an isotropic
glassy state at 100 K. By the temperature jump from 100 K to 330 K, there
occurred the crystallization into the fiber structure, during the process of
which we observed the developments of various order parameters. The real space
image and its Fourier transform revealed that a hexagonally ordered domain was
initially formed, and then highly ordered crystalline state with stacked
lamellae developed after further adjustment of the relative heights of the
chains along their axes. | cond-mat_soft |
Elastohydrodynamic Synchronization of Adjacent Beating Flagella: It is now well established that nearby beating pairs of eukaryotic flagella
or cilia typically synchronize in phase. A substantial body of evidence
supports the hypothesis that hydrodynamic coupling between the active
filaments, combined with waveform compliance, provides a robust mechanism for
synchrony. This elastohydrodynamic mechanism has been incorporated into
`bead-spring' models in which flagella are represented by microspheres tethered
by radial springs as internal forces drive them about orbits. While these
low-dimensional models reproduce the phenomenon of synchrony, their parameters
are not readily relatable to those of flagella. More realistic models which
reflect the elasticity of the axonemes and active force generation take the
form of fourth-order nonlinear PDEs. While computational studies have shown
synchrony, the effects of hydrodynamic coupling between nearby filaments
governed by such models have been theoretically examined only in the regime of
interflagellar distances $d$ large compared to flagellar length $L$. Yet, in
many biological situations $d/L \ll 1$. Here, we first present an asymptotic
analysis of the hydrodynamic coupling between two filaments in the regime $d/L
\ll 1$, and find that the form of the coupling is independent of the details of
the internal forces that govern the motion of the filaments. The analysis is
like the localized induction approximation for vortex filament motion, extended
to the case of mutual induction. To understand how the coupling mechanism leads
to synchrony of extended objects, we introduce a heuristic model of flagellar
beating, a single fourth-order nonlinear PDE whose form is derived from
symmetry considerations, the physics of elasticity, and the overdamped nature
of the dynamics. Analytical and numerical studies of this model illustrate how
synchrony between two filaments is achieved through the asymptotic coupling. | cond-mat_soft |
Conjugation-Length Dependence of Spin-Dependent Exciton Formation Rates
in Pi-Conjugated Oligomers and Polymers: We have measured the ratio, r = $\sigma_S/\sigma_T$ of the formation cross
section, $\sigma$ of singlet ($\sigma_S$) and triplet ($\sigma_T$) excitons
from oppositely charged polarons in a large variety of $\pi$-conjugated
oligomer and polymer films, using the photoinduced absorption and optically
detected magnetic resonance spectroscopies. The ratio r is directly related to
the singlet exciton yield, which in turn determines the maximum
electroluminescence quantum efficiency in organic light emitting diodes (OLED).
We discovered that r increases with the conjugation length, CL; in fact a
universal dependence exists in which $r^{-1}$ depends linearly on $CL^{-1}$,
irrespective of the chain backbone structure. These results indicate that
$\pi$-conjugated polymers have a clear advantage over small molecules in OLED
applications. | cond-mat_soft |
Kinetic Arrest Originating in Competition between\linebreak Attractive
Interaction and Packing Force: We discuss the situation where attractive and repulsive portions of the
inter-particle potential both contribute significantly to glass formation. We
introduce the square-well potential as prototypical model for this situation,
and {\it reject} the Baxter as a useful model for comparison to experiment on
glasses, based on our treatment within mode coupling theory. We present
explicit results for various well-widths, and show that, for narrow wells,
there is a useful analytical formula that would be suitable for experimentalist
working in the field of colloidal science. We raise the question as to whether,
in a more exact treatment, the sticky sphere limit might have an infinite glass
transition temperature, or a high but finite one. | cond-mat_soft |
A theory of magneto-elastic nanorods obtained through rigorous dimension
reduction: Starting from a two-dimensional theory of magneto-elasticity for
fiber-reinforced magnetic elastomers we carry out a rigorous dimension
reduction to derive a rod model that describes a thin magneto-elastic strip
undergoing planar deformations. The main features of the theory are the
following: a magneto-elastic interaction energy that manifests itself through a
distributed torque; a penalization term that prevent local interpenetration of
matter; a regularization that depends on the second gradient of the deformation
and models microstructure-induced size effects. As an application, we study a
problem involving magnetically-induced buckling and we show that the intensity
of the field at the onset of the instability increases if the length of the rod
is decreased. Finally, we assess the accuracy of the deduced model by
performing numerical simulations where we compare the two-dimensional and the
one-dimensional theory in some special cases and we observe excellent
agreement. | cond-mat_soft |
Cluster structure and dynamics in gels and glasses: The dynamical arrest of gels is the consequence of a well defined structural
phase transition, leading to the formation of a spanning cluster of bonded
particles. The dynamical glass transition, instead, is not accompanied by any
clear structural signature. Nevertheless, both transitions are characterized by
the emergence of dynamical heterogeneities. Reviewing recent results from
numerical simulations, we discuss the behavior of dynamical heterogeneities in
different systems and show that a clear connection with the structure exists in
the case of gels. The emerging picture may be also relevant for the more
elusive case of glasses. We show, as an example, that the relaxation process of
a simple glass-forming model can be related to a reverse percolation transition
and discuss further perspective in this direction. | cond-mat_soft |
Wrinkling composite sheets: We examine the buckling shape and critical compression of confined
inhomogeneous composite sheets lying on a liquid foundation. The buckling modes
are controlled by the bending stiffness of the sheet, the density of the
substrate, and the size and the spatially dependent elastic coefficients of the
sheet. We solve the (linearized) F\"oppl-von K\'arm\'an equations describing
the mechanical equilibrium of a sheet when its bending stiffness varies
parallel to the direction of confinement. The case of a homogeneous bending
stiffness exhibits a degeneracy of wrinkled states for certain sizes of the
confined sheet. This degeneracy disappears for spatially dependent elastic
coefficients. Medium length sheets buckle similarly to their homogeneous
counterparts, whereas the wrinkled states in large length sheets localize the
bending energy towards the soft regions of the sheet. | cond-mat_soft |
Leak-rate of seals: effective medium theory and comparison with
experiment: Seals are extremely useful devices to prevent fluid leakage. We present an
effective medium theory of the leak-rate of rubber seals, which is based on a
recently developed contact mechanics theory. We compare the theory with
experimental results for seals consisting of silicon rubber in contact with
sandpaper and sand-blasted PMMA surfaces. | cond-mat_soft |
Salt Modulated Structure of Polyelectrolyte-Macroion Complex Fibers: The structure and stability of strongly charged complex fibers formed by
complexation of a single long semi-flexible polyelectrolyte (PE) chain and many
oppositely charged spherical macroions are investigated numerically at the
ground-state level using a chain-sphere cell model. The model takes into
account chain elasticity as well as electrostatic interactions between charged
spheres and chain segments. Using a numerical optimization method based on a
periodically repeated unit cell, we obtain fiber configurations that minimize
the total energy. The optimal configurations exhibit a variety of helical
structures for the arrangement of macroions including zig-zag, solenoidal and
beads-on-a-string patterns. These structures are determined by a competition
between attraction between spheres and the PE chain (which favors chain
wrapping around the spheres), chain bending and electrostatic repulsion between
chain segments (which favor unwrapping of the chain), and the interactions
between neighboring sphere-chain complexes which can be attractive or repulsive
depending on the system parameters such as medium salt concentration, macroion
charge and chain length per macroion (linker size). At about physiological salt
concentration, dense zig-zag patterns are found to be energetically most stable
when parameters appropriate for the DNA-histone system in a chromatin fiber are
adopted. In fact, the predicted fiber diameter in this regime is found to be
around 30nm, which appears to agree with the thickness observed in in vitro
experiments on chromatin. We also find a macroion density of 5-6 per 11nm which
agrees with the zig-zag or cross-linker models of chromatin. Since our study
deals primarily with a generic model, these findings suggest that
chromatin-like structures should also be observable for PE-macroion complexes
formed in solutions of DNA and synthetic nano-colloids of opposite charge. | cond-mat_soft |
Local order variations in confined hard-sphere fluids: Pair distributions of fluids confined between two surfaces at close distance
are of fundamental importance for a variety of physical, chemical, and
biological phenomena, such as interactions between macromolecules in solution,
surface forces, and diffusion in narrow pores. However, in contrast to bulk
fluids, properties of inhomogeneous fluids are seldom studied at the
pair-distribution level. Motivated by recent experimental advances in
determining anisotropic structure factors of confined fluids, we analyze
theoretically the underlying anisotropic pair distributions of the archetypical
hard-sphere fluid confined between two parallel hard surfaces using
first-principles statistical mechanics of inhomogeneous fluids. For this
purpose, we introduce an experimentally accessible ensemble-averaged local
density correlation function and study its behavior as a function of confining
slit width. Upon increasing the distance between the confining surfaces, we
observe an alternating sequence of strongly anisotropic versus more isotropic
local order. The latter is due to packing frustration of the spherical
particles. This observation highlights the importance of studying inhomogeneous
fluids at the pair-distribution level. | cond-mat_soft |
A Discrete Packing Model of Granular Material Confined in A Vertical
Column: The transmission rules of interparticle forces between granular particles
were analyzed, and a discrete packing model was proposed to calculate the
static pressure at the bottom of granular material confined in a vertical
column. The mechanical analysis and numerical simulation results indicate that
the silo effect is caused by the frictional contacts between border particles
and inner walls, the static pressure at the bottom depends on the external load
initially, and then tends to a saturation pressure (Pn) in an exponential form.
The saturation pressure (Pn) is positive linear related to the container radius
(R) with the same granular matter and stacking manner. The saturation pressure
(Pn) is directly proportional to the particle size (ra), and the increasing or
decreasing characteristic depends on the frictional property of inner walls,
the friction and stacking angle of grains. At last, the predictions of the
aforementioned model are compared with the experimental results from the
literature, and good agreement is achieved. | cond-mat_soft |
Exact results for deposition of binary mixtures of superdisks on the
plane: We investigate the deposition of binary mixtures of oriented superdisks on a
plane. Superdisks are chosen as objects bounded by $|x|^{2p}+|y|^{2p}=1$, where
parameter $p$ controls their size and shape. For single-type superdisks, the
maximum packing and jamming densities are known to be nonanalytic at $p=0.5$.
For binary mixtures, we discover that nonanalyticities form a locus of points
separating "phase diagram" of shape combinations into regions with different
excluded-area constructions. An analytical expression for this phase boundary
and exact constructions of the excluded-areas are presented. | cond-mat_soft |
A Mechanistic Model of the Organization of Cell Shapes in Epithelial
Tissues: The organization of cells within tissues plays a vital role in various
biological processes, including development and morphogenesis. As a result,
understanding how cells self-organize in tissues has been an active area of
research. In our study, we explore a mechanistic model of cellular organization
that represents cells as force dipoles that interact with each other via the
tissue, which we model as an elastic medium. By conducting numerical
simulations using this model, we are able to observe organizational features
that are consistent with those obtained from vertex model simulations. This
approach provides valuable insights into the underlying mechanisms that govern
cellular organization within tissues, which can help us better understand the
processes involved in development and disease. | cond-mat_soft |
Relativistic separable dual-space Gaussian Pseudopotentials from H to Rn: We generalize the concept of separable dual-space Gaussian pseudopotentials
to the relativistic case. This allows us to construct this type of
pseudopotential for the whole periodic table and we present a complete table of
pseudopotential parameters for all the elements from H to Rn. The relativistic
version of this pseudopotential retains all the advantages of its
nonrelativistic version. It is separable by construction, it is optimal for
integration on a real space grid, it is highly accurate and due to its analytic
form it can be specified by a very small number of parameters. The accuracy of
the pseudopotential is illustrated by an extensive series of molecular
calculations. | cond-mat_soft |
Minimal distance transformations between links and polymers: Principles
and examples: The calculation of Euclidean distance between points is generalized to
one-dimensional objects such as strings or polymers. Necessary and sufficient
conditions for the minimal transformation between two polymer configurations
are derived. Transformations consist of piecewise rotations and translations
subject to Weierstrass-Erdmann corner conditions. Numerous examples are given
for the special cases of one and two links. The transition to a large number of
links is investigated, where the distance converges to the polymer length times
the mean root square distance (MRSD) between polymer configurations, assuming
curvature and non-crossing constraints can be neglected. Applications of this
metric to protein folding are investigated. Potential applications are also
discussed for structural alignment problems such as pharmacophore
identification, and inverse kinematic problems in motor learning and control. | cond-mat_soft |
Entropy-Driven Phase Transitions in Colloidal Systems: This thesis can be divided into two independent parts. In the first part of
this thesis, we focus on studying the kinetic pathways of nucleation in
colloidal systems. In Chapter 2, we briefly introduce the relevant theory of
nucleation, i.e., classic nucleation theory. Then in Chapter 3, we investigate
the crystal nucleation in the "simplest" model system for colloids, i.e., the
monodisperse hard-sphere system, by using three different simulation methods,
i.e., molecular dynamics, forward flux sampling and umbrella sampling
simulations. Subsequently, we apply our simulation methods to a more realistic
system of colloidal hard spheres in Chapter 4. Furthermore, we study the
nucleation in a variety of systems consisting of hard particles, i.e., hard
dumbbells (Chapter 5), hard rods (Chapter 6), hard colloidal polymers (Chapter
7) and binary hard-sphere mixtures (Chapter 8). In the second part of this
thesis, we study the phase behavior of several colloidal systems. In Chapter 9,
we study the equilibrium phase diagram of colloidal hard superballs whose shape
interpolates from cubes to octahedra via spheres. We investigate the
micellization of asymmetric patchy dumbbells induced by the depletion
attraction in Chapter 10. | cond-mat_soft |
Manipulating the Speed of Sound in a Two-Component Bose-Einstein
Condensate: We consider a two-component weakly interacting Bose-Einstein condensate in
the presence of an external field which couples the two components. We express
the Hamiltonian in terms of the energy eigenstates of the single-body part of
the Hamiltonian. These eigenstates are the atomic dressed states of quantum
optics. When the energy difference between the two dressed states is much
larger than the mean-field interactions, two-body interactions in the dressed
state basis that do not conserve the number of atoms in each of the two dressed
states are highly suppressed. The two-body interactions then take on a
simplified form in the dressed basis with effective coupling constants that
depend on the intensity and frequency of the external field. This implies that
the chemical potential as well as the quasiparticle spectrum may be controlled
experimentally in a simple manner. We demonstrate this by showing that one may
achieve significant variations in the speed of sound in the condensate, a
quantity which has been measured experimentally. | cond-mat_soft |
Photodynamics of stress in clamped nematic elastomers: We describe the complex time-dependence of the build up of force exerted by a
clamped photo-elastomer under illumination. Nonlinear (non-Beer) absorption
leads to a bleaching wave of a significant cis isomer dye concentration deeply
penetrating the solid with a highly characteristic dynamics. We fit our
experimental response at one temperature to obtain material parameters.
Force-time data can be matched at all other temperatures with no fitting
required -- our model provides a universal description of this unusual
dynamics. The description is unambiguous since these are clamped systems where
gross polymer motion is suppressed as a possible source of anomalous dynamics.
Future experiments are suggested. | cond-mat_soft |
Robust fabrication of ultra-soft tunable PDMS microcapsules as a
biomimetic model for red blood cells: Microcapsules with liquid cores encapsulated by thin membranes have many
applications in science, medicine and industry. In this paper, we design a
suspension of microcapsules which flow and deform like red blood cells (RBCs),
as a valuable tool to investigate microhaemodynamics. A reconfigurable and
easy-to-assemble 3D nested glass capillary device is used to robustly fabricate
water-oil-water double emulsions which are then converted into spherical
microcapsules with hyperelastic membranes by cross-linking the
polydimethylsiloxane (PDMS) layer coating the droplets. The resulting capsules
are monodisperse to within 1% and can be made in a wide range of size and
membrane thickness. We use osmosis to deflate by 36% initially spherical
capsules of diameter 350 {\mu}m and a membrane thickness of 4% of their radius,
in order to match the reduced volume of biconcave RBCs. We compare the
propagation of initially spherical and deflated capsules under constant
volumetric flow in cylindrical capillaries of different confinements. We find
that only deflated capsules deform broadly similarly to RBCs over a similar
range of capillary numbers (Ca) -- the ratio of viscous to elastic forces.
Similarly to the RBCs, the microcapsules transition from a symmetric
'parachute' to an asymmetric 'slipper'-like shape as Ca increases within the
physiological range, demonstrating intriguing confinement-dependent dynamics.
In addition to biomimetic RBC properties, high-throughput fabrication of
tunable ultra-soft microcapsules could be further functionalized and find
applications in other areas of science and engineering | cond-mat_soft |
Statistics of Shear-induced Rearrangements in a Model Foam: Under steady shear, a foam relaxes stress through intermittent rearrangements
of bubbles accompanied by sudden drops in the stored elastic energy. We use a
simple model of foam that incorporates both elasticity and dissipation to study
the statistics of bubble rearrangements in terms of energy drops, the number of
nearest neighbor changes, and the rate of neighbor-switching (T1) events. We do
this for a two-dimensional system as a function of system size, shear rate,
dissipation mechanism, and gas area fraction. We find that for dry foams, there
is a well-defined quasistatic limit at low shear rates where localized
rearrangements occur at a constant rate per unit strain, independent of both
system size and dissipation mechanism. These results are in good qualitative
agreement with experiments on two-dimensional and three-dimensional foams. In
contrast, we find for progessively wetter foams that the event size
distribution broadens into a power law that is cut off only by system size.
This is consistent with criticality at the melting transition. | cond-mat_soft |
On The Critical Casimir Interaction Between Anisotropic Inclusions On A
Membrane: Using a lattice model and a versatile thermodynamic integration scheme, we
study the critical Casimir interactions between inclusions embedded in a
two-dimensional critical binary mixtures. For single-domain inclusions we
demonstrate that the interactions are very long range, and their magnitudes
strongly depend on the affinity of the inclusions with the species in the
binary mixtures, ranging from repulsive when two inclusions have opposing
affinities to attractive when they have the same affinities. When one of the
inclusions has no preference for either of the species, we find negligible
critical Casimir interactions. For multiple-domain inclusions, mimicking the
observations that membrane proteins often have several domains with varying
affinities to the surrounding lipid species, the presence of domains with
opposing affinities does not cancel the interactions altogether. Instead we can
observe both attractive and repulsive interactions depending on their relative
orientations. With increasing number of domains per inclusion, the range and
magnitude of the effective interactions decrease in a similar fashion to those
of electrostatic multipoles. Finally, clusters formed by multiple-domain
inclusions can result in an effective affinity patterning due to the
anisotropic character of the Casimir interactions between the building blocks. | cond-mat_soft |
On the pressure exerted by a bundle of independent living filaments: The properties of a bundle of grafted semi-flexible living filaments in ideal
solution facing an obstacle wall, under supercritical conditions, are explored.
For this purpose, we make use of the discrete wormlike chain model
characterized by the monomer size $d$, a size dependent contour length $L_{\rm
c}$ and a persistence length $l_{\rm p}$. The calculation of the equilibrium
filament size distribution and the average equilibrium force require the
knowledge of the wall effect on the single filament partition function of any
size, which can be computed by Metropolis Monte-Carlo methods. The force
exerted by a living filament on a fixed wall turns out to be the weighted
average of the dead grafted filament forces computed for sizes hitting the
wall, multiplied by the probability of occurrence of the corresponding filament
size. As the distance to the wall is varied, the resultant force shows large
variations whose amplitude decrease with increasing gap sizes and/or with
decreasing persistence length. Also, its average over a gap interval of precise
size $d$ gives an average force close to what is expected by the ratchet model
for actin growth against a wall. The osmotic pressure exerted by $N_f$
filaments is the average equilibrium force per filament times the grafting
surface density. | cond-mat_soft |
Topological instabilities of spherical vesicles: Within the framework of the Helfrich elastic theory of membranes and of
differential geometry we study the possible instabilities of spherical vesicles
towards double bubbles. We find that not only temperature, but also magnetic
fields can induce topological transformations between spherical vesicles and
double bubbles and provide a phase diagram for the equilibrium shapes. | cond-mat_soft |
Far From Threshold Buckling Analysis of Thin Films: Thin films buckle easily and form wrinkled states in regions of well defined
size. The extent of a wrinkled region is typically assumed to reflect the zone
of in-plane compressive stresses prior to buckling, but recent experiments on
ultrathin sheets have shown that wrinkling patterns are significantly longer
and follow different scaling laws than those predicted by standard buckling
theory. Here we focus on a simple setup to show the striking differences
between near-threshold buckling and the analysis of wrinkle patterns in very
thin films, which are typically far from threshold. | cond-mat_soft |
Periodic orbits, pair nucleation, and unbinding of active nematic
defects on cones: Geometric confinement and topological constraints present promising means of
controlling active materials. By combining analytical arguments derived from
the Born-Oppenheimer approximation with numerical simulations, we investigate
the simultaneous impact of confinement together with curvature singularity by
characterizing the dynamics of an active nematic on a cone. Here, the
Born-Oppenheimer approximation means that textures can follow defect positions
rapidly on the time scales of interest. Upon imposing strong anchoring boundary
conditions at the base of a cone, we find a a rich phase diagram of
multi-defect dynamics including exotic periodic orbits of one or two $+1/2$
flank defects, depending on activity and non-quantized geometric charge at the
cone apex. By characterizing the transitions between these ordered dynamical
states, we can understand (i) defect unbinding, (ii) defect absorption and
(iii) defect pair nucleation at the apex. Numerical simulations confirm
theoretical predictions of not only the nature of the circular orbits but also
defect unbinding from the apex. | cond-mat_soft |
Loops in One Dimensional Random Walks: Distribution of loops in a one-dimensional random walk (RW), or,
equivalently, neutral segments in a sequence of positive and negative charges
is important for understanding the low energy states of randomly charged
polymers. We investigate numerically and analytically loops in several types of
RWs, including RWs with continuous step-length distribution. We show that for
long walks the probability density of the longest loop becomes independent of
the details of the walks and definition of the loops. We investigate crossovers
and convergence of probability densities to the limiting behavior, and obtain
some of the analytical properties of the universal probability density. | cond-mat_soft |
Nonlinear dielectric response at the excess wing of glass-forming
liquids: We present nonlinear dielectric measurements of glass-forming glycerol and
propylene carbonate applying electrical fields up to 671 kV/cm. The
measurements extend to sufficiently high frequencies to allow for the
investigation of the nonlinear behavior in the regime of the so-far mysterious
excess wing, showing up in the loss spectra of many glass formers as a second
power law at high frequencies. Surprisingly, we find a complete lack of
nonlinear behavior in the excess wing, in marked contrast to the
alpha-relaxation where, in agreement with previous reports, a strong increase
of dielectric constant and loss is found. | cond-mat_soft |
Geometry-dependent constitutive law for granular slow frictional drag: Frictional constitutive law for very slow vertical withdrawing of a thin rod
from a granular bed is experimentally studied. Using a very precise creep
meter, geometry-dependent granular frictional constitutive law is particularly
examined. In some previous works, a dimensionless number
$I=\dot{\gamma}D_g/\sqrt{p/\rho_g}$ has been used to characterize granular
frictional constitutive laws, where $\dot{\gamma}$, $D_g$, $p$, and $\rho_g$
are the shear strain rate, grain diameter, confining pressure, and bulk density
of granular bed, respectively. It has been considered that granular frictional
constitutive law expressed by $I$ is universal (almost geometry-independent) in
dense flow regime. In this study, however, we find that the geometry of the
system is much more crucial to characterize granular friction in a very slow
withdrawing regime. Specifically, the ratio between rod and grain diameters
must be an essential parameter to describe the granular frictional constitutive
law. Physical meaning of the geometry-dependent constitutive law is discussed
on the basis of grains-contact-number dependence of granular behavior. | cond-mat_soft |
Plastic deformations in crystal, polycrystal, and glass in binary
mixtures under shear: Collective yielding: Using molecular dynamics simulation, we examine the dynamics of crystal,
polycrystal, and glass in a Lennard-Jones binary mixture composed of small and
large particles in two dimensions. The crossovers occur among these states as
the composition c is varied at fixed size ratio. Shear is applied to a system
of 9000 particles in contact with moving boundary layers composed of 1800
particles. The particle configurations are visualized with a sixfold
orientation angle alpha_j(t) and a disorder variable D_j(t) defined for
particle j, where the latter represents the deviation from hexagonal order.
Fundamental plastic elements are classified into dislocation gliding and grain
boundary sliding. At any c, large-scale yielding events occur on the acoustic
time scale. Moreover, they multiply occur in narrow fragile areas, forming
shear bands. The dynamics of plastic flow is highly hierarchical with a wide
range of time scales for slow shearing. We also clarify the relationship
between the shear stress averaged in the bulk region and the wall stress
applied at the boundaries. | cond-mat_soft |
Controlling anomalous stresses in soft field-responsive systems: We report a new phenomenon occurring in field-responsive suspensions:
shear-induced anomalous stresses. Competition between a rotating field and a
shear flow originates a multiplicity of anomalous stress behaviors in
suspensions of bounded dimers constituted by induced dipoles. The great variety
of stress regimes includes non-monotonous behaviors, multi-resonances, negative
viscosity effect and blockades. The reversibility of the transitions between
the different regimes and the self-similarity of the stresses make this
phenomenon controllable and therefore applicable to modify macroscopic
properties of soft condensed matter phases | cond-mat_soft |
Uniform line fillings: Deterministic fabrication of random metamaterials requires filling of a space
with randomly oriented and randomly positioned chords with an on-average
homogenous density and orientation, which is a nontrivial task. We describe a
method to generate fillings with such chords, lines that run from edge to edge
of the space, in any dimension. We prove that the method leads to random but
on-average homogeneous and rotationally invariant fillings of circles, balls
and arbitrary-dimensional hyperballs from which other shapes such as rectangles
and cuboids can be cut. We briefly sketch the historic context of Bertrand's
paradox and Jaynes' solution by the principle of maximum ignorance. We analyse
the statistical properties of the produced fillings, mapping out the density
profile and the line-length distribution and comparing them to analytic
expressions. We study the characteristic dimensions of the space in between the
chords by determining the largest enclosed circles and balls in this pore
space, finding a lognormal distribution of the pore sizes. We apply the
algorithm to the direct-laser-writing fabrication design of optical
multiple-scattering samples as three-dimensional cubes of random but
homogeneously positioned and oriented chords. | cond-mat_soft |
Myelin figures from microbial glycolipid biosurfactant amphiphiles: Myelin figures (MFs) -- cylindrical lyotropic liquid crystalline structures
consisting of concentric arrays of bilayers and aqueous media -- arise from the
hydration of the bulk lamellar phase of many common amphiphiles. Prior efforts
have concentrated on the formation, structure, and dynamics of myelin produced
by phosphatidylcholine (PC)-based amphiphiles. Here, we study the myelinization
of glycolipid microbial amphiphiles, commonly addressed as biosurfactants,
produced through the process of fermentation. The hydration characteristics
(and phase diagrams) of these biological amphiphiles are atypical (and thus
their capacity to form myelin) because unlike typical amphiphiles, their
molecular structure is characterized by two hydrophilic groups (sugar,
carboxylic acid) on both ends with a hydrophobic moiety in the middle. We
tested three different glycolipid molecules: C18:1 sophorolipids and
single-glucose C18:1 and C18:0 glucolipids, all in their nonacetylated acidic
form. Neither sophorolipids (too soluble) nor C18:0 glucolipids (too insoluble)
displayed myelin growth at room temperature (RT, 25 C). The glucolipid C18:1
(G-C18:1), on the other hand, showed dense myelin growth at RT below pH 7.0.
Examining their growth rates, we find that they display a linear L $\alpha$ t
(L, myelin length; t, time) growth rate, suggesting ballistic growth,
distinctly different from the L $\alpha$ t^(1/2) dependence, characterizing
diffusive growth such as what occurs in more conventional phospholipids. These
results offer some insight into lipidic mesophases arising from a previously
unexplored class of amphiphiles with potential applications in the field of
drug delivery. | cond-mat_soft |
Memory-induced alignment of colloidal dumbbells: When a colloidal probe is forced through a viscoelastic fluid which is
characterized by a long stress-relaxation time, the fluid is excited out of
equilibrium. This is leading to a number of interesting effects including a
non-trivial recoil of the probe when the driving force is removed. Here, we
experimentally and theoretically investigate the transient recoil dynamics of
non-spherical particles, i.e., colloidal dumbbells. In addition to a
translational recoil of the dumbbells, we also find a pronounced angular
reorientation which results from the relaxation of the surrounding fluid. Our
findings are in good agreement with a Langevin description based on the
symmetries of a director (dumbbell) as well as a microscopic bath-rod model.
Remarkably, we find a frustrated state with amplified fluctuations when the
dumbbell is oriented perpendicular to the direction of driving. Our results
demonstrate the complex behavior of non-spherical objects within a relaxing
environment which are of immediate interest for the motion of externally but
also self-driven asymmetric objects in viscoelastic fluids. | cond-mat_soft |
Texture-induced modulations of friction force: the fingerprint effect: Dry solid friction is often accompanied by force modulations originating from
stick-slip instabilities. Here a distinct, quasi-static mechanism is evidenced
leading to quasi-periodic force oscillations during sliding contact between an
elastomer block, whose surface is patterned with parallel grooves, and finely
abraded glass slides. The dominant oscillation frequency is set by the ratio
between the sliding velocity and the period of the grooves. A mechanical model
is proposed that provides a quantitative prediction for the amplitude of the
force modulations as a function of the normal load, the period of the grooves
and the roughness characteristics of the substrate. The model's main ingredient
is the non-linearity of the friction law. Since such non-linearity is
ubiquitous for soft solids, this "fingerprint effect" should be relevant to a
large class of frictional configurations and might in particular have important
consequences in human (or humanoid) active digital touch. | cond-mat_soft |
Local stress and elastic properties of lipid membranes obtained from
elastic energy variation: A theory and computational method are provided for the calculation of lipid
membranes elastic parameters, which overcomes the difficulties of the existing
approaches and can be applied not only to single-component but also to
multi-component membranes. It is shown that the major elastic parameters can be
determined as the derivatives of the stress-profile moments with respect to
stretching. The more general assumption of the global incompressibility,
instead of the local one, is employed, which allows the measurement of the
local Poisson's ratio from the response of the stress profile to the isotropic
ambient pressure. In the case of the local incompressibility and quadratic
energy law, a direct relation between the bending modulus and Gaussian
curvature modulus is established. | cond-mat_soft |
Spreading of triboelectrically charged granular matter: We report on the spreading of triboelectrically charged glass particles on an
oppositely charged surface of a plastic cylindrical container in the presence
of a constant mechanical agitation. The particles spread via sticking, as a
monolayer on the cylinder's surface. Continued agitation initiates a sequence
of instabilities of this monolayer, which first forms periodic
wavy-stripe-shaped transverse density modulation in the monolayer and then
ejects narrow and long particle-jets from the tips of these stripes. These jets
finally coalesce laterally to form a homogeneous spreading front that is
layered along the spreading direction. These remarkable growth patterns are
related to a time evolving frictional drag between the moving charged glass
particles and the countercharges on the plastic container. The results provide
insight into the multiscale time-dependent tribolelectric processes and
motivates further investigation into the microscopic causes of these
macroscopic dynamical instabilities and spatial structures. | cond-mat_soft |
Evidence for marginal stability in emulsions: We report the first measurements of the effect of pressure on vibrational
modes in emulsions, which serve as a model for soft frictionless spheres at
zero temperature. As a function of the applied pressure, we find that the
density of states D(omega) exhibits a low-frequency cutoff omega*, which scales
linearly with the number of extra contacts per particle dz. Moreover, for
omega<omega*, D(omega)~ omega^2/omega*^2; a quadratic behavior whose prefactor
is larger than what is expected from Debye theory. This surprising result
agrees with recent theoretical findings. Finally, the degree of localization of
the softest low frequency modes increases with compression, as shown by the
participation ratio as well as their spatial configurations. Overall, our
observations show that emulsions are marginally stable and display
non-plane-wave modes up to vanishing frequencies. | cond-mat_soft |
On Mechanical Behavior of Elastomeric Networks: Effects of Random Porous
Microstructure: An assumption in micromechanical analysis of polymers is that the
constitutive polymeric media is non-porous. Non-porosity of media, however, is
merely a simplifying assumption. In this paper, we neglect this assumption and
studied polymer networks with a different porosity volume fraction. A random
morphology description function is used to model the porosity of the network
and nonlinear finite element analyses are conducted to perform structural
analysis of porous polymer networks. The results show that the porosity effect
is significant in mechanical behavior of polymer networks and may increase the
maximum Von-Mises stress drastically. | cond-mat_soft |
Emergent colloidal edge currents generated via exchange dynamics in a
broken dimer state: Controlling the flow of matter down to micrometer-scale confinement is of
central importance in materials and environmental sciences, with direct
applications in nano-microfluidics, drug delivery and biothechnology. Currents
of microparticles are usually generated with external field gradients of
different nature [e.g., electric, magnetic, optical, thermal or chemical ones]
which are difficult to control over spatially extended regions and samples.
Here we demonstrate a general strategy to assemble and transport polarizable
microparticles in fluid media through combination of confinement and magnetic
dipolar interactions. We use a homogeneous magnetic modulation to assemble
dispersed particles into rotating dimeric state and frustrated binary lattices,
and generate collective edge currents which arise from a novel,
field-synchronized particle exchange process. These dynamic states are similar
to cyclotron and skipping orbits in electronic and molecular systems, thus
paving the way toward understanding and engineering similar processes at
different length scales across condensed matter. | cond-mat_soft |
The role of dilation and confining stresses in shear thickening of dense
suspensions: Many densely packed suspensions and colloids exhibit a behavior known as
Discontinuous Shear Thickening in which the shear stress jumps dramatically and
reversibly as the shear rate is increased. We performed rheometry and video
microscopy measurements on a variety of suspensions to determine the mechanism
for this behavior. Shear profiles and normal stress measurements indicate that,
in the shear thickening regime, stresses are transmitted through frictional
rather than viscous interactions, and come to the surprising conclusion that
the local constitutive relation between stress and shear rate is not
necessarily shear thickening. If the suspended particles are heavy enough to
settle we find the onset stress of shear thickening tau_min corresponds to a
hydrostatic pressure from the weight of the particle packing where neighboring
particles begin to shear relative to each other. Above tau_min, dilation is
seen to cause particles to penetrate the liquid-air interface of the sheared
sample. The upper stress boundary tau_max of the shear thickening regime is
shown to roughly match the ratio of surface tension divided by a radius of
curvature on the order of the particle size. These results suggest a new model
in which the increased dissipation in the shear thickening regime comes from
frictional stresses that emerge as dilation is frustrated by a confining stress
from surface tension at the liquid-air interface. When instead the suspensions
are confined by solid walls and have no liquid-air interface, we find tau_max
is set by the stiffness of the most compliant boundary which frustrates
dilation. This rheology can be described by a non-local constitutive relation
in which the local relation between stress and shear rate is shear thinning,
but where the stress increase comes from a normal stress term which depends on
the global dilation. | cond-mat_soft |
Nonlinear, electrocatalytic swimming in the presence of salt: A small, bimetallic particle in a hydrogen peroxide solution can propel
itself by means of an electrocatalytic reaction. The swimming is driven by a
flux of ions around the particle. We model this process for the presence of a
monovalent salt, where reaction-driven proton currents induce salt ion
currents. A theory for thin diffuse layers is employed, which yields nonlinear,
coupled transport equations. The boundary conditions include a compact Stern
layer of adsorbed ions. Electrochemical processes on the particle surface are
modeled with a first order reaction of the Butler-Volmer type. The equations
are solved numerically for the swimming speed. An analytical approximation is
derived under the assumption that the decomposition of hydrogen peroxide occurs
mainly without inducing an electric current. We find that the swimming speed
increases linearly with hydrogen peroxide concentration for small
concentrations. The influence of ion diffusion on the reaction rate can lead to
a concave shape of the function of speed vs. hydrogen peroxide concentration.
The compact layer of ions on the particle diminishes the reaction rate and
consequently reduces the speed. Our results are consistent with published
experimental data. | cond-mat_soft |
Clusters in sedimentation equilibrium for an experimental
hard-sphere-plus-dipolar Brownian colloidal system: In this work, we use structure and dynamics in sedimentation equilibrium, in
the presence of gravity, to examine, $via$ confocal microscopy, a Brownian
colloidal system in the presence of an external electric field. The zero field
equation of state (EOS) is hard sphere without any re-scaling of particle size,
and the hydrodynamic corrections to the long-time self-diffusion coefficient
are quantitatively consistent with the expected value for hard spheres. Care is
taken to ensure that both the dimensionless gravitational energy, which is
equivalent to a Peclet number $Pe_g$, and dipolar strength $\Lambda$ are of
order unity. In the presence of an external electric field, anisotropic
chain-chain clusters form; this cluster formation manifests itself with the
appearance of a plateau in the diffusion coefficient when the dimensionless
dipolar strength $\Lambda \sim 1$. The structure and dynamics of this
chain-chain cluster state is examined for a monodisperse system for two
particle sizes. | cond-mat_soft |
Deposition of a particle-laden film on the inner wall of a tube: The withdrawal of a liquid or the translation of a liquid slug in a capillary
tube leads to the deposition of a thin film on the inner wall. When particles
or contaminants are present in the liquid, they deposit and contaminate the
tube if the liquid film is sufficiently thick. In this article, we
experimentally investigate the condition under which particles are deposited
during the air invasion in a capillary tube initially filled with a dilute
suspension. We show that the entrainment of particles in the film is controlled
by the ratio of the particle and the tube radii and the capillary number
associated with the front velocity. We also develop a model which suggests
optimal operating conditions to avoid contamination during withdrawal of a
suspension. This deposition mechanism can also be leveraged in coating
processes by controlling the deposition of particles on the inner walls of
channels. | cond-mat_soft |
Evidence of growing spatial correlations during the aging of glassy
glycerol: We have measured, as a function of the age $t_a$, the aging of the nonlinear
dielectric susceptibility $\chi_3$ of glycerol below the glass transition.
Whereas the linear susceptibility can be accurately accounted for in terms of
an age dependent relaxation time $\tau_{\alpha}(t_a)$, this scaling breaks down
for $\chi_3$, suggesting an increase of the amplitude of $\chi_3$. This is a
strong indication that the number $N_{corr}$ of molecules involved in
relaxation events increases with $t_a$. For $T=0.96 \times T_g$, we find that
$N_{corr}$ increases by $\sim 10%$ when $t_a$ varies from $1\mathrm{ks}$ to
$100\mathrm{ks}$. This sheds new light on the relation between length scales
and time scales in glasses. | cond-mat_soft |
Creep and drainage in the fast destabilization of emulsions Creep and
drainage in the fast destabilization of emulsions: The destabilization of emulsions is important for many applications but
remains incompletely understood. We perform squeeze flow measurements on
oil-in-water emulsions, finding that the spontaneous destabilization of
emulsions is generally very slow under normal conditions, with a characteristic
time scale given by the drainage of the continuous phase and the coalescence of
the dispersed phase. We show that if the emulsion is compressed between two
plates, the destabilization can be sped up significantly; on the one hand, the
drainage is faster due to the application of the squeezing force. On the other
hand, creep processes lead to rearrangements that also contribute to the
destabilization. | cond-mat_soft |
Renewal-anomalous-heterogeneous files: Renewal-anomalous-heterogeneous files are solved. A simple file is made of
Brownian hard spheres that diffuse stochastically in an effective 1D channel.
Generally, Brownian files are heterogeneous: the spheres' diffusion
coefficients are distributed and the initial spheres' density is non-uniform.
In renewal-anomalous files, the distribution of waiting times for individual
jumps is exponential as in Brownian files, yet obeys: {\psi}_{\alpha}
(t)~t^(-1-{\alpha}), 0<{\alpha}<1. The file is renewal as all the particles
attempt to jump at the same time. It is shown that the mean square displacement
(MSD) in a renewal-anomalous-heterogeneous file, <r^2>, obeys,
<r^2>~[<r^2>_{nrml}]^{\alpha}, where <r^2 >_{nrml} is the MSD in the
corresponding Brownian file. This scaling is an outcome of an exact relation
(derived here) connecting probability density functions of Brownian files and
renewal-anomalous files. It is also shown that non-renewal-anomalous files are
slower than the corresponding renewal ones. | cond-mat_soft |
Dynamics of Three-Dimensional Vesicles in DC Electric fields: A numerical and systematic parameter study of three-dimensional vesicle
electrohydrodynamics is presented to investigate the effects of different fluid
and membrane properties. The dynamics of vesicles in the presence of DC
electric fields is considered, both in the presence and absence of linear shear
flow. For suspended vesicles it is shown that the conductivity ratio and
viscosity ratio between the interior and exterior fluids, as well as the
vesicle membrane capacitance, substantially affect the minimum electric field
strength required to induce a full Prolate-Oblate-Prolate transition.In
addition, there exists a critical electric field strength above which a vesicle
will no longer tumble when exposed to linear shear flow. | cond-mat_soft |
Brownian Motion of Boomerang Colloidal Particles: We investigate the Brownian motion of boomerang colloidal particles confined
between two glass plates. Our experimental observations show that the mean
displacements are biased towards the center of hydrodynamic stress (CoH), and
that the mean-square displacements exhibit a crossover from short time faster
to long time slower diffusion with the short-time diffusion coefficients
dependent on the points used for tracking. A model based on Langevin theory
elucidates that these behaviors are ascribed to a superposition of two
diffusive modes: the ellipsoidal motion of the CoH and the rotational motion of
the tracking point with respect to the CoH. | cond-mat_soft |
All Optical Formation of an Atomic Bose-Einstein Condensate: We have created a Bose-Einstein condensate of 87Rb atoms directly in an
optical trap. We employ a quasi-electrostatic dipole force trap formed by two
crossed CO_2 laser beams. Loading directly from a sub-doppler laser-cooled
cloud of atoms results in initial phase space densities of ~1/200.
Evaporatively cooling through the BEC transition is achieved by lowering the
power in the trapping beams over ~ 2 s. The resulting condensates are F=1
spinors with 3.5 x 10^4 atoms distributed between the m_F = (-1,0,1) states. | cond-mat_soft |
Stiffest Elastic Networks: The rigidity of a network of elastic beams crucially depends on the specific
details of its structure. We show both numerically and theoretically that there
is a class of isotropic networks which are stiffer than any other isotropic
network with same density. The elastic moduli of these \textit{stiffest elastic
networks} are explicitly given. They constitute upper-bounds which compete or
improve the well-known Hashin-Shtrikman bounds. We provide a convenient set of
criteria (necessary and sufficient conditions) to identify these networks, and
show that their displacement field under uniform loading conditions is affine
down to the microscopic scale. Finally, examples of such networks with periodic
arrangement are presented, in both two and three dimensions. | cond-mat_soft |
The rheology of dense, polydisperse granular fluids under shear: The solution of the Enskog equation for the one-body velocity distribution of
a moderately dense, arbitrary mixture of inelastic hard spheres undergoing
planar shear flow is described. A generalization of the Grad moment method,
implemented by means of a novel generating function technique, is used so as to
avoid any assumptions concerning the size of the shear rate. The result is
illustrated by using it to calculate the pressure, normal stresses and shear
viscosity of a model polydisperse granular fluid in which grain size, mass and
coefficient of restitution varies amoungst the grains. The results are compared
to a numerical solution of the Enskog equation as well as molecular dynamics
simulations. Most bulk properties are well described by the Enskog theory and
it is shown that the generalized moment method is more accurate than the simple
(Grad) moment method. However, the description of the distribution of
temperatures in the mixture predicted by Enskog theory does not compare well to
simulation, even at relatively modest densities. | cond-mat_soft |
Surface waves in orthotropic incompressible materials: The secular equation for surface acoustic waves propagating on an orthotropic
incompressible half-space is derived in a direct manner, using the method of
first integrals. | cond-mat_soft |
Derivation of a constitutive model for the rheology of jammed soft
suspensions from particle dynamics: Considering the rheology of two-dimensional soft suspensions above the
jamming density, we derive a tensorial constitutive model from the microscopic
particle dynamics. Starting from the equation governing the $N$-particle
distribution, we derive an evolution equation for the stress tensor. This
evolution equation is not closed, as it involves the pair and three-particle
correlation functions. To close this equation, we first employ the standard
Kirkwood closure relation to express the three-particle correlation function in
terms of the pair correlation function. Then we use a simple and physically
motivated parametrization of the pair correlation function to obtain a closed
evolution equation for the stress tensor. The latter is naturally expressed as
separate evolution equations for the pressure and for the deviatoric part of
the stress tensor. These evolution equations provide us with a non-linear
tensorial constitutive model describing the rheological response of a jammed
soft suspension to an arbitrary uniform deformation. One of the advantages of
this microscopically-rooted description is that the coefficients appearing in
the constitutive model are known in terms of packing fraction and microscopic
parameters. | cond-mat_soft |
Formation of shear-bands in drying colloidal dispersions: In directionally-dried colloidal dispersions regular bands can appear behind
the drying front, inclined at $\pm45^\circ$ to the drying line. Although these
features have been noted to share visual similarities to shear bands in metal,
no physical mechanism for their formation has ever been suggested, until very
recently. Here, through microscopy of silica and polystyrene dispersions, dried
in Hele-Shaw cells, we demonstrate that the bands are indeed associated with
local shear strains. We further show how the bands form, that they scale with
the thickness of the drying layer, and that they are eliminated by the addition
of salt to the drying dispersions. Finally, we reveal the origins of these
bands in the compressive forces associated with drying, and show how they
affect the optical properties (birefringence) of colloidal films and coatings. | cond-mat_soft |
Critical wetting in power-law wedge geometries: We investigate critical wetting transitions for fluids adsorbed in wedge-like
geometries where the substrate height varies as a power-law, $z(x,y) \sim |x|
^\gamma$, in one direction. As $\gamma$ is increased from 0 to 1, the substrate
shape is smoothly changed from a planar-wall to a linear wedge. The continuous
wetting and filling transitions pertinent to these limiting geometries are
known to have distinct phase boundaries and critical singularities. We predict
that the intermediate critical wetting behaviour occurring for $0<\gamma< 1$
falls into one of {\it{three}} possible regimes depending on the values of
$\gamma$, p and q. The unbinding behaviour is characterised by a high degree of
non-universality, strongly anisotropic correlations and enhanced interfacial
roughness. The shift in phase boundary and emergence of universal critical
behaviour in the linear wedge limit is discussed in detail. | cond-mat_soft |
Diffusion and velocity correlations of the phase transitions in a system
of macroscopic rolling spheres: We study an air-fluidized granular monolayer, composed of plastic spheres
which roll on a metallic grid. The air current is adjusted so that the
spheres never loose contact
with the grid, so that the dynamics may be regarded as pseudo two-dimensional
(or two-dimensional,
if the effects of sphere rolling are not taken into account). We find two
surprising continuous
transitions, both of them displaying two coexisting phases. Moreover, in all
cases, we found the
coexisting phases display strong energy non-equipartition. In the first
transition, at weak
fludization, a glassy phase coexists with a disordered fluid-like phase. In
the second transition,
a hexagonal crystal coexists with the fluid phase. We analyze, for these
two-phase systems, the
specific diffusive properties of each phase, as well as the velocity
correlations. Surprisingly,
we find a glass phase at very low packing fraction and for a wide range of
granular
temperatures. Both phases are characterized also by a strong anti-correlated
velocities upon
collision. Thus, the dynamics observed for this quasi two-dimensional system
unveils phase
transitions with peculiar properties, very different from the predicted
behavior in well know
theories for their equilibrium counterparts. | cond-mat_soft |
Tension dynamics in semiflexible polymers. Part II: Scaling solutions
and applications: In Part I of this contribution, a systematic coarse-grained description of
the dynamics of a weakly-bending semiflexible polymer was developed. Here, we
discuss analytical solutions of the established deterministic partial
integro-differential equation for the spatio-temporal relaxation of the
backbone tension. For prototypal experimental situations, such as the sudden
application or release of a strong external pulling force, it is demonstrated
that the tensile dynamics reflects the self-affine conformational fluctuation
spectrum in a variety of intermediate asymptotic power laws. Detailed and
explicit analytical predictions for the tension propagation and relaxation and
corresponding results for common observables, such as the end-to-end distance,
are obtained. | cond-mat_soft |
Role of disorder in finite-amplitude shear of a 2D jammed material: A material's response to small but finite deformations can reveal the roots
of its response to much larger deformations. Here, we identify commonalities in
the responses of 2D soft jammed solids with different amounts of disorder. We
cyclically shear the materials while tracking their constituent particles, in
experiments that feature a stable population of repeated structural
relaxations. Using bidisperse particle sizes creates a more amorphous material,
while monodisperse sizes yield a more polycrystalline one. We find that the
materials' responses are very similar, both at the macroscopic, mechanical
level and in the microscopic motions of individual particles. However, both
locally and in bulk, crystalline arrangements of particles are stiffer (greater
elastic modulus) and less likely to rearrange. Our work supports the idea of a
common description for the responses of a wide array of materials. | cond-mat_soft |
Spatial distribution of core monomers in acrylamide-based core-shell
microgels with linear swelling behaviour: The peculiar linear temperature-dependent swelling of core-shell microgels
has been conjectured to be linked to the core-shell architecture combining
materials of different transition temperatures. Here the structure of
pNIPMAM-core and pNNPAM-shell microgels in water is studied as a function of
temperature using small-angle neutron scattering with selective deuteration.
Photon correlation spectroscopy is used to scrutinize the swelling behaviour of
the colloidal particles and reveals linear swelling. Moreover, these
experiments are also employed to check the influence of deuteration on
swelling. Using a form free multi-shell reverse Monte Carlo approach, the
small-angle scattering data are converted into radial monomer density profiles.
The comparison of 'core-only' particles consisting of identical cores to fully
hydrogenated core-shell microgels, and finally to H core/D shell architectures
unambiguously shows that core and shell monomers display gradient profiles with
strong interpenetration, leading to cores embedded in shells which are bigger
than their isolated 'core only' precursor particles. This surprising result is
further generalized to different core cross linker contents, for temperature
ranges encompassing both transitions. Our analysis demonstrates that the
internal structure of pNIPMAM-core and pNNPAM-shell microgels is heterogeneous
and strongly interpenetrated, presumably allowing only progressive core
swelling at temperatures intermediate to both transition temperatures, thus
promoting linear swelling behaviour. | cond-mat_soft |
Antibubbles: evidences of a critical pressure: We present experimental investigations of antibubbles. Such an unusual fluid
object is a thin spherical air shell surrounding a liquid globule. We explain
how to produce them and we study their stability. By overweighting antibubbles
with a small amount of salt, they sink and pop at a definite depth. A critical
depth related to a critical pressure has been found. A modified Laplace law
describes the air shell thickness evolution with respect to pressure. This law
combined with surfactant layers interaction allows to explain the critical
depth for antibubble stability. | cond-mat_soft |
Time correlations and persistence probability of a Brownian particle in
a shear flow: In this article, results have been presented for the two-time correlation
functions for a free and a harmonically confined Brownian particle in a simple
shear flow. For a free Brownian particle, the motion along the direction of
shear exhibit two distinct dynamics, with the mean-square-displacement being
diffusive at short times while at late times scales as $t^3$. In contrast the
cross-correlation $\la x(t) y(t) \ra $ scales quadratically for all times. In
the case of a harmonically trapped Brownian particle, the
mean-square-displacement exhibits a plateau determined by the strength of the
confinement and the shear. Further, the analysis is extended to a chain of
Brownian particles interacting via a harmonic and a bending potential. Finally,
the persistence probability is constructed from the two-time correlation
functions. | cond-mat_soft |
Tracer dynamics in dense soft-colloidal suspensions: From free diffusion
to hopping: Tracking of individual particle and studying their motion serves as a direct
means to understand the dynamics in crowded and complex environments. In this
study, the dynamics of tracer particles in the matrix of dense soft-colloidal
suspensions in fluid phase is studied by means of dissipative particle dynamics
simulations. By considering relatively large tracer (three times that of
colloid) we systematically explore the interplay between the environment in
which the tracer undergoes motion and interaction with the environment on the
dynamics for temperatures close to the thermodynamic freezing transition where
the effect of pair-wise interaction is significant compared to thermal energy.
To this end we consider three fluid systems differing in the degree of softness
(i.e., ultra-soft, intermediate, and hard) of the constituent colloidal
particles, also change tracer types in the sense that we vary the degree of
softness of the tracer w.r.t. colloids from ultra-soft to very hard. It is
found that for tracer in ultra-soft colloidal fluid, at long times, the motion
is diffusive for all tracer types, however the relaxation time (or diffusion
constant) increases (or decreases) with increasing hardness of tracer at a
given temperature. Interestingly, for tracer in hard colloidal fluid, the
motion changes from a free diffusion (continuous trajectory) to that of hopping
where there is intermittent jumps following a long period of localized
vibrations and consequently displacement distribution function show higher
order peaks indicating different dynamics at different time (or length) scales. | cond-mat_soft |
Estimation and Uncertainty Quantification of Yield Via Strain Recovery
Simulations: In computational materials science, predicting the yield strain of
crosslinked polymers remains a challenging task. A common approach is to
identify yield via the first critical point of stress-strain curves produced by
molecular dynamics simulations. However, the simulated data can be excessively
noisy, making it difficult to extract meaningful results. In this work, we
discuss an alternate method for identifying yield on the basis of residual
strain computations. Notably, the associated raw data produce a sharper signal
for yield through a transition in their global behavior. As we show, this
transition can be analyzed in terms of simple functions (e.g. hyperbolas) that
admit straightforward uncertainty quantification techniques. | cond-mat_soft |
Crossover from Equilibration to Aging: (Non-equilibrium) Theory vs.
Simulations: Understanding glasses and the glass transition requires comprehending the
nature of the crossover from the ergodic (or equilibrium) regime, in which the
stationary properties of the system have no history dependence, to the
mysterious glass transition region, where the measured properties are
non-stationary and depend on the protocol of preparation. In this work we use
non-equilibrium molecular dynamics simulations to test the main features of the
crossover predicted by the \emph{molecular} version of the recently-developed
multicomponent non-equilibrium self-consistent generalized Langevin equation
(NE-SCGLE) theory. According to this theory, the glass transition involves the
abrupt passage from the ordinary pattern of full equilibration to the aging
scenario characteristic of glass-forming liquids. The same theory explains that
this abrupt transition will always be observed as a blurred crossover by the
unavoidable finiteness of the time window of any experimental observation. We
find that within their finite waiting-time window, the simulations confirm the
general trends predicted by the theory. | cond-mat_soft |
Fluctuating hydrodynamics in a vertically vibrated granular fluid with
gravity: We investigate hydrodynamic fluctuations in a 2D granular fluid excited by a
vibrating base and in the presence of gravity, focusing on the transverse
velocity modes. Since the system is inhomogeneous, we measure fluctuations in
horizontal layers whose width is smaller than the characteristic hydrodynamic
lengths: they can be considered as almost-homogeneous subsystems. The large
time decay of autocorrelations of modes is exponential and compatible with
vorticity diffusion due to shear viscosity, as in equilibrium fluids. The
velocity structure factor, which strongly deviates from the equilibrium
constant behavior, is well reproduced by an effective fluctuating hydrodynamics
described by two noise terms: the first associated with vorticity diffusion and
the second with the local energy exchange, which have internal and external
character, respectively. | cond-mat_soft |
Shear band healing in amorphous materials by small-amplitude oscillatory
shear deformation: The effect of small-amplitude periodic shear on annealing of a shear band in
binary glasses is investigated using molecular dynamics simulations. The shear
band is first introduced in stable glasses via large-amplitude periodic shear,
and then amorphous samples are subjected to repeated loading during thousands
of cycles at strain amplitudes below the yield strain. It was found that with
increasing strain amplitude, the glasses are relocated to deeper potential
energy levels, while the energy change upon annealing is not affected by the
glass initial stability. The results of mechanical tests indicate that the
shear modulus and yield stress both increase towards plateau levels during the
first few hundred cycles, and their magnitudes are largest when samples are
loaded at strain amplitudes close to the yield strain. The analysis of
nonaffine displacements reveals that the shear band breaks up into isolated
clusters that gradually decay over time, leading to nearly reversible
deformation within the elastic range. These results might be useful for
mechanical processing of metallic glasses and additive manufacturing. | cond-mat_soft |
Ions at hydrophobic interfaces: We argue that the kosmotropes remain strongly hydrated in the vicinity of a
hydrophobic surface, while the chaotropes lose their hydration shell and can
become adsorbed to the interface. The mechanism of adsorption is still a
subject of debate. We argue that there are two driving forces for anionic
adsorption: the hydrophobic cavitational energy and the interfacial
electrostatic surface potential of water. While the cavitational contribution
to ionic adsorption is now well accepted, the role of the electrostatic surface
potential is much less clear. The difficulty is that even the sign of this
potential is a subject of debate, with the ab initio and the classical force
field simulations predicting electrostatic surface potentials of opposite sign.
In this paper, we will argue that the strong anionic adsorption found in the
polarizable force field simulations is the result of the artificial
electrostatic surface potential present in the classical water models. We will
show that if the adsorption of anions were as large as predicted by the
polarizable force field simulations, the excess surface tension of the NaI
solution would be strongly negative, contrary to the experimental measurements.
While the large polarizability of heavy halides is a fundamental property and
must be included in realistic modeling of the electrolyte solutions, we argue
that the point charge water models, studied so far, are incompatible with the
polarizable ionic force fields when the translational symmetry is broken. The
goal for the future should be the development of water models with very low
electrostatic surface potential. We believe that such water models will be
compatible with the polarizable force fields, which can then be used to study
the interaction of ions with hydrophobic surfaces and proteins. | cond-mat_soft |
Re-entrant spin glass and magnetoresistance in
Co_{0.2}Zn_{0.8}Fe_{1.6}Ti_{0.4}O_4 spinel oxide: We have investigated the static and dynamic response of magnetic clusters in
Co_{0.2}Zn_{0.8}Fe_{1.6}Ti_{0.4}O_4 spinel oxide, where a sequence of magnetic
phase transitions, i.e., paramagnetic (PM) to ferromagnetic at T_{C} $\leq$
270K and ferromagnetic to canted spin glass state at T_f$ $\leq$ 125K is
observed. | cond-mat_soft |
Crazing of Nanocomposites with Polymer-Tethered Nanoparticles: The crazing behavior of polymer nanocomposites formed by blending polymer
grafted nanoparticles with an entangled polymer melt is studied by molecular
dynamics simulations. We focus on the three key differences in the crazing
behavior of a composite relative to the pure homopolymer matrix, namely, a
lower yield stress, a smaller extension ratio and a grafted chain length
dependent failure stress. The yield behavior is found to be mostly controlled
by the local nanoparticle-grafted polymer interfacial energy, with the grafted
polymer-polymer matrix interfacial structure being of little to no relevance.
Increasing the attraction between nanoparticle core and the grafted polymer
inhibits void nucleation and leads to a higher yield stress. In the craze
growth regime, the presence of grafted chain sections of 100 monomers alters
the mechanical response of composite samples, giving rise to smaller extension
ratios and higher drawing stresses than for the homopolymer matrix. The
dominant failure mechanism of composite samples depends strongly on the length
of the grafted chains, with disentanglement being the dominant mechanism for
short chains, while bond breaking is the failure mode for chain lengths greater
than 10Ne, where Ne is the entanglement length. | cond-mat_soft |
Structural Properties of the Sliding Columnar Phase in Layered Liquid
Crystalline Systems: Under appropriate conditions, mixtures of cationic and neutral lipids and DNA
in water condense into complexes in which DNA strands form local 2D smectic
lattices intercalated between lipid bilayer membranes in a lamellar stack.
These lamellar DNA-cationic-lipid complexes can in principle exhibit a variety
of equilibrium phases, including a columnar phase in which parallel DNA strands
from a 2D lattice, a nematic lamellar phase in which DNA strands align along a
common direction but exhibit no long-range positional order, and a possible new
intermediate phase, the sliding columnar (SC) phase, characterized by a
vanishing shear modulus for relative displacement of DNA lattices but a
nonvanishing modulus for compressing these lattices. We develop a model capable
of describing all phases and transitions among them and use it to calculate
structural properties of the sliding columnar phase. We calculate displacement
and density correlation functions and x-ray scattering intensities in this
phase and show, in particular, that density correlations within a layer have an
unusual $\exp(- {\rm const.} \ln^2 r)$ dependence on separation r. We
investigate the stability of the SC phase with respect to shear couplings
leading to the columnar phase and dislocation unbinding leading to the lamellar
nematic phase. For models with interactions only between nearest neighbor
planes, we conclude that the SC phase is not thermodynamically stable.
Correlation functions in the nematic lamellar phase, however, exhibit SC
behavior over a range of length scales | cond-mat_soft |
Theoretical model of viscous friction inside steadily sheared foams and
concentrated emulsions: In a recent letter (Denkov et al., Phys. Rev. Lett., vol. 100 (2008) p.
138301) we calculated theoretically the macroscopic viscous stress of steadily
sheared foam/emulsion from the energy dissipated inside the transient planar
films, formed between neighboring bubbles/drops in the shear flow. The model
predicts that the viscous stress in these systems should be a proportional to
Ca^1/2, where Ca is the capillary number and n = 1/2 is the power-law index. In
the current paper we explain in detail our model and develop it further in
several aspects: First, we extend the model to account for the effects of
viscous friction in the curved meniscus regions, surrounding the planar films,
on the dynamics of film formation and on the total viscous stress. Second, we
consider the effects of surface forces (electrostatic, van der Waals, etc.)
acting between the surfaces of the neighboring bubbles/drops and show that
these forces could be important in emulsions, due to the relatively small
thickness of emulsion films (often comparable to the range of action of the
surface forces). Third, additional consideration is made for bubbles/drops
exhibiting high surface viscosity, for which we demonstrate an additional
contribution to the macroscopic viscous stress, created by the surface
dissipation of energy. The new upgraded model predicts that the energy
dissipation at the bubble/drop surface leads to power-law index n < 1/2,
whereas the contribution of the surface forces leads to n > 1/2, which explains
the rich variety of foam/emulsion behaviors observed upon steady shear. Various
comparisons are made between model predictions and experimental results for
both foams and emulsions, and a very good agreement is found. | cond-mat_soft |
Temperatures in Grains and Plasma: Grains are widely assumed to be characterized by a single temperature --
derived either from the configurational entropy, or employing the kinetic
theory. Yet granular media do have two temperatures, $T_g$ and $T$, pertaining
to the grains and atoms. It is argued here that a two-temperature plasma yields
a more useful analogy for grains than a molecular gas: (1)~Irreversible
collisions also occur in plasma, to reach the equilibrium of equal temperature.
(2)~The plasma energy is not linear in the two temperatures; it is quadratic in
the temperature difference, minimal at equilibrium. Both points have valid
analogues in grains, yielding useful insights. | cond-mat_soft |
Flows and heterogeneities with a vane tool: Magnetic resonance imaging
measurements: We study the local flow properties of various materials in a vane-in-cup
geometry. We use magnetic resonance imaging techniques to measure velocities
and particle concentrations in flowing Newtonian fluid, yield stress fluid, and
in a concentrated suspension of noncolloidal particles in a yield stress fluid.
In the Newtonian fluid, we observe that the $\theta$-averaged strain rate
component $d_{r,\theta}$ decreases as the inverse squared radius in the gap, in
agreement with a Couette analogy. This allows direct comparison (without
end-effect corrections) of the resistances to shear in vane and Couette
geometries. Here, the mean shear stress in the vane-in-cup geometry is slightly
lower than in a Couette cell of same dimensions, and a little higher than when
the vane is embedded in an infinite medium. We also observe that the flow
enters deeply the region between the blades, leading to significant extensional
flow. In the yield stress fluid, in contrast with the usually accepted picture
based on simulation results from the literature, we find that the layer of
material that is sheared near the blades at low velocity is not cylindrical.
There is thus a significant extensional component of shear that should be taken
into account in the analysis. Finally and surprisingly, in the suspension, we
observe that a thin non-cylindrical slip layer made of the pure interstitial
yield stress fluid appears quickly at the interface between the sheared
material and the material that moves as a rigid body between the blades. This
feature can be attributed to the non-symmetric trajectories of the noncolloidal
particles around the edges of the blades. This new important observation is in
sharp contradiction with the common belief that the vane tool prevents slippage
and may preclude the use of the vane tool for studying the flows of pasty
materials with large particles. | cond-mat_soft |
Global Defect Topology in Nematic Liquid Crystals: We give the global homotopy classification of nematic textures for a general
domain with weak anchoring boundary conditions and arbitrary defect set in
terms of twisted cohomology, and give an explicit computation for the case of
knotted and linked defects in $\mathbb{R}^3$, showing that the distinct
homotopy classes have a 1-1 correspondence with the first homology group of the
branched double cover, branched over the disclination loops. We show further
that the subset of those classes corresponding to elements of order 2 in this
group have representatives that are planar and characterise the obstruction for
other classes in terms of merons. The planar textures are a feature of the
global defect topology that is not reflected in any local characterisation.
Finally, we describe how the global classification relates to recent
experiments on nematic droplets and how elements of order 4 relate to the
presence of $\tau$ lines in cholesterics. | cond-mat_soft |
Generation of vector beams with liquid crystal disclination lines: We report that guiding light beams, ranging from continuous beams to
femtosecond pulses, along liquid crystal defect lines can transform them into
vector beams with various polarization profiles. Using Finite Difference Time
Domain numerical solving of Maxwell equations, we confirm that the defect in
the orientational order of the liquid crystal induces a defect in the light
field with twice the winding number of the liquid crystal defect, coupling the
topological invariants of both fields. For example, it is possible to transform
uniformly-polarized light into light with a radial polarization profile. Our
approach also correctly yields a zero-intensity region near the defect core,
which is always present in areas of discontinuous light polarization or phase.
Using circularly polarized incident light, we show that defects with
non-integer winding numbers can be obtained, where topological constants are
preserved by phase vortices, demonstrating coupling between the light's spin,
orbital angular momentum and polarization profile. Further, we find an
ultrafast femtosecond laser pulse travelling along a defect line splits into
multiple intensity regions, again depending on the defect's winding number,
allowing applications in beam steering and filtering. Finally, our approach
describing generation of complex optical fields via coupling with topological
defect lines in optically birefringent nematic fluids can be easily extended to
high-intensity beams that affect nematic ordering. | cond-mat_soft |
Jeffery Orbits with Noise Revisited: The behavior of non-spherical particles in a shear-flow is of significant
practical and theoretical interest. These systems have been the object of
numerous investigations since the pioneering work of Jeffery a century ago. His
eponymous orbits describe the deterministic motion of an isolated, rod-like
particle in a shear flow. Subsequently, the effect of adding noise was
investigated. The theory has been applied to colloidal particles,
macromolecules, anisometric granular particles and most recently to
microswimmers, for example bacteria. We study the Jeffery orbits of elongated
particles subject to noise using Langevin simulations and a Fokker-Planck
equation. We extend the analytical solution for infinitely thin needles
($\beta=1$) obtained by Doi and Edwards to particles with arbitrary shape
factor ($0\le \beta\le 1$) and validate the theory by comparing it with
simulations. We examine the rotation of the particle around the vorticity axis
and study the orientational order matrix. We use the latter to obtain scalar
order parameters $s$ and $r$ describing nematic ordering and biaxiality from
the orientational distribution function. The value of $s$ (nematic ordering)
increases monotonically with increasing P\'eclet number, while $r$ (measure of
biaxiality) displays a maximum value. From perturbation theory we obtain simple
expressions that provide accurate descriptions at low noise (or large P\'eclet
numbers). We also examine the orientational distribution in the v-grad v plane
and in the perpendicular direction. Finally we present the solution of the
Fokker-Planck equation for a strictly two-dimensional (2D) system. For the same
noise amplitude the average rotation speed of the particle in 3D is larger than
in 2D. | cond-mat_soft |
Dodecagonal spherical quasicrystals: We argue that 2D dodecagonal spherical quasicrystalls (QCs) will be
discovered in the nearest future and investigate how the planar QC order
becomes compatible with the spherical geometry. We show that the appearance of
curvature-induced topological defects and low-energy structural rearrangements
are sufficient to obtain the regular spherical QC structures. Minimization of
total energy required for the order reconstruction determines the number of
topological defects, which are located near the vertices of snub cub or
icosahedron in dependence on type of the initial dodecagonal order. | cond-mat_soft |
Giant Amplification of Small Perturbations in Frictional Amorphous Solid: Catastrophic events in Nature can be often triggered by small perturbations,
with "remote triggering" of earthquakes being an important example. Here we
present a mechanism for the giant amplification of small perturbations that is
expected to be generic in systems whose dynamics is not derivable from a
Hamiltonian. We offer a general discussion of the typical instabilities
involved (being oscillatory with an exponential increase of noise) and examine
in detail the normal forms that determine the relevant dynamics. The high
sensitivity to external perturbations is explained for systems with and without
dissipation. Numerical examples are provided using the dynamics of frictional
granular matter. Finally we point out the relationship of the presently
discussed phenomenon to the highly topical issue of "exceptional points" in
quantum models with non-Hermitian Hamiltonians. | cond-mat_soft |
Strain localization in planar shear of granular media: the role of
porosity and boundary conditions: Shear strain localization into shear bands is associated with velocity
weakening instabilities and earthquakes. Here, we simulate steady-state
plane-shear flow of numerical granular material (gouge), confined between
parallel surfaces. Both constant shear stress and constant strain-rate boundary
conditions are tested and the two types of boundary conditions are found to
yield distinct velocity profiles and friction laws. The inertial number, $I$,
exerts the largest control on the layers' behavior, but additional dependencies
of friction on normal stress and thickness of the layer are observed under
constant stress boundary condition. We find that shear-band localization, which
is present in the quasistatic regime ($I<10^{-3}$) in rate-controlled shear, is
absent under stress-controlled loading. In the latter case, flow ceases when
macroscopic friction coefficient approaches the quasistatic friction value. The
inertial regime that occurs at higher inertial numbers ($I>10^{-3}$) is
associated with distributed shear, and friction and porosity that increase with
shear rate (rate-strengthening regime). The finding that shear under constant
stress boundary condition produces the inertial, distributed shear but never
quasistatic, localized deformation is rationalized based on low fluctuations of
shear forces in granular contacts for stress-controlled loading. By examining
porosity within and outside a shear band, we also provide a mechanical reason
why the transition between quasistatic and inertial shear coincides with the
transition between localized and distributed strain. | cond-mat_soft |
The non-centrosymmetric lamellar phase in blends of ABC triblock and ac
diblock copolymers: The phase behaviour of blends of ABC triblock and ac diblock copolymers is
examined using self-consistent field theory. Several equilibrium lamellar
structures are observed, depending on the volume fraction of the diblocks,
phi_2, the monomer interactions, and the degrees of polymerization of the
copolymers. For segregations just above the order-disorder transition the
triblocks and diblocks mix together to form centrosymmetric lamellae. As the
segregation is increased the triblocks and diblocks spatially separate either
by macrophase-separating, or by forming a non-centrosymmetric (NCS) phase of
alternating layers of triblock and diblock (...ABCcaABCca...). The NCS phase is
stable over a narrow region near phi_2=0.4. This region is widest near the
critical point on the phase coexistence curve and narrows to terminate at a
triple point at higher segregation. Above the triple point there is two-phase
coexistence between almost pure triblock and diblock phases. The theoretical
phase diagram is consistent with experiments. | cond-mat_soft |
Analyzing mechanisms and microscopic reversibility of self-assembly: We use computer simulations to investigate self-assembly in a system of model
chaperonin proteins, and in an Ising lattice gas. We discuss the mechanisms
responsible for rapid and efficient assembly in these systems, and we use
measurements of dynamical activity and assembly progress to compare their
propensities for kinetic trapping. We use the analytic solution of a simple
minimal model to illustrate the key features associated with such trapping,
paying particular attention to the number of ways that particles can misbind.
We discuss the relevance of our results for the design and control of
self-assembly in general. | cond-mat_soft |
Tetris Model for Granular Drag: Motivated by recent experiments on objects moved vertically through a bed a
glass beads, a simple model to study granular drag is proposed. The model
consists of dimers on a slanted two-dimensional lattice through which objects
are dragged very slowly to obtain full relaxation between moves. Such an
approach avoids complications due to static friction in more realistic
off-lattice models, and provides for fast simulations at large system sizes.
The upward motion of objects of various diameters embedded in the lattice is
simulated and close resemblance with the experiments is found. | cond-mat_soft |
Non-linear Osmotic Brush Regime: Simulations and mean-field theory: We investigate polyelectrolyte brushes in the osmotic regime using both
theoretical analysis and molecular dynamics simulation techniques. In the
simulations at moderate Bjerrum length, we observe that the brush height varies
weakly with grafting density, in contrast to the accepted scaling law, which
predicts a brush thickness independent of the grafting density. We show that
such behavior can be explained by considering lateral electrostatic effects
(within the non-linear Poisson-Boltzmann theory) combined with the coupling
between lateral and longitudinal degrees of freedom due to the conserved
polymer volume (which are neglected in scaling arguments). We also take the
non-linear elasticity of polyelectrolyte chains into consideration, which makes
significant effects as chains are almost fully stretched in the osmotic regime.
It is shown that all these factors lead to a non-monotonic behavior for the
brush height as a function of the grafting density. At large grafting
densities, the brush height increases with increasing the grafting density due
to the volume constraint. At small grafting densities, we obtain a
re-stretching of the chains for decreasing grafting density, which is caused by
lateral electrostatic contributions and the counterion-condensation process at
polyelectrolyte chains. These results are obtained assuming all counterions to
be trapped within the brush, which is valid for sufficiently long chains of
large charge fraction. | cond-mat_soft |
Towards a soft magnetoelastic twist actuator: Soft actuators allow to transform external stimuli to mechanical
deformations. Because of their deformational response to external magnetic
fields, magnetic gels and elastomers represent ideal candidates for such tasks.
Mostly, linear magnetostrictive deformations, that is, elongations or
contractions along straight axes are discussed in this context. In contrast to
that, we here suggest to realize a twist actuator that responds by torsional
deformations around the axis of the applied magnetic field. For this purpose,
we theoretically investigate the overall mechanical response of a basic model
system containing discrete magnetizable particles in a soft elastic matrix. Two
different types of discrete particle arrangements are used as starting
conditions in the nonmagnetized state. These contain globally twisted
anisotropic particle arrangements on the one hand, and groups of discrete
helical-like particle structures positioned side by side on the other hand.
Besides the resulting twist upon magnetization, we also evaluate different
other modes of deformation. Our analysis supports the construction of
magnetically orientable and actuatable torsional mixing devices in fluidic
applications or other types of soft actuators that initiate relative rotations
between different components. | cond-mat_soft |
Importance of many-body correlations in glass transition: an example
from polydisperse hard spheres: Most of the liquid-state theories, including glass-transition theories, are
constructed on the basis of two-body density correlations. However, we have
recently shown that many-body correlations, in particular bond orientational
correlations, play a key role in both the glass transition and the
crystallization transition. Here we show, with numerical simulations of
supercooled polydisperse hard spheres systems, that the lengthscale associated
with any two-point spatial correlation function does not increase toward the
glass transition. A growing lengthscale is instead revealed by considering
many-body correlation functions, such as correlators of orientational order,
which follows the lengthscale of the dynamic heterogeneities. Despite the
growing of crystal-like bond orientational order, we reveal that the stability
against crystallization with increasing polydispersity is due to an increasing
population of icosahedral arrangements of particles. Our results suggest that,
for this type of systems, many-body correlations are a manifestation of the
link between the vitrification and the crystallization phenomena. Whether a
system is vitrified or crystallized can be controlled by the degree of
frustration against crystallization, polydispersity in this case. | cond-mat_soft |
Mean-field theory of random close packings of axisymmetric particles: Finding the optimal random packing of non-spherical particles is an open
problem with great significance in a broad range of scientific and engineering
fields. So far, this search has been performed only empirically on a
case-by-case basis, in particular, for shapes like dimers, spherocylinders and
ellipsoids of revolution. Here, we present a mean-field formalism to estimate
the packing density of axisymmetric non-spherical particles. We derive an
analytic continuation from the sphere that provides a phase diagram predicting
that, for the same coordination number, the density of monodisperse random
packings follows the sequence of increasing packing fractions: spheres < oblate
ellipsoids < prolate ellipsoids < dimers < spherocylinders. We find the maximal
packing densities of 73.1% for spherocylinders and 70.7% for dimers, in good
agreement with the largest densities found in simulations. Moreover, we find a
packing density of 73.6% for lens-shaped particles, representing the densest
random packing of the axisymmetric objects studied so far. | cond-mat_soft |
Glass and Jamming Rheology in Soft Particles Made of PNIPAM and
Polyacrylic Acid: The phase behaviour of soft colloids has attracted great attention due to the
large variety of new phenomenologies emerging from their ability to pack at
very high volume fractions. Here we report rheological measurements on
interpenetrated polymer network microgels composed of
poly(Nisopropylacrylamide) (PNIPAM) and polyacrylic acid (PAAc) at fixed PAAc
content as a function of weight concentration. We found three different
rheological regimes characteristic of three different states: a Newtonian
shear-thinning fluid, an attractive glass characterized by a yield stress, and
a jamming state. We discuss the possible molecular mechanisms driving the
formation of these states. | cond-mat_soft |
Non-equilibrium dynamics of a confined colloidal bilayer in planar shear
flow: Using Brownian dynamics (BD) simulations we investigate the impact of shear
flow on structural and dynamical properties of a system of charged colloids
confined to a narrow slit pore. Our model consists of spherical microions
interacting through a Derjaguin-Landau-Verwey-Overbeek (DLVO) and a soft-sphere
potential. The DLVO parameters were chosen according to a system of moderately
charged silica particles (with valence Z$\sim$35) in a solvent of low ionic
strength. At the confinement conditions considered, the colloids form two
well-pronounced layers. In the present study we investigate shear-induced
transitions of the translational order and dynamics in the layers, including a
discussion of the translational diffusion. In particular, we show that
diffusion in the shear-melted state can be described by an analytical model
involving a single shear-driven particle in a harmonic trap. We also explore
the emergence of zig-zag motion characterized by spatio-temporal oscillations
of the particles in the layers in the vorticity direction. Similar behavior has
been recently observed in experiments of much thicker colloidal films. | cond-mat_soft |
Orientational and induced contributions to the depolarized Rayleigh
spectra of liquid and supercooled ortho-terphenyl: The depolarized light scattering spectra of the glass forming liquid
ortho-terphenyl have been calculated in the low frequency region using
molecular dynamics simulation. Realistic system's configurations are produced
by using a recent flexible molecular model and combined with two limiting
polarizability schemes, both of them using the dipole-induced-dipole
contributions at first and second order. The calculated Raman spectral shape
are in good agreement with the experimental results in a large temperature
range. The analysis of the different contributions to the Raman spectra
emphasizes that the orientational and the collision-induced (translational)
terms lie on the same time-scale and are of comparable intensity. Moreover, the
cross terms are always found to be an important contribution to the scattering
intensity. | cond-mat_soft |
Directed Motion of Elongated Active Polymers: Previous work shows that a net directed motion arises from a system of
individual particles undergoing run-and-tumble dynamics in the presence of an
array of asymmetric barriers. Here, we show that when the individual particle
is replaced by a chain of particles linked to each other by spring forces
(polymer), the rectification is enhanced. It is found that the rectification
increases when the number of particles in each polymer, as well as its length,
increases. In addition, the rectification increases when the size of the
opening between neighboring funnel tips, lo, decreases. Interestingly, if the
conformal entropic difference exceeds the thermal diffusion, net directed
motion is observed even when the run-and-tumble dynamics approaches Brownian
motion. Also, when the inelastic collisions between the particles and the
barriers are replaced by elastic collisions, a reversed rectification is
observed. | cond-mat_soft |
Entropic elasticity and dynamics of the bacterial chromosome: a
simulation study: We study the compression and extension dynamics of a DNA-like polymer
interacting with non-DNA binding and DNA-binding proteins, by means of computer
simulations. The geometry we consider is inspired by recent experiments probing
the compressional elasticity of the bacterial nucleoid (DNA plus associated
proteins), where DNA is confined into a cylindrical container and subjected to
the action of a "piston" - a spherical bead to which an external force is
applied. We quantify the effect of steric interactions (excluded volume) on the
force-extension curves as the polymer is compressed. We find that
non-DNA-binding proteins, even at low densities, exert an osmotic force which
can be a lot larger than the entropic force exerted by the compressed DNA. The
trends we observe are qualitatively robust with respect to changes in protein
size, and are similar for neutral and charged proteins (and DNA). We also
quantify the dynamics of DNA expansion following removal of the "piston": while
the expansion is well fitted by power laws, the apparent exponent depends on
protein concentration, and protein-DNA interaction in a significant way. We
further highlight an interesting kinetic process which we observe during the
expansion of DNA interacting with DNA-binding proteins when the interaction
strength is intermediate: the proteins bind while the DNA is packaged by the
compression force, but they "pop-off" one-by-one as the force is removed,
leading to a slow unzipping kinetics. Finally, we quantify the importance of
supercoiling, which is an important feature of bacterial DNA in vivo. | cond-mat_soft |
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