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Electroadhesion for soft adhesive pads and robotics: theory and
numerical results: Soft adhesive pads are needed for many robotics applications, and one
approach is based on electroadhesion. Here we present a general analytic model
and numerical results for electroadhesion for soft solids with arbitrary
time-dependent applied voltage, and arbitrary dielectric response of the
solids, and including surface roughness. We consider the simplest
coplanar-plate-capacitor model with a periodic array of conducting strips
located close to the surface of the adhesive pad, and discuss the optimum
geometrical arrangement to obtain the maximal electroadhesion force. For
surfaces with roughness the (non-contact) gap between the solids will strongly
influence the electroadhesion, and we show how the electroadhesion force can be
calculated using a contact mechanics theory for elastic solids. The theory and
models we present can be used to optimize the design of adhesive pads for
robotics application. | cond-mat_soft |
Charge-induced conformational changes of dendrimers: We study the effect of chargeable monomers on the conformation of dendrimers
of low generation by computer simulations, employing bare Coulomb interactions.
The presence of the latter leads to an increase in size of the dendrimer due to
a combined effect of electrostatic repulsion and the presence of counterions
within the dendrimer, and also enhances a shell-like structure for the monomers
of different generations. In the resulting structures the bond-length between
monomers, especially near the center, will increase to facilitate a more
effective usage of space in the outer-regions of the dendrimer. | cond-mat_soft |
Large frequency range of negligible transmission in 1D photonic quantum
well structures: We show that it is possible to enlarge the range of low transmission in 1D
photonic crystals by using photonic quantum well structures. If a defect is
introduced in the photonic quantum well structures, defect modes with a very
high quality factor may appear. The transmission of the defect mode is due to
the coupling between the eigenmodes of the defect and those at the band edges
of the constituent photonic crystals. | cond-mat_soft |
Dielectric behaviors of corrugated membranes: We have employed our recently developed Green's function formalism to study
the dielectric behavior of a model membrane, formed by two periodic interfaces
separating two media of different dielectric constants. The Maxwell's equations
are converted into a surface integral equation; thus it greatly simplifies the
solutions and yields accurate results for membranes of arbitrary shape. The
integral equation is solved and dielectric dispersion spectrum is obtained for
a model corrugated membrane. We report a giant dielectric dispersion as the
amplitude of corrugation becomes large. | cond-mat_soft |
The effect of attractions on the local structure of liquids and
colloidal fluids: We revisit the role of attractions in liquids and apply these concepts to
colloidal suspensions. Two means are used to investigate the structure; the
pair correlation function and a recently developed topological method. The
latter identifies structures topologically equivalent to ground state clusters
formed by isolated groups of 5 < m < 13 particles, which are specific to the
system under consideration. Our topological methodology shows that, in the case
of Lennard-Jones, the addition of attractions increases the system's ability to
form larger (m>8) clusters, although pair-correlation functions are almost
identical. Conversely, in the case of short-ranged attractions, pair
correlation functions show a significant response to adding attraction, while
the liquid structure exhibits a strong decrease in clustering upon adding
attractions. Finally, a compressed, weakly interacting system shows a similar
pair structure and topology. | cond-mat_soft |
A Knowledge-driven Physics-Informed Neural Network model; Pyrolysis and
Ablation of Polymers: In aerospace applications, multiple safety regulations were introduced to
address associated with pyrolysis. Predictive modeling of pyrolysis is a
challenging task since multiple thermo-chemo-mechanical laws need to be
concurrently solved at each time step. So far, classical modeling approaches
were mostly focused on defining the basic chemical processes (pyrolysis and
ignite) at micro-scale by decoupling them from thermal solution at the
micro-scale and then validating them using meso-scale experimental results. The
advent of Machine Learning (ML) and AI in recent years has provided an
opportunity to construct quick surrogate ML models to replace high fidelity
multi-physics models, which have a high computational cost and may not be
applicable for high nonlinear equations. This serves as the motivation for the
introduction of innovative Physics informed neural networks (PINNs) to simulate
multiple stiff, and semi-stiff ODEs that govern Pyrolysis and Ablation. Our
Engine is particularly developed to calculate the char formation and degree of
burning in the course of pyrolysis of crosslinked polymeric systems. A
multi-task learning approach is hired to assure the best fitting to the
training data. The proposed Hybrid-PINN (HPINN) solver was bench-marked against
finite element high fidelity solutions on different examples. We developed PINN
architectures using collocation training to forecast temperature distributions
and the degree of burning in the course of pyrolysis in multiple one- and
two-dimensional examples. By decoupling thermal and mechanical equations, we
can predict the loss of performance in the system by predicting the char
formation pattern and localized degree of burning at each continuum. | cond-mat_soft |
A Thermodynamic Model for Receptor Clustering: Intracellular signaling often arises from ligand-induced oligomerization of
cell surface receptors. This oligomerization or clustering process is
fundamentally a cooperative behavior between near-neighbor receptor molecules;
the properties of this cooperative process clearly affects the signal
transduction. Recent investigations have revealed the molecular basis of
receptor-receptor interactions, but a simple theoretical framework for using
this data to predict cluster formation has been lacking. Here, we propose a
simple, coarse-grained, phenomenological model for ligand-modulated receptor
interactions and discuss its equilibrium properties via mean-field theory. The
existence of a first-order transition for this model has immediate implications
regarding the robustness of the cellular signaling response. | cond-mat_soft |
Exactly solvable model for self-assembly of hard core - soft shell
particles at interfaces: A lattice model with soft repulsion followed by attraction is developed for a
monolayer of hybrid core-shell particles self-assembling at an interface. The
model is solved exactly in one dimension. One, two or three periodic structures
and variety of shapes of the pressure-density isotherms may occur in different
versions of the model. For strong interactions the isotherm consists of
vertical segments separated by plateaus. The range of order depends strongly on
the strength of attraction and on the density. Our results agree with
experimental observations. | cond-mat_soft |
Spreading of Fluids on Solids Under Pressure: Effect of Slip: Spreading of different types of fluid on substrates under an impressed force
is an interesting problem. Here we study spreading of four fluids, having
different hydrophilicity and viscosity on two substrates - glass and perspex,
under an external force. The area of contact of fluid and solid is
video-photographed and its increase with time is measured. The results for
different external forces can be scaled onto a common curve. We try to explain
the nature of this curve on the basis of existing theoretical treatment where
either the no-slip condition is used or slip between fluid and substrate is
introduced. We find that of the eight cases under study, in five cases
quantitative agreement is obtained using a slip coefficient. | cond-mat_soft |
Adhesion of membranes with competing specific and generic interactions: Biomimetic membranes in contact with a planar substrate or a second membrane
are studied theoretically. The membranes contain specific adhesion molecules
(stickers) which are attracted by the second surface. In the absence of
stickers, the trans--interaction between the membrane and the second surface is
assumed to be repulsive at short separations. It is shown that the interplay of
specific attractive and generic repulsive interactions can lead to the
formation of a potential barrier. This barrier induces a line tension between
bound and unbound membrane segments which results in lateral phase separation
during adhesion. The mechanism for adhesion--induced phase separation is rather
general, as is demonstrated by considering two distinct cases involving: (i)
stickers with a linear attractive potential, and (ii) stickers with a
short--ranged square--well potential. In both cases, membrane fluctuations
reduce the potential barrier and, therefore, decrease the tendency of phase
separation. | cond-mat_soft |
Chiral Twisting of a Smectic-A Liquid Crystal: Chiral twisting of the molecular orientation within the layer of a smectic-A
liquid crystal has been investigated using circular dichroism spectroscopy. The
results indicate that a rotation of the layers away from the alignment
direction is induced by the surface electroclinic effect. This leads to an
interfacial region where the molecular director twists from the alignment
direction until it reaches the layer normal direction. A theory is presented to
explain the observed field and temperature dependence of the circular
dichroism. | cond-mat_soft |
Quantification of plasticity via particle dynamics above and below yield
in a 2D jammed suspension: Failure of amorphous materials is characterized by the emergence of
dissipation. The connection between particle dynamics, dissipation, and overall
material rheology, however, has still not been elucidated. Here, we take a new
approach relating trajectories to yielding, using a custom built interfacial
stress rheometer, which allows for measurement of shear moduli (G',G'') of a
dense athermal suspension's microstructure while simultaneously tracking
particle trajectories undergoing cyclic shear. We find an increase in total
area traced by particle trajectories as the system is stressed well below to
well above yield. Trajectories may be placed into three categories: reversibly
elastic paths; reversibly plastic paths, associated with smooth limit cycles;
and irreversibly plastic paths, in which particles do not return to their
original position. We find that above yield, reversibly plastic trajectories
are predominantly found near to the shearing surface, whereas reversibly
elastic paths are more prominent near the stationary wall. This spatial
transition between particles acting as solids to those acting as liquids is
characteristic of a 'melting front', which is observed to shift closer to the
wall with increasing strain. We introduce a non-dimensional measure of plastic
dissipation based on particle trajectories that scales linearly with strain
amplitude both above and below yield, and that is unity at the rheological
yield point. Surprisingly, this relation collapses for three systems of varying
degrees of disorder. | cond-mat_soft |
Computational Study of Mechanochemical Activation in Nanostructured
Triblock Copolymers: Force-driven chemical reactions have emerged as an attractive platform for
diverse applications in polymeric materials. However, the network topologies
necessary for efficiently transducing macroscopic forces to the molecular scale
are not well-understood. In this work, we use coarse-grained molecular dynamics
simulations to investigate the impact of network topology on mechanochemical
activation in a self-assembled triblock copolymers. We find that
mechanochemical activation during tensile deformation depends strongly on both
the polymer composition and chain conformation in these materials, with
activation requiring higher stress in materials with a higher glassy block
content, and most activation occurring in the tie chains connecting different
glassy domains. Our work suggests that changes in the network topology
significantly impact mechanochemical activation efficiencies in these
materials, suggesting that this area will be a fruitful avenue for further
experimental research. | cond-mat_soft |
Gel rupture in a dynamic environment: Hydrogels have had a profound impact in the fields of tissue engineering,
drug delivery, and materials science as a whole. Due to the network
architecture of these materials, imbibement with water often results in uniform
swelling and isotropic expansion which scales with the degree of cross-linking.
However, the development of internal stresses during swelling can have dramatic
consequences, leading to surface instabilities as well as rupture or bursting
events. To better understand hydrogel behavior, macroscopic mechanical
characterization techniques (e.g.\ tensile testing, rheometry) are often used,
however most commonly these techniques are employed on samples that are in two
distinct states: (1) unswollen and without any solvent, or (2) in an
equilibrium swelling state where the maximum amount of water has been imbibed.
Rarely is the dynamic process of swelling studied, especially in samples where
rupture or failure events are observed. To address this gap, here we focus on
rupture events in poly(ethylene glycol)-based networks that occur in response
to swelling with water. Rupture events were visualized using high-speed
imaging, and the influence of swelling on material properties was characterized
using dynamic mechanical analysis. We find that rupture events follow a
three-stage process that includes a waiting period, a slow fracture period, and
a final stage in which a rapid increase in the velocity of crack propagation is
observed. We describe this fracture behavior based on changes in material
properties that occur during swelling, and highlight how this rupture behavior
can be controlled by straight-forward modifications to the hydrogel network
structure. | cond-mat_soft |
Decoupling of dipolar and hydrophobic motions in biological membranes: Cells use homeostatic mechanisms to maintain an optimal composition of
distinct types of phospholipids in cellular membranes. The hydrophilic dipolar
layer at the membrane interface, composed of phospholipid headgroups, regulates
the interactions between cell membranes and incoming molecules, nanoparticles,
and viruses. On the other hand, the membrane hydrophobic core determines
membrane thickness and forms an environment for membrane-bound molecules such
as transmembrane proteins. A fundamental open question is to what extent the
motions of these regions are coupled and, consequently, how strongly the
interactions of lipid headgroups with other molecules depend on the properties
and composition of the membrane hydrophobic core. We combine advanced
solid-state nuclear magnetic resonance spectroscopy methodology with
high-fidelity molecular dynamics simulations to demonstrate how the rotational
dynamics of choline headgroups remain nearly unchanged (slightly faster) with
incorporation of cholesterol into a phospholipid membrane, contrasting the well
known extreme slowdown of the other phospholipid segments. Notably, our results
suggest a new paradigm where phospholipid headgroups interact as quasi-freely
rotating flexible dipoles at the interface, independent of the properties in
the hydrophobic region. | cond-mat_soft |
Chiral active matter: microscopic `torque dipoles' have more than one
hydrodynamic description: Many biological systems, such as bacterial suspensions and actomyosin
networks, form polar liquid crystals. These systems are `active' or
far-from-equilibrium, due to local forcing of the solvent by the constituent
particles. In many cases the source of activity is chiral; since forcing is
internally generated, some sort of `torque dipole' is then present locally. But
it is not obvious how `torque dipoles' should be encoded in the hydrodynamic
equations that describe the system at continuum level: different authors have
arrived at contradictory conclusions on this issue. In this work, we resolve
the paradox by presenting a careful derivation, from linear irreversible
thermodynamics, of the general equations of motion of a single-component chiral
active fluid with spin degrees of freedom. We find that there is no unique
hydrodynamic description for such a fluid in the presence of torque dipoles of
a given strength. Instead, at least three different hydrodynamic descriptions
emerge, depending on whether we decompose each torque dipole as two point
torques, two force pairs, or one point torque and one force pair -- where point
torques create internal angular momenta of the chiral bodies (spin), whereas
force pairs impart centre of mass motion that contributes to fluid velocity. By
considering a general expansion of the Onsager coefficients, we also derive a
new shear-elongation parameter and cross-coupling viscosity, which can lead to
unpredicted phenomena even in passive polar liquid crystals. Finally,
elimination of the angular variables gives an effective polar hydrodynamics
with renormalized active stresses, viscosities and kinetic coefficients.
Remarkably, this can include a direct contribution of chiral activity to the
equation of motion for the polar order parameter, which survives even in `dry'
active systems where the fluid velocity is set to zero. | cond-mat_soft |
Non-local kinetic theory of inhomogeneous liquid mixtures: In this work we investigate the dynamical properties of a mixture of mutually
interacting spherical molecules of different masses and sizes. From an analysis
of the microscopic laws governing the motion of the molecules we derive a set
of non-local self-consistent equations for the singlet phase-space distribution
functions. The theory is shown to reproduce the hydrodynamic equations for the
densities of each species, the total momentum and the local temperature. The
non ideal gas interaction term is separated into a contribution due to the
repulsive part, which is treated by means of the revised Enskog theory for hard
spheres, and an attractive contribution treated within the random phase
approximation. The present formulation accounts for the effects of the density
and velocity inhomogeneities both on the thermodynamic and transport properties
of the fluid. In a special limit, where one species is massive and diluted, the
theory leads to a description which is formally identical to the dynamic
density functional equation governing the time evolution of a colloidal system.
The derivation also determines the dependence of the friction coefficient,
appearing in the dynamic density functional theory, on the microscopic
parameters of the solvent. However, the predicted value takes into account only
the collisional contributions to the friction and not the Stokes friction of
hydrodynamic origin, suggesting that velocity correlations should be
incorporated in a more complete treatment. | cond-mat_soft |
Mechanical rejuvenation and over-aging in the soft glassy rheology model: Mechanical rejuvenation and over-aging of glasses is investigated through
stochastic simulations of the soft glassy rheology (SGR) model. Strain- and
stress-controlled deformation cycles for a wide range of loading conditions are
analyzed and compared to molecular dynamics simulations of a model polymer
glass. Results indicate that deformation causes predominantly rejuvenation,
whereas over-aging occurs only at very low temperature, small strains, and for
high initial energy states. Although the creep compliance in the SGR model
exhibits full aging independent of applied load, large stresses in the
nonlinear creep regime cause configurational changes leading to rejuvenation of
the relaxation time spectrum probed after a stress cycle. During recovery,
however, the rejuvenated state rapidly returns to the original aging trajectory
due to collective relaxations of the internal strain. | cond-mat_soft |
Network effects lead to self-organization in metabolic cycles of
self-repelling catalysts: Mixtures of particles that interact through phoretic effects are known to
aggregate if they belong to species that exhibit attractive self-interactions.
We study self-organization in a model metabolic cycle composed of three species
of catalytically-active particles that are chemotactic towards the chemicals
that define their connectivity network. We find that the self-organization can
be controlled by the network properties, as exemplified by a case where a
collapse instability is achieved by design for self-repelling species. Our
findings highlight a possibility for controlling the intricate functions of
metabolic networks by taking advantage of the physics of phoretic active
matter. | cond-mat_soft |
Anti-Diffusion in an Algae-Bacteria Microcosm: Photosynthesis,
Chemotaxis, and Expulsion: In Nature there are significant relationships known between microorganisms
from two kingdoms of life, as in the supply of vitamin B$_{12}$ by bacteria to
algae. Such interactions motivate general investigations into the
spatio-temporal dynamics of metabolite exchanges. Here we study by experiment
and theory a model system: a coculture of the bacterium $B. subtilis$, an
obligate aerobe that is chemotactic to oxygen, and a nonmotile mutant of the
alga $C. reinhardtii$, which photosynthetically produces oxygen when
illuminated. Strikingly, when a shaft of light illuminates a thin, initially
uniform suspension of the two, the chemotactic influx of bacteria to the
photosynthetically active region leads to expulsion of the algae from that
area. This effect arises from algal transport due to spatially-varying
collective behavior of bacteria, and is mathematically related to the
``turbulent diamagnetism" associated with magnetic flux expulsion in stars. | cond-mat_soft |
Computational Study of Mechanochemical Activation in Nanostructured
Triblock Copolymers: Force-driven chemical reactions have emerged as an attractive platform for
diverse applications in polymeric materials. However, the network topologies
necessary for efficiently transducing macroscopic forces to the molecular scale
are not well-understood. In this work, we use coarse-grained molecular dynamics
simulations to investigate the impact of network topology on mechanochemical
activation in a self-assembled triblock copolymers. We find that
mechanochemical activation during tensile deformation depends strongly on both
the polymer composition and chain conformation in these materials, with
activation requiring higher stress in materials with a higher glassy block
content, and most activation occurring in the tie chains connecting different
glassy domains. Our work suggests that changes in the network topology
significantly impact mechanochemical activation efficiencies in these
materials, suggesting that this area will be a fruitful avenue for further
experimental research. | cond-mat_soft |
Diffusion of an Inhomogeneous Vortex Tangle: The spatial diffusion of an inhomogeneous vortex tangle is studied
numerically with the vortex filament model. A localized initial tangle is
prepared by applying a counterflow, and the tangle is allowed to diffuse freely
after the counterflow is turned off. Comparison with the solution of a
generalization of the Vinen equation that takes diffusion into account leads to
a very small diffusion constant, as expected from simple theoretical
considerations. The relevance of this result to recent experiments on the
generation and decay of superfluid turbulence at very low temperatures is
discussed. | cond-mat_soft |
Electrochemically controlled polymeric device: a memristor (and more)
found two years ago: We report the fabrication and properties of a polymeric memristor, i.e. an
electronic element with memory of its previous history. We show how this
element can be viewed as a functional analog of a synaptic junction and how it
can be used as a critical node in adaptive networks capable of bioinspired
intelligent signal processing. | cond-mat_soft |
Mixtures of Hard Ellipsoids and Spheres: Stability of the Nematic Phase: The stability of liquid crystal phases in presence of small amount of
non-mesogenic impurities is of general interest for a large spectrum of
technological applications and in the theories of binary mixtures. Starting
from the known phase diagram of the hard ellipsoids systems, we propose a
simple model and method to explore the stability of the nematic phase in
presence of small impurities represented by hard spheres. The study is
performed in the isobaric ensemble with Monte Carlo simulations. | cond-mat_soft |
Effective mass overshoot in single degree of freedom mechanical systems
with a particle damper: We study the response of a single degree of freedom mechanical system
composed of a primary mass, M, a linear spring, a viscous damper and a particle
damper. The particle damper consists in a prismatic enclosure of variable
height that contains spherical grains (total mass m_p). Contrary to what it has
been discussed in previous experimental and simulation studies, we show that,
for small containers, the system does not approach the fully detuned mass limit
in a monotonous way. Rather, the system increases its effective mass up and
above M+m_p before reaching this expected limiting value (which is associated
with the immobilization of the particles due to a very restrictive container).
Moreover, we show that a similar effect appears in the tall container limit
where the system reaches effective masses below the expected asymptotic value
M. We present a discussion on the origin of these overshoot responses and the
consequences for industrial applications. | cond-mat_soft |
Two-Time Correlations for Probing the Aging Dynamics of Jammed Colloids: We present results for the aging dynamics of a jammed 2D colloidal system
obtained with molecular dynamics simulations. We performed extensive
simulations to gather detailed statistics about rare rearrangement events. With
a simple criterion for identifying irreversible events based on Voronoi
tessellations, we find that the rate of those events decelerates
hyperbolically. We track the probability density function for particle
displacements, the van-Hove function, with sufficient statistics as to reveal
its two-time dependence that is indicative of aging. Those displacements,
measured from a waiting time $t_{w}$ after the quench up to times
$t=t_{w}+\Delta t$, exhibit a data collapse as a function of $\Delta t/t_{w}$.
These findings can be explained comprehensively as manifestations of "record
dynamics", i.e., a relaxation dynamic driven by record-breaking fluctuations.
We show that an on-lattice model of a colloid that was built on record dynamics
indeed reproduces the experimental results in great detail. | cond-mat_soft |
Colloidal supported lipid bilayers for self-assembly: The use of colloidal supported lipid bilayers (CSLBs) has recently been
extended to create colloidal joints, that - in analogy to their macroscopic
counterparts - can flexibly connect colloidal particles. These novel elements
enable the assembly of structures with internal degrees of flexibility, but
rely on previously unappreciated properties: the simultaneous fluidity of the
bilayer, lateral mobility of inserted (linker) molecules and colloidal
stability. Here we characterize every step in the manufacturing of CSLBs in
view of these requirements using confocal microscopy and fluorescence recovery
after photobleaching (FRAP). Specifically, we have studied the influence of
different particle properties (roughness, surface charge, chemical composition,
polymer coating) on the quality and mobility of the supported bilayer. We find
that the insertion of lipopolymers in the bilayer can affect its homogeneity
and fluidity. We improve the colloidal stability by inserting lipopolymers or
double-stranded inert DNA into the bilayer. Finally, we include surface-mobile
DNA linkers and use FRAP to characterize their lateral mobility both in their
freely diffusive and bonded state. Our work offers a collection of experimental
tools for working with CSLBs in applications ranging from controlled bottom-up
self-assembly to model membrane studies. | cond-mat_soft |
Preliminary experiments demonstrating a "directed" Maxwell's granular
demon: In this paper, we design a system of two symmetrical containers communicated
by an aperture, in which a granular gas of glass spheres is created by shaking
laterally the whole system in a planetary mill. If the aperture consists in a
symmetrical hole, the two halves end up with the same number of grains after
some time when initially all particles are into in one of the containers.
However, when a funnel-like aperture is used, a robust symmetry breaking is
induced: if all the grains are originally deposited in the container facing the
wide side 95% of the grains pass to the opposite side in a relatively small
time. | cond-mat_soft |
Nonlinear dynamics and rheology of active fluids: simulations in two
dimensions: We report simulations of a continuum model for (apolar, flow aligning) active
fluids in two dimensions. Both free and anchored boundary conditions are
considered, at parallel confining walls that are either static or moving at
fixed relative velocity. We focus on extensile materials and find that steady
shear bands, previously shown to arise ubiquitously in 1D for the active
nematic phase at small (or indeed zero) shear rate, are generally replaced in
2D by more complex flow patterns that can be stationary, oscillatory, or
apparently chaotic. The consequences of these flow patterns for time-averaged
steady-state rheology are examined. | cond-mat_soft |
Two-dimensional skyrmions and other solitonic structures in
confinement-frustrated chiral nematics: We explore spatially localized solitonic configurations of a director field,
generated using optical realignment and laser-induced heating, in frustrated
chiral nematic liquid crystals confined between substrates with perpendicular
surface anchoring. We demonstrate that, in addition to recently studied torons
and Hopf-fibration solitonic structures (hopfions), one can generate a host of
other axially symmetric stable and metastable director field configurations
where local twist is matched to the surface boundary conditions through
introduction of point defects and loops of singular and nonsingular
disclinations. The experimentally demonstrated structures include the so-called
"baby-skyrmions" in the form of double twist cylinders oriented perpendicular
to the confining substrates where their double twist field configuration is
matched to the perpendicular boundary conditions by loops of twist
disclinations. We also generate complex textures with arbitrarily large
skyrmion numbers. A simple back-of-the-envelope theoretical analysis based on
free energy considerations and the nonpolar nature of chiral nematics provides
insights into the long-term stability and diversity of these inter-related
solitonic field configurations, including different types of torons,
cholestric-finger loops, two-dimensional skyrmions, and more complex structures
comprised of torons, hopfions, and various disclination loops that are
experimentally observed in a confinement-frustrated chiral nematic system. | cond-mat_soft |
Non-Equilibrium in Adsorbed Polymer Layers: High molecular weight polymer solutions have a powerful tendency to deposit
adsorbed layers when exposed to even mildly attractive surfaces. The
equilibrium properties of these dense interfacial layers have been extensively
studied theoretically. A large body of experimental evidence, however,
indicates that non-equilibrium effects are dominant whenever monomer-surface
sticking energies are somewhat larger than kT, a common case. Polymer
relaxation kinetics within the layer are then severely retarded, leading to
non-equilibrium layers whose structure and dynamics depend on adsorption
kinetics and layer ageing. Here we review experimental and theoretical work
exploring these non-equilibrium effects, with emphasis on recent developments.
The discussion addresses the structure and dynamics in non-equilibrium polymer
layers adsorbed from dilute polymer solutions and from polymer melts and more
concentrated solutions. Two distinct classes of behaviour arise, depending on
whether physisorption or chemisorption is involved. A given adsorbed chain
belonging to the layer has a certain fraction of its monomers bound to the
surface, f, and the remainder belonging to loops making bulk excursions. A
natural classification scheme for layers adsorbed from solution is the
distribution of single chain f values, P(f), which may hold the key to
quantifying the degree of irreversibility in adsorbed polymer layers. Here we
calculate P(f) for equilibrium layers; we find its form is very different to
the theoretical P(f) for non-equilibrium layers which are predicted to have
infinitely many statistical classes of chain. Experimental measurements of P(f)
are compared to these theoretical predictions. | cond-mat_soft |
Thermomechanics of DNA: A theory for thermomechanical behavior of homogeneous DNA at thermal
equilibrium predicts critical temperatures for denaturation under torque and
stretch, phase diagrams for stable B--DNA, supercoiling, optimally stable
torque, and the overstretching transition as force-induced DNA melting.
Agreement with available single molecule manipulation experiments is excellent. | cond-mat_soft |
How roughness affects the depletion mechanism: We develop a simple model, in the spirit of the Asakura-Oosawa theory, able
to describe the effects of surface roughness on the depletion potential as a
function of a small set of parameters. The resulting explicit expressions are
easily computed, without free parameters, for a wide range of physically
interesting conditions. Comparison with the recent numerical simulations [M.
Kamp et al., Langmuir, 2016, 32, 1233] shows an encouraging agreement and
allows to predict the onset of colloidal aggregation in dilute suspensions of
rough particles. Furthermore, the model proves to be suitable to investigate
the role of the geometry of the roughness. | cond-mat_soft |
Effect of topology on dynamics of knots in polymers under tension: We use computer simulations to compare the dynamical behaviour of torus and
even-twist knots in polymers under tension. The knots diffuse through a
mechanism similar to reptation. Their friction coefficients grow linearly with
average knot length for both knot types. For similar complexity, however, the
torus knots diffuse faster than the even twist knots. The knot-length
auto-correlation function exhibits a slow relaxation time that can be linked to
a breathing mode. Its timescale depends on knot type, being typically longer
for torus than for even-twist knots. These differences in dynamical behaviour
are interpreted in terms of topological features of the knots. | cond-mat_soft |
Stress correlations in glasses: We rigorously establish that, in disordered three-dimensional (3D) isotropic
solids, the stress autocorrelation function presents anisotropic terms that
decay as $1/r^3$ at long-range, with $r$ the distance, as soon as either
pressure or shear stress fluctuations are normal. By normal, we mean that the
fluctuations of stress, as averaged over spherical domains, decay as the
inverse domain volume. Since this property is required for macroscopic stress
to be self-averaging, it is expected to hold generically in all glasses and we
thus conclude that the presence of $1/r^3$ stress correlation tails is the rule
in these systems. Our proof follows from the observation that, in an infinite
medium, when both material isotropy and mechanical balance hold, (i) the stress
autocorrelation matrix is completely fixed by just two radial functions: the
pressure autocorrelation and the trace of the autocorrelation of stress
deviators; furthermore, these two functions (ii) fix the decay of the
fluctuations of sphere-averaged pressure and deviatoric stresses for windows of
increasing volume. Our conclusion is reached because, due to the precise
analytic relation (i) fixed by isotropy and mechanical balance, the constraints
arising via (ii) from the normality of stress fluctuations demand the spatially
anisotropic stress correlation terms to decay as $1/r^3$ at long-range. For the
sake of generality, we also examine situations when stress fluctuations are not
normal. | cond-mat_soft |
A Bhatnagar-Gross-Krook-like Model Kinetic Equation for a Granular Gas
of Inelastic Rough Hard Spheres: The Boltzmann collision operator for a dilute granular gas of inelastic rough
hard spheres is much more intricate than its counterpart for inelastic smooth
spheres. Now the one-body distribution function depends not only on the
translational velocity of the center of mass but also on the angular velocity
of the particle. Moreover, the collision rules couple both velocities,
involving not only the coefficient of normal restitution but also the
coefficient of tangential restitution. The aim of this paper is to propose an
extension to inelastic rough particles of a Bhatnagar-Gross-Krook-like kinetic
model previously proposed for inelastic smooth particles. The Boltzmann
collision operator is replaced by the sum of three terms representing: (i) the
relaxation to a two-temperature local equilibrium distribution, (ii) the action
of a nonconservative drag force proportional to the peculiar velocity, and
(iii) the action of a nonconservative torque equal to a linear combination of
the angular velocity and its mean value. The three coefficients in the force
and torque are fixed to reproduce the Boltzmann collisional rates of change of
the mean angular velocity and of the two granular temperatures (translational
and rotational). A simpler version of the model is also constructed in the form
of two coupled kinetic equations for the translational and rotational velocity
distributions. The kinetic model is applied to the simple shear flow steady
state and the combined influence of the two coefficients of restitution on the
shear and normal stresses and on the translational velocity distribution
function is analyzed. | cond-mat_soft |
The cage effect in systems of hard spheres: The cage effect is generally invoked when discussing the delay in the decay
of time correlation functions of dense fluids. In an attempt to examine the
role of caging more closely we consider the spread of the displacement
distributions of Brownian particles. These distributions are necessarily biased
by the presence of neighbouring particles. Accommodation of this bias by those
neighbours conserves the displacement distribution locally and presents a
collective mechanism for exploring configuration space that is more efficient
than the intrinsic Brownian motion. Caging of some particles incurs, through
the impost of global conservation of the displacement distribution, a delayed,
non-local collective process. This non-locality compromises the efficiency with
which configuration space is explored. Both collective mechanisms incur delay
or stretching of time correlation functions, in particular the particle number
and flux densities. This paper identifies and distinguishes these mechanisms in
existing data from experiments and computer simulations on systems of particles
with hard sphere interactions. | cond-mat_soft |
Likely striping in stochastic nematic elastomers: For monodomain nematic elastomers, we construct generalised elastic-nematic
constitutive models combining purely elastic and neoclassical-type
strain-energy densities. Inspired by recent developments in stochastic
elasticity, we extend these models to stochastic-elastic-nematic forms where
the model parameters are defined by spatially-independent probability density
functions at a continuum level. To investigate the behaviour of these systems
and demonstrate the effects of the probabilistic parameters, we focus on the
classical problem of shear striping in a stretched nematic elastomer for which
the solution is given explicitly. We find that, unlike in the neoclassical case
where the inhomogeneous deformation occurs within a universal interval that is
independent of the elastic modulus, for the elastic-nematic models, the
critical interval depends on the material parameters. For the stochastic
extension, the bounds of this interval are probabilistic, and the homogeneous
and inhomogeneous states compete in the sense that both have a a given
probability to occur. We refer to the inhomogeneous pattern within this
interval as "likely striping". | cond-mat_soft |
Light-Induced Manipulation of Passive and Active Microparticles: We consider sedimented at a solid wall particles that are immersed in water
containing small additives of photosensitive ionic surfactants. It is shown
that illumination with an appropriate wavelength, a beam intensity profile,
shape and size could lead to a variety of dynamic, both unsteady and
steady-state, configurations of particles. These dynamic, well-controlled and
switchable particle patterns at the wall are due to an emerging
diffusio-osmotic flow that takes its origin in the adjacent to the wall
electrostatic diffuse layer, where the concentration gradients of surfactant
are induced by light. The conventional nonporous particles are passive and can
move only with already generated flow. However, porous colloids actively
participate themselves in the flow generation mechanism at the wall, which also
sets their interactions that can be very long ranged. This light-induced
diffusio-osmosis opens novel avenues to manipulate colloidal particles and
assemble them to various patterns. We show in particular how to create and
split optically the confined regions of particles of tunable size and shape,
where well controlled flow-induced forces on the colloids could result in their
cristalline packing, formation of dilute lattices of well-separated particles,
and other states. | cond-mat_soft |
Effect of Branching on Phase Behaviors of ABC Triblock Copolymers in
Nonfrustrated Systems: The phase behavior of linear dendritic triblock copolymer melts(AB2C4) is
studied by self-consistent-field theory (SCFT) in order to find the effects of
branching on the phase behavior of ABC linear triblock copolymer melts. We
focus on a nonfrustrated parameters that \c{hi}NAB=\c{hi}NBC=\c{hi}NAC=40 where
A/C interface will not form. Frank-Kasper phases, asymmetric alternative sphere
as well as traditional phases observed in diblock copolymer melts are found to
be stable in the calculation. | cond-mat_soft |
Competition between Born solvation, dielectric exclusion, and Coulomb
attraction in spherical nanopores: The recent measurement of a very low dielectric constant, $\epsilon$, of
water confined in nanometric slit pores leads us to reconsider the physical
basis of ion partitioning into nanopores. For confined ions in chemical
equilibrium with a bulk of dielectric constant $\epsilon_b>\epsilon$, three
physical mechanisms, at the origin of ion exclusion in nanopores, are expected
to be modified due to this dielectric mismatch: dielectric exclusion at the
water-pore interface (with membrane dielectric constant,
$\epsilon_m<\epsilon$), the solvation energy related to the difference in
Debye-H\"uckel screening parameters in the pore, $\kappa$, and in the bulk
$\kappa_b$, and the classical Born solvation self-energy proportional to
$\epsilon^{-1}-\epsilon_b^{-1}$. Our goal is to clarify the interplay between
these three mechanisms and investigate the role played by the Born contribution
in ionic liquid-vapor (LV) phase separation in confined geometries. We first
compute analytically the potential of mean force (PMF) of an ion of radius
$R_i$ located at the center of a nanometric spherical pore of radius $R$.
Computing the variational grand potential for a solution of confined ions, we
then deduce the partition coefficients of ions in the pore. Phase diagrams of
the LV transition are established for various parameter values and we show that
a signature of this phase transition can be detected by monitoring the total
osmotic pressure. For charged nanopores, these exclusion effects compete with
the electrostatic attraction that imposes the entry of counterions into the
pore to enforce electro-neutrality. This study will therefore help in
deciphering the respective roles of the Born self-energy and dielectric
mismatch in experiments and simulations of ionic transport through nanopores. | cond-mat_soft |
Aging of CKN: Modulus versus conductivity analysis: It was recently reported that the electrical modulus peaks narrows upon
annealing of the ionic system CKN [Paluch et al., Phys. Rev. Lett. 110, 015702
(2013)], which was interpreted as providing evidence of dynamic heterogeneity
of this glass-forming liquid. An analysis of the same data in terms of the ac
conductivity shows no shape changes, however. We discuss the relation between
both findings and show further that the ac conductivity conforms to the
prediction of the random barrier model (RBM) at all times during the annealing. | cond-mat_soft |
Force-induced rupture of a DNA duplex: The rupture of double-stranded DNA under stress is a key process in
biophysics and nanotechnology. In this article we consider the shear-induced
rupture of short DNA duplexes, a system that has been given new importance by
recently designed force sensors and nanotechnological devices. We argue that
rupture must be understood as an activated process, where the duplex state is
metastable and the strands will separate in a finite time that depends on the
duplex length and the force applied. Thus, the critical shearing force required
to rupture a duplex within a given experiment depends strongly on the time
scale of observation. We use simple models of DNA to demonstrate that this
approach naturally captures the experimentally observed dependence of the
critical force on duplex length for a given observation time. In particular,
the critical force is zero for the shortest duplexes, before rising sharply and
then plateauing in the long length limit. The prevailing approach, based on
identifying when the presence of each additional base pair within the duplex is
thermodynamically unfavorable rather than allowing for metastability, does not
predict a time-scale-dependent critical force and does not naturally
incorporate a critical force of zero for the shortest duplexes. Additionally,
motivated by a recently proposed force sensor, we investigate application of
stress to a duplex in a mixed mode that interpolates between shearing and
unzipping. As with pure shearing, the critical force depends on the time scale
of observation; at a fixed time scale and duplex length, the critical force
exhibits a sigmoidal dependence on the fraction of the duplex that is subject
to shearing. | cond-mat_soft |
Elastic instabilities in a layered cerebral cortex: A revised axonal
tension model for cortex folding: We model the elasticity of the cerebral cortex as a layered material with
bending energy along the layers and elastic energy between them in both planar
and polar geometries. The cortex is also subjected to axons pulling from the
underlying white matter. Above a critical threshold force, a "flat" cortex
configuration becomes unstable and periodic unduluations emerge, i.e. a
buckling instability occurs. These undulations may indeed initiate folds in the
cortex. We identify analytically the critical force and the critical wavelength
of the undulations. Both quantities are physiologically relevant values. Our
model is a revised version of the axonal tension model for cortex folding, with
our version taking into account the layered structure of the cortex. Moreover,
our model draws a connection with another competing model for cortex folding,
namely the differential growth-induced buckling model. For the polar geometry,
we study the relationship between brain size and the critical force and
wavelength to understand why small mice brains exhibit no folds, while larger
human brains do, for example. Finally, an estimate of the bending rigidity
constant for the cortex can be made based on the critical wavelength. | cond-mat_soft |
Escherichia coli as a model active colloid: a practical introduction: The flagellated bacterium Escherichia coli is increasingly used
experimentally as a self-propelled swimmer. To obtain meaningful, quantitative
results that are comparable between different laboratories, reproducible
protocols are needed to control, `tune' and monitor the swimming behaviour of
these motile cells. We critically review the knowledge needed to do so, explain
methods for characterising the colloidal and motile properties of E.coli,
cells, and propose a protocol for keeping them swimming at constant speed at
finite bulk concentrations. In the process of establishing this protocol, we
use motility as a high-throughput probe of aspects of cellular physiology via
the coupling between swimming speed and the proton motive force. | cond-mat_soft |
Defect turbulence in a dense suspension of polar, active swimmers: We study the effects of inertia in dense suspensions of polar swimmers. The
hydrodynamic velocity field and the polar order parameter field describe the
dynamics of the suspension. We show that a dimensionless parameter $R$ (ratio
of the swimmer self-advection speed to the active stress invasion speed)
controls the stability of an ordered swimmer suspension. For $R$ smaller than a
threshold $R_1$, perturbations grow at a rate proportional to their wave number
$q$. Beyond $R_1$, we show that the growth rate is $\mathcal{O}(q^2)$ until a
second threshold $R=R_2$ is reached. The suspension is stable for $R>R_2$. We
perform direct numerical simulations to investigate the steady state properties
and observe defect turbulence for $R<R_2$. An investigation of the spatial
organisation of defects unravels a hidden transition: for small $R\approx 0$
defects are uniformly distributed and cluster as $R\to R_1$. Beyond $R_1$,
clustering saturates and defects are arranged in nearly string-like structures. | cond-mat_soft |
Isotropic-nematic interfacial tension of hard and soft rods: application
of advanced grand canonical biased sampling techniques: Coexistence between the isotropic and the nematic phase in suspensions of
rods is studied using grand canonical Monte Carlo simulations with a bias on
the nematic order parameter. The biasing scheme makes it possible to estimate
the interfacial tension gamma in systems of hard and soft rods. For hard rods
with L/D=15, we obtain gamma ~ 1.4 kB T/L^2, with L the rod length, D the rod
diameter, T the temperature, and kB the Boltzmann constant. This estimate is in
good agreement with theoretical predictions, and the order of magnitude is
consistent with experiments. | cond-mat_soft |
Modelling Polymer Self Diffusion: An Alternative Model to Reptation: Herein an alternative model to reptation to describe concentrated polymer
dynamics is developed. The model assumes that the chains act as blobs that are
able to diffuse past each other in a compressed state. Allowing that the local
viscosity experienced by a blob is less than the macroscopic viscosity allows
the viscosity-molecular weight and diffusion coefficient-molecular weight
relationships to be determined. | cond-mat_soft |
Strongly inelastic granular gases: The expansion of the velocity distribution function for the homogeneous
cooling state (HCS) in a Sonine polynomial series around a Maxwellian is shown
to be divergent, though Borel resummable. A convergent expansion for the HCS
has been devised and employed to obtain the HCS velocity distribution function
and (using it) the linear transport coefficients for a three dimensional
monodisperse granular gas of smooth inelastic spheres, for all physical values
of the coefficient of normal restitution. The results are in very good
agreement with findings of DSMC simulations. | cond-mat_soft |
Rain water transport and storage in a model sandy soil with hydrogel
particle additives: We study rain water infiltration and drainage in a dry model sandy soil with
superabsorbent hydrogel particle additives by measuring the mass of retained
water for non-ponding rainfall using a self-built 3D laboratory set-up. In the
pure model sandy soil, the retained water curve measurements indicate that
instead of a stable horizontal wetting front that grows downward uniformly, a
narrow fingered flow forms under the top layer of water-saturated soil. This
rain water channelization phenomenon not only further reduces the available
rain water in the plant root zone, but also affects the efficiency of soil
additives, such as superabsorbent hydrogel particles. Our studies show that the
shape of the retained water curve for a soil packing with hydrogel particle
additives strongly depends on the location and the concentration of the
hydrogel particles in the model sandy soil. By carefully choosing the particle
size and distribution methods, we may use the swollen hydrogel particles to
modify the soil pore structure, to clog or extend the water channels in sandy
soils, or to build water reservoirs in the plant root zone. | cond-mat_soft |
Role of rotational inertia for collective phenomena in active matter: We investigate the effect of rotational inertia on the collective phenomena
of underdamped active systems and show that the increase of the moment of
inertia of each particle favors non-equilibrium phase coexistence, known as
motility induced phase separation, and counteracts its suppression due to
translational inertia. Our conclusion is supported by a non-equilibrium phase
diagram (in the plane spanned by rotational inertial time and translational
inertial time) whose transition line is understood theoretically through
scaling arguments. In addition, rotational inertia increases the correlation
length of the spatial velocity correlations in the dense cluster. The fact that
rotational inertia enhances collective phenomena, such as motility induced
phase separation and spatial velocity correlations, is strongly linked to the
increase of rotational persistence. Moreover, large moments of inertia induce
non-monotonic temporal (cross) correlations between translational and
rotational degrees of freedom truly absent in non-equilibrium systems. | 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 |
Flexible polymer confined inside a cone-shaped nano-channel: Nano-scale confinement of polymer in cone-shaped geometries occurs in many
experimental situations. A flexible polymer confined in a cone-shaped
nano-channel is studied theoretically and using molecular dynamics simulations.
Distribution of the monomers inside the channel, configuration of the confined
polymer, entropic force acting on the polymer, and their dependence on the
channel and the polymer parameters are investigated. The theory and the
simulation results are in very good agreement. The entropic force on the
polymer that results from the asymmetric shape of the channel is measured in
the simulations and its magnitude is found to be significant relative to
thermal energy. The obtained dependence of the force on the channel parameters
may be useful in the design of cone-shaped nano-channels. | cond-mat_soft |
Simultaneous concentration and velocity maps in particle suspensions
under shear from rheo-ultrasonic imaging: We extend a previously developed ultrafast ultrasonic technique [Gallot et
al., Rev. Sci. Instrum. 84, 045107 (2013)] to concentration field measurements
in non-Brownian particle suspensions under shear. The technique provides access
to time-resolved concentration maps within the gap of a Taylor-Couette cell
simultaneously to local velocity measurements and standard rheological
characterization. Benchmark experiments in homogeneous particle suspensions are
used to calibrate the system. We then image heterogeneous concentration fields
that result from centrifugation effects, from the classical Taylor-Couette
instability and from sedimentation or shear-induced resuspension. | cond-mat_soft |
Polymer translocation out of confined environments: We consider the dynamics of polymer translocation out of confined
environments. Analytic scaling arguments lead to the prediction that the
translocation time scales like $\tau\sim
N^{\beta+\nu_{2D}}R^{1+(1-\nu_{2D})/\nu}$ for translocation out of a planar
confinement between two walls with separation $R$ into a 3D environment, and
$\tau \sim N^{\beta+1}R$ for translocation out of two strips with separation
$R$ into a 2D environment. Here, $N$ is the chain length, $\nu$ and $\nu_{2D}$
are the Flory exponents in 3D and 2D, and $\beta$ is the scaling exponent of
translocation velocity with $N$, whose value for the present choice of
parameters is $\beta \approx 0.8$ based on Langevin dynamics simulations. These
scaling exponents improve on earlier predictions. | cond-mat_soft |
Landau theory of bending-to-stretching transition: Transition from bending-dominated to stretching-dominated elastic response in
semi-flexible fibrous networks plays an important role in the mechanical
behavior of cells and tissues. It is induced by changes in network connectivity
and relies on construction of new cross-links. We propose a simple continuum
model of this transition with macroscopic strain playing the role of order
parameter. An unusual feature of this Landau-type theory is that it is based on
a single-well potential. We predict that bending-to-stretching transition
proceeds through propagation of the localized fronts separating domains with
affine and non-affine elastic response. | cond-mat_soft |
The vortex-driven dynamics of droplets within droplets: Understanding the fluid-structure interaction is crucial for an optimal
design and manufacturing of soft mesoscale materials. Multi-core emulsions are
a class of soft fluids assembled from cluster configurations of deformable
oil-water double droplets (cores), often employed as building-blocks for the
realisation of devices of interest in bio-technology, such as drug-delivery,
tissue engineering and regenerative medicine. Here, we study the physics of
multi-core emulsions flowing in microfluidic channels and report numerical
evidence of a surprisingly rich variety of driven non-equilibrium states (NES),
whose formation is caused by a dipolar fluid vortex triggered by the sheared
structure of the flow carrier within the microchannel. The observed dynamic
regimes range from long-lived NES at low core-area fraction, characterised by a
planetary-like motion of the internal drops, to short-lived ones at high
core-area fraction, in which a pre-chaotic motion results from multi-body
collisions of inner drops, as combined with self-consistent hydrodynamic
interactions. The onset of pre-chaotic behavior is marked by transitions of the
cores from one vortex to another, a process that we interpret as manifestations
of the system to maximize its entropy by filling voids, as they arise
dynamically within the capsule. | cond-mat_soft |
Absence of Dipole Glass Transition for Randomly Dilute Classical Ising
Dipoles: Dilute dipolar systems in three dimensions are expected to undergo a spin
glass transition as the temperature decreases. Contrary to this, we find from
Wang-Landau Monte Carlo simulations that at low concentrations $x$, dipoles
randomly placed on a cubic lattice with dipolar interactions do not undergo a
phase transition. We find that in the thermodynamic limit the ``glass''
transition temperature $T_g$ goes to zero as $1/\sqrt{N}$ where $N$ is the
number of dipoles. The entropy per particle at low temperatures is larger for
lower concentrations ($x=4.5%$) than for higher concentrations ($x=20%$). | cond-mat_soft |
Self-assembled multi-layer simple cubic photonic crystals of oppositely
charged colloids in confinement: Designing and fabricating self-assembled open colloidal crystals have become
one major direction in soft matter community because of many promising
applications associated with open colloidal crystals. However, most of the
self-assembled crystals found in experiments are not open but close-packed.
Here by using computer simulation, we systematically investigate the
self-assembly of oppositely charged colloidal hard spheres confined between two
parallel hard walls, and we find that the confinement can stabilize multi-layer
NaCl-like (simple cubic) open crystals. The maximal layers of stable NaCl-like
crystal increases with decreasing the inverse screening length. More
interestingly, at finite low temperature, the large vibrational entropy can
stabilize some multi-layer NaCl-like crystals against the most energetically
favoured close-packed crystals. In the parameter range studied, we find upto
4-layer NaCl-like crystal to be stable in confinement. Our photonic calculation
shows that the inverse 4-layer NaCl-like crystal can already reproduce the
large photonic band gaps of the bulk simple cubic crystal, which open at low
frequency range with low dielectric contrast. This suggests new possibilities
of using confined colloidal systems to fabricate open crystalline materials
with novel photonic properties. | cond-mat_soft |
Visualizing the strain evolution during the indentation of colloidal
glasses: We use an analogue of nanoindentation on a colloidal glass to elucidate the
incipient plastic deformation of glasses. By tracking the motion of the
individual particles in three dimensions, we visualize the strain field and
glass structure during the emerging deformation. At the onset of flow, we
observe a power-law distribution of strain indicating strongly correlated
deformation, and reflecting a critical state of the glass. At later stages, the
strain acquires a Gaussian distribution, indicating that plastic events become
uncorrelated. Investigation of the glass structure using both static and
dynamic measures shows a weak correlation between the structure and the
emerging strain distribution. These results indicate that the onset of
plasticity is governed by strong power-law correlations of strain, weakly
biased by the heterogeneous glass structure. | cond-mat_soft |
Drag Law of Two Dimensional Granular Fluids: The drag force law acting on a moving circular disk in a two-dimensional
granular medium is analyzed based on the discrete element method (DEM). It is
remarkable that the drag force on the moving disk in moderate dense and pure
two-dimensional granular medium can be well reproduced by a perfect fluid with
separation from the surface of the tracer. A yield force, being independent of
the moving speed of the disk, appears if a dry friction between the granular
disks and the bottom plate exists. The perfect fluidity is violated in this
case. The yield force and the drag force diverge at the jamming point. | cond-mat_soft |
Positronium-positronium interaction: Resonance, scattering length, and
Bose-Einstein condensation: The low-energy scattering of ortho positronium (Ps) by ortho Ps has been
studied in a full quantum mechanical coupled-channel approach. In the singlet
channel (total spin $s_T=0$) we find S- and P-wave resonances at 3.35 eV (width
0.02 eV) and 5.05 eV (width 0.04 eV), respectively and a binding of 0.44 eV of
Ps$_2$. The scattering length for $s_T=0$ is 3.95 \AA and for $s_T=2$ is 0.83
\AA . The small $s_T=2$ scattering length makes the spin-polarized ortho Ps
atoms an almost noninteracting ideal gas which may undergo Bose-Einstein
condensation. | cond-mat_soft |
Layering and position-dependent diffusive dynamics of confined fluids: We study the diffusive dynamics of a hard-sphere fluid confined between
parallel smooth hard walls. The position-dependent diffusion coefficient normal
to the walls is larger in regions of high local packing density. High density
regions also have the largest available volume, consistent with the fast local
diffusivity. Indeed, local and global diffusivities as a function of the Widom
insertion probability approximately collapse onto a master curve. Parallel and
average normal diffusivities are strongly coupled at high densities and deviate
from bulk fluid behavior. | cond-mat_soft |
Josephson Current Flowing in Cyclically Coupled Bose-Einstein
Condensates: The Josephson effect in cyclically coupled Bose-Einstein condensates is
studied theoretically. We analyze the simultaneous Gross-Pitaevskii equations
with coupling terms between adjacent condensates. Depending on the initial
relative phases between condensates, Josephson current flows cyclically to make
a quantized vortex. Reducing the coupling between condensates changes the
motion from periodic to chaotic, thus suppressing the cyclic current. The
relation to the Kibble-Zurek mechanism is discussed. | cond-mat_soft |
Comment on "Minimal size of a barchan dune": It is now an accepted fact that the size at which dunes form from a flat sand
bed as well as their `minimal size' scales on the flux saturation length. This
length is by definition the relaxation length of the slowest mode toward
equilibrium transport. The model presented by Parteli, Duran and Herrmann
[Phys. Rev. E 75, 011301 (2007)] predicts that the saturation length decreases
to zero as the inverse of the wind shear stress far from the threshold. We
first show that their model is not self-consistent: even under large wind, the
relaxation rate is limited by grain inertia and thus can not decrease to zero.
A key argument presented by these authors comes from the discussion of the
typical dune wavelength on Mars (650 m) on the basis of which they refute the
scaling of the dune size with the drag length evidenced by Claudin and
Andreotti [Earth Pla. Sci. Lett. 252, 30 (2006)]. They instead propose that
Martian dunes, composed of large grains (500 micrometers), were formed in the
past under very strong winds. We show that this saltating grain size, estimated
from thermal diffusion measurements, is not reliable. Moreover, the microscopic
photographs taken by the rovers on Martian aeolian bedforms show a grain size
of 87 plus or minus 25 micrometers together with hematite spherules at
millimetre scale. As those so-called ``blueberries'' can not be entrained by
reasonable winds, we conclude that the saltating grains on Mars are the small
ones, which gives a second strong argument against the model of Parteli et al. | cond-mat_soft |
Simulating structured fluids with tensorial viscoelasticity: We consider an immersed elastic body that is actively driven through a
structured fluid by a motor or an external force. The behavior of such a system
generally cannot be solved analytically, necessitating the use of numerical
methods. However, current numerical methods omit important details of the
microscopic structure and dynamics of the fluid, which can modulate the
magnitudes and directions of viscoelastic restoring forces. To address this
issue, we develop a simulation platform for modeling viscoelastic media with
tensorial elasticity. We build on the lattice Boltzmann algorithm and
incorporate viscoelastic forces, elastic immersed objects, a microscopic
orientation field, and coupling between viscoelasticity and the orientation
field. We demonstrate our method by characterizing how the viscoelastic
restoring force on a driven immersed object depends on various key parameters
as well as the tensorial character of the elastic response. We find that the
restoring force depends non-monotonically on the rate of diffusion of the
stress and the size of the object. We further show how the restoring force
depends on the relative orientation of the microscopic structure and the
pulling direction. These results imply that accounting for previously neglected
physical features, such as stress diffusion and the microscopic orientation
field, can improve the realism of viscoelastic simulations. We discuss possible
applications and extensions to the method. | cond-mat_soft |
Scaling in rupture of polymer chains: We consider the rupture dynamics of a homopolymer chain pulled at one end at
a constant loading rate r. Compared to single bond breaking, the existence of
the chain introduces two new aspects into rupture dynamics: the non-Markovian
aspect in the barrier crossing and the slow-down of the force propagation to
the breakable bond. The relative impact of both these processes is
investigated, and the second one was found to be the most important at moderate
loading rates. The most probable rupture force is found to decrease with the
number of bonds as f_{max} const-(ln(N/r))^(2/3) and finally to approach a
saturation value independent on N. All our analytical findings are confirmed by
extensive numerical simulations. | cond-mat_soft |
Photocontrol of Protein Conformation in a Langmuir Monolayer: We report a method to control the conformation of a weak polyampholyte (the
protein $\beta$-casein) in Langmuir monolayers by light, even though the
protein is not photosensitive. Our approach is to couple the monolayer state to
a photochemical reaction excited in the liquid subphase. The conformational
transition of the protein molecule is triggered through its sensitivity to a
subphase bulk field (pH in this study), changing in the course of the
photochemical process. Thus, reaction of photoaquation of the ferrocyanide ion,
which increases the subphase pH from 7.0 to about 8.3, produces a change in the
surface monolayer pressure, $\Delta\Pi$, between -0.5 and +1.5 ${\rm mN/m}$
(depending on the surface concentration), signalling a conformational switch.
The approach proposed here can be used to selectively target and influence
different interfacial properties by light, without embedding photosensitisers
in the matrix. | cond-mat_soft |
Effect of shape anisotropy on the phase diagram of the Gay-Berne fluid: We have used the density functional theory to study the effect of molecular
elongation on the isotropic-nematic, isotropic-smectic A and nematic-smectic A
phase transitions of a fluid of molecules interacting via the Gay-Berne
intermolecular potential. We have considered a range of length-to-width
parameter $3.0\leq x_0\leq 4.0$ in steps of 0.2 at different densities and
temperatures. Pair correlation functions needed as input information in density
functional theory are calculated using the Percus-Yevick integral equation
theory. Within the small range of elongation, the phase diagram shows
significant changes. The fluid at low temperature is found to freeze directly
from isotropic to smectic A phase for all the values of $x_0$ considered by us
on increasing the density while nematic phase stabilizes in between isotropic
and smectic A phases only at high temperatures and densities. Both
isotropic-nematic and nematic-smectic A transition density and pressure are
found to decrease as we increase $x_0$. The phase diagram obtained is compared
with computer simulation result of the same model potential and is found to be
in good qualitative agreement. | cond-mat_soft |
Nonlinear modes disentangle glassy and Goldstone modes in structural
glasses: One outstanding problem in the physics of glassy solids is understanding the
statistics and properties of the low-energy excitations that stem from the
disorder that characterizes these systems' microstructure. In this work we
introduce a family of algebraic equations whose solutions represent collective
displacement directions (modes) in the multi-dimensional configuration space of
a structural glass. We explain why solutions of the algebraic equations, coined
nonlinear glassy modes, are quasi-localized low-energy excitations. We present
an iterative method to solve the algebraic equations, and use it to study the
energetic and structural properties of a selected subset of their solutions
constructed by starting from a normal mode analysis of the potential energy of
a model glass. Our key result is that the structure and energies associated
with harmonic glassy vibrational modes and their nonlinear counterparts
converge in the limit of very low frequencies. As nonlinear modes never suffer
hybridizations, our result implies that the presented theoretical framework
constitutes a robust alternative definition of `soft glassy modes' in the
thermodynamic limit, in which Goldstone modes overwhelm and destroy the
identity of low-frequency harmonic glassy modes. | cond-mat_soft |
Bond formation and slow heterogeneous dynamics in adhesive spheres with
long--ranged repulsion: Quantitative test of Mode Coupling Theory: A colloidal system of spheres interacting with both a deep and narrow
attractive potential and a shallow long-ranged barrier exhibits a prepeak in
the static structure factor. This peak can be related to an additional
mesoscopic length scale of clusters and/or voids in the system. Simulation
studies of this system have revealed that it vitrifies upon increasing the
attraction into a gel-like solid at intermediate densities. The dynamics at the
mesoscopic length scale corresponding to the prepeak represents the slowest
mode in the system. Using mode coupling theory with all input directly taken
from simulations, we reveal the mechanism for glassy arrest in the system at
40% packing fraction. The effects of the low-q peak and of polydispersity are
considered in detail. We demonstrate that the local formation of physical bonds
is the process whose slowing down causes arrest.
It remains largely unaffected by the large-scale heterogeneities, and sets
the clock for the slow cluster mode. Results from mode-coupling theory without
adjustable parameters agree semi-quantitatively with the local density
correlators but overestimate the lifetime of the mesoscopic structure (voids). | cond-mat_soft |
Transformation between elastic dipoles, quadrupoles, octupoles and
hexadecapoles driven by surfactant self-assembly in nematic emulsion: Emulsions comprising isotropic fluid drops within a nematic host are of
interest for applications ranging from biodetection to smart windows, which
rely on changes of molecular alignment structures around the drops in response
to chemical, thermal, electric and other stimuli. We show that absorption or
desorption of trace amounts of common surfactants can drive continuous
transformations of elastic multipoles induced by the droplets within the
uniformly aligned nematic host. Out-of-equilibrium dynamics of director
structures emerge from a controlled self-assembly or desorption of different
surfactants at the drop-nematic interfaces, with ensuing forward and reverse
transformations between elastic dipoles, quadrupoles, octupoles and
hexadecapoles. We characterize inter-transformations of droplet-induced surface
and bulk defects, probe elastic pair interactions and discuss emergent
prospects for fundamental science and applications of the reconfigurable
nematic emulsions. | cond-mat_soft |
Equilibrating temperature-like variables in jammed granular subsystems: Although jammed granular systems are athermal, several thermodynamic-like
descriptions have been proposed which make quantitative predictions about the
distribution of volume and stress within a system and provide a corresponding
temperature-like variable. We perform experiments with an apparatus designed to
generate a large number of independent, jammed, two-dimensional configurations.
Each configuration consists of a single layer of photoelastic disks supported
by a gentle layer of air. New configurations are generated by alternately
dilating and re-compacting the system through a series of boundary
displacements. Within each configuration, a bath of particles surrounds a
smaller subsystem of particles with a different inter-particle friction
coefficient than the bath. The use of photoelastic particles permits us to find
all particle positions as well as the vector forces at each inter-particle
contact. By comparing the temperature-like quantities in both systems, we find
compactivity (conjugate to the volume) does not equilibrate between the
systems, while the angoricity (conjugate to the stress) does. Both independent
components of the angoricity are linearly dependent on the hydrostatic
pressure, in agreement with predictions of the stress ensemble. | cond-mat_soft |
Magnetization of polydisperse colloidal ferrofluids: Effect of
magnetostriction: We exploit magnetostriction in polydisperse ferrofluids in order to generate
nonlinear responses, and apply a thermodynamical method to derive the desired
nonlinear magnetic susceptibility. For an ideal gas, this method has been
demonstrated to be in excellent agreement with a statistical method. In the
presence of a sinusoidal ac magnetic field, the magnetization of the
polydisperse ferrofluid contains higher-order harmonics, which can be extracted
analytically by using a perturbation approach. We find that the harmonics are
sensitive to the particle distribution and the degree of field-induced
anisotropy of the system. In addition, we find that the magnetization is higher
in the polydisperse system than in the monodisperse one, as also found by a
recent Monte Carlo simulation. Thus, it seems possible to detect the size
distribution in a polydisperse ferrofluid by measuring the harmonics of the
magnetization under the influence of magnetostriction. | cond-mat_soft |
Conformation of a Polyelectrolyte Complexed to a Like-Charged Colloid: We report results from a molecular dynamics (MD) simulation on the
conformations of a long flexible polyelectrolyte complexed to a charged sphere,
\textit{both negatively charged}, in the presence of neutralizing counterions
in the strong Coulomb coupling regime. The structure of this complex is very
sensitive to the charge density of the polyelectrolyte. For a fully charged
polyelectrolyte the polymer forms a dense two-dimensional "disk", whereas for a
partially charged polyelectrolyte the monomers are spread over the colloidal
surface. A mechanism involving the \textit{overcharging} of the polyelectrolyte
by counterions is proposed to explain the observed conformations. | cond-mat_soft |
Reaction Kinetics in Polymer Melts: We study the reaction kinetics of end-functionalized polymer chains dispersed
in an unreactive polymer melt. Starting from an infinite hierarchy of coupled
equations for many-chain correlation functions, a closed equation is derived
for the 2nd order rate constant $k$ after postulating simple physical bounds.
Our results generalize previous 2-chain treatments (valid in dilute reactants
limit) by Doi, de Gennes, and Friedman and O'Shaughnessy, to arbitrary initial
reactive group density $n_0$ and local chemical reactivity $Q$. Simple mean
field (MF) kinetics apply at short times, $k \sim Q$. For high $Q$, a
transition occurs to diffusion-controlled (DC) kinetics with $k \approx
x_t^3/t$ (where $x_t$ is rms monomer displacement in time $t$) leading to a
density decay $n_t \approx n_0 - n_0^2 x_t^3$. If $n_0$ exceeds the chain
overlap threshold, this behavior is followed by a regime where $n_t \approx
1/x_t^3$ during which $k$ has the same power law dependence in time, $k \approx
x_t^3/t$, but possibly different numerical coefficient. For unentangled melts
this gives $n_t \sim t^{-3/4}$ while for entangled cases one or more of the
successive regimes $n_t \sim t^{-3/4}$, $t^{-3/8}$ and $t^{-3/4}$ may be
realized depending on the magnitudes of $Q$ and $n_0$. Kinetics at times longer
than the longest polymer relaxation time $\tau$ are always MF. If a DC regime
has developed before $\tau$ then the long time rate constant is $k \approx
R^3/\tau$ where $R$ is the coil radius. We propose measuring the above kinetics
in a model experiment where radical end groups are generated by photolysis. | cond-mat_soft |
Optimized Large Hyperuniform Binary Colloidal Suspensions in Two
Dimensions: The creation of disordered hyperuniform materials with potentially
extraordinary optical properties requires a capacity to synthesize large
samples that are effectively hyperuniform down to the nanoscale. Motivated by
this challenge, we propose a fabrication protocol using binary
superparamagnetic colloidal particles confined in a 2D plane. The strong and
long-ranged dipolar interaction induced by a tunable magnetic field is free
from screening effects that attenuates long-ranged electrostatic interactions
in charged colloidal systems. Specifically, we find a family of optimal size
ratios that makes the two-phase system effectively hyperuniform. We show that
hyperuniformity is a general consequence of low isothermal compressibilities,
which makes our protocol suitable to systems with other long-ranged soft
interactions, dimensionalities and/or polydispersity. Our methodology paves the
way to synthesize large photonic hyperuniform materials that function in the
visible to infrared range and hence may accelerate the discovery of novel
photonic materials. | cond-mat_soft |
Role of reversibility in viral capsid growth: A paradigm for
self-assembly: Self-assembly at submicroscopic scales is an important but little understood
phenomenon. A prominent example is virus capsid growth, whose underlying
behavior can be modeled using simple particles that assemble into polyhedral
shells. Molecular dynamics simulation of shell formation in the presence of an
atomistic solvent provides new insight into the self-assembly mechanism,
notably that growth proceeds via a cascade of strongly reversible steps and,
despite the large variety of possible intermediates, only a small fraction of
highly bonded forms appear on the pathway. | cond-mat_soft |
Martini coarse-grained model for clay-polymer nanocomposites: We have developed a coarse-grained (CG) model of a polymer-clay system
consisting of organically modified montmorillonite nanoclay as the nanoparticle
in accordance with the MARTINI forcefield. We have used mechanical properties
and cleavage free energy of clay particle to respectively parameterize bonded
and nonbonded interaction parameters for an organically modified
montmorillonite (oMMT) clay particle where intergallery Na+ ions are replaced
by tetramethylammonium (TMA) ions. The mechanical properties were determined
from the slope of stress-strain curve and cleavage free energy was determined
by allowing for full surface reconstruction corresponding to a slow equilibrium
cleavage process. Individual dispersive and polar contributions to oMMT
cleavage energy were used for determination of appropriate MARTINI bead types
for CG oMMT sheet. The self-consistency of developed MARTINIFF parameters for
TMA-MMT-polymer system was verified by comparing estimates for select
structural, thermodynamic, and dynamic properties obtained in all-atomistic
simulations with that obtained in coarse-grained simulations. We have
determined the influence of clay particle on properties of three polymer melts
(polyethylene, polypropylene, and polystyrene) at two temperatures to establish
transferability of the developed parameters. We have also shown that the effect
of clay-polymer interactions on structure-property relationships in this
nanocomposite system is well captured by Rosenfeld's excess entropy scaling. | cond-mat_soft |
Thermodynamic consistency between the energy and virial routes in the
mean spherical approximation for soft potentials: It is proven that, for any soft potential characterized by a finite Fourier
transform $\widetilde{\phi}(k)$, the virial and energy thermodynamic routes are
equivalent for approximations such that the Fourier transform of the total
correlation function divided by the density $\rho$ is an arbitrary function of
$\rho\beta\widetilde{\phi}(k)$, where $\beta$ is the inverse temperature. This
class includes the mean spherical approximation as a particular case. | cond-mat_soft |
Wetting, Spreading, and Adsorption on Randomly Rough Surfaces: The wetting properties of solid substrates with customary (i.e., macroscopic)
random roughness are considered as a function of the microscopic contact angle
of the wetting liquid and its partial pressure in the surrounding gas phase.
Analytic expressions are derived which allow for any given lateral correlation
function and height distribution of the roughness to calculate the wetting
phase diagram, the adsorption isotherms, and to locate the percolation
transition in the adsorbed liquid film. Most features turn out to depend only
on a few key parameters of the roughness, which can be clearly identified. It
is shown that a first order transition in the adsorbed film thickness, which we
term 'Wenzel prewetting', occurs generically on typical roughness topographies,
but is absent on purely Gaussian roughness. It is thereby shown that even
subtle deviations from Gaussian roughness characteristics may be essential for
correctly predicting even qualitative aspects of wetting. | cond-mat_soft |
Transitional cylindrical swirling flow in presence of a flat free
surface: This article is devoted to the study of an incompressible viscous flow of a
fluid partly enclosed in a cylindrical container with an open top surface and
driven by the constant rotation of the bottom wall. Such type of flows belongs
to a group of recirculating lid-driven cavity flows with geometrical
axisymmetry and of the prescribed boundary conditions of Dirichlet type --
no-slip on the cavity walls. The top surface of the cylindrical cavity is left
open with an imposed stress-free boundary condition, while a no-slip condition
with a prescribed rotational velocity is imposed on the bottom wall. The
Reynolds regime corresponds to transitional flows with some incursions in the
fully laminar regime. The approach taken here revealed new flow states that
were investigated based on a fully three-dimensional solution of the
Navier--Stokes equations for the free-surface cylindrical swirling flow,
without resorting to any symmetry property unlike all other results available
in the literature. Theses solutions are obtained through direct numerical
simulations based on a Legendre spectral element method. | cond-mat_soft |
Guiding microscale swimmers using teardrop-shaped posts: The swimming direction of biological or artificial microscale swimmers tends
to be randomised over long time-scales by thermal fluctuations. Bacteria use
various strategies to bias swimming behaviour and achieve directed motion
against a flow, maintain alignment with gravity or travel up a chemical
gradient. Herein, we explore a purely geometric means of biasing the motion of
artificial nanorod swimmers. These artificial swimmers are bimetallic rods,
powered by a chemical fuel, which swim on a substrate printed with
teardrop-shaped posts. The artificial swimmers are hydrodynamically attracted
to the posts, swimming alongside the post perimeter for long times before
leaving. The rods experience a higher rate of departure from the higher
curvature end of the teardrop shape, thereby introducing a bias into their
motion. This bias increases with swimming speed and can be translated into a
macroscopic directional motion over long times by using arrays of
teardrop-shaped posts aligned along a single direction. This method provides a
protocol for concentrating swimmers, sorting swimmers according to different
speeds, and could enable artificial swimmers to transport cargo to desired
locations. | cond-mat_soft |
Nucleation of liquid droplets in a fluid with competing interactions: Using a simple density functional theory (DFT) we determine the height of the
free energy barrier for forming a droplet of the liquid phase from the
metastable gas phase for a model colloidal fluid exhibiting competing
interactions. The pair potential has a hard core of diameter {\sigma}, is
attractive Yukawa at intermediate separations, and is repulsive Yukawa at large
separations. We find that even a very weak long-range repulsive tail in the
pair potential has a profound effect on nucleation: increasing the amplitude of
the repulsive Yukawa tail reduces significantly the free energy barrier height
and therefore increases the liquid droplet nucleation rate. The method we
introduce for calculating the droplet density profile and free energy employs a
fictitious external potential to stabilize a liquid droplet of the desired
size, i.e. with a given excess number of particles. For the critical droplet,
corresponding to an extremum of the grand potential, this fictitious potential
is everywhere zero. We examine the decay of the droplet density profiles into
the bulk gas. For a range of nucleation state points the DFT predicts
exponentially damped, long wavelength oscillatory decay for systems exhibiting
long-range repulsion, contrasting sharply with the monotonic decay found when
the pair potential has only an attractive Yukawa piece. The changes in
nucleation properties that we find for small amplitudes of the repulsive Yukawa
tail reflect the propensity of the fluid to form modulated structures such as
clusters or stripes. | cond-mat_soft |
Heterogeneities and Topological Defects in Two-Dimensional Pinned
Liquids: We simulate a model of repulsively interacting colloids on a commensurate
two-dimensional triangular pinning substrate where the amount of heterogeneous
motion that appears at melting can be controlled systematically by turning off
a fraction of the pinning sites. We correlate the amount of heterogeneous
motion with the average topological defect number, time dependent defect
fluctuations, colloid diffusion, and the form of the van Hove correlation
function. When the pinning sites are all off or all on, the melting occurs in a
single step. When a fraction of the sites are turned off, the melting becomes
considerably broadened and signatures of a two-step melting process appear. The
noise power associated with fluctuations in the number of topological defects
reaches a maximum when half of the pinning sites are removed, and the noise
spectrum has a pronounced 1/f^\alpha structure in the heterogeneous regime. We
find that regions of high mobility are associated with regions of high
dislocation densities. | cond-mat_soft |
Interference of a thermal Tonks gas on a ring: A nonzero temperature generalization of the Fermi-Bose mapping theorem is
used to study the exact quantum statistical dynamics of a one-dimensional gas
of impenetrable bosons on a ring. We investigate the interference produced when
an initially trapped gas localized on one side of the ring is released, split
via an optical-dipole grating, and recombined on the other side of the ring.
Nonzero temperature is shown not to be a limitation to obtaining high
visibility fringes. | cond-mat_soft |
Granular Dynamics during Impact: We study the impact of a projectile onto a bed of 3 mm grains immersed in an
index-matched fluid. Specifically, we vary the amount of prestrain on the
sample, strengthening the force chains within the system. We find this affects
only the prefactor of linear depth-dependent term in the stopping force. We
therefore attribute this term to pressure within the material, and not the
grain-intruder friction as is sometimes suggested. Using a laser sheet scanning
technique to visualize internal grain motion, a high-speed camera, and particle
tracking, we can measure the trajectory of each grain throughout an impact
event. Microscopically, our results indicate that weaker initial force chains
result in more irreversible, plastic rearrangements during impact, suggesting
static friction between grains does play a substantial role in the energy
dissipation within the granular material. | cond-mat_soft |
The Boson Peak and Disorder in Hard Sphere Colloidal Systems: The Boson peak is believed to be the key to the fundamental understanding of
the anomalous thermodynamic properties of glasses, notably the anomalous peak
in the heat capacity at low temperatures; it is believed to be due to an excess
of low frequency vibrational modes and a manifestation of the structural
disorder in these systems. We study the thermodynamics and vibrational dynamics
of colloidal glasses and (defected) crystals. The experimental determination of
the vibrational density of states allows us to directly observe the Boson peak
as a strong enhancement of low frequency modes. Using a novel method [Zargar et
al., Phys. Rev. Lett. 110, 258301 (2013)] to determine the free energy, we also
determine the entropy and the specific heat experimentally. It follows that the
emergence of the Boson peak and high values of the specific heat are directly
related and are specific to the glass: for a very defected crystal with a
disorder that is only slightly smaller than for the glass, both the
low-frequency density of states and the specific heat are significantly smaller
than in the glass. | cond-mat_soft |
Vector Formalism for Active Nematics in Two Dimensions: Specific features of two-dimensional nematodynamics give rise to shortfalls
of the tensor representation of the nematic order parameter commonly used in
computations, especially in theory of active matter. The alternative
representation in terms of the vector order parameter follows with small
adjustments the classical director-based theory, but is applicable to 2D
problems where both nematic alignment and deviation from the isotropic state
are variable. Stability analysis of nematic alignment and flow is used as a
testing ground. A director-based analysis demonstrates a shortfall of the
standard theory, which does not ensure relaxation to equilibrium in a passive
system. It also demonstrates the inadequacy of the director-based description,
which misses a stabilizing effect of perturbations of the modulus ensuring
stability of a passive system on scales far exceeding the healing length. | cond-mat_soft |
Universality in Driven and Equilibrium Hard Sphere Liquid Dynamics: We demonstrate that the time evolution of the van Hove dynamical pair
correlation function is governed by adiabatic forces that arise from the free
energy and by superadiabatic forces that are induced by the flow of the van
Hove function. The superadiabatic forces consist of drag, viscous, and
structural contributions, as occur in active Brownian particles, in liquids
under shear and in lane forming mixtures. For hard sphere liquids we present a
power functional theory that predicts these universal force fields in
quantitative agreement with our Brownian dynamics simulation results. | cond-mat_soft |
Kinetics of isotropic to string-like phase switching in
electrorheological fluids of nanocubes: Applying an electric field to polarisable colloidal particles, whose
permittivity differs from that of the dispersing medium, generates induced
dipoles that promote the formation of string-like clusters and ultimately alter
the fluid mechanical and rheological properties. Complex systems of this kind,
whose electric-field-induced rheology can be manipulated between that of
viscous and elastic materials, are referred to as electrorheological fluids. By
dynamic Monte Carlo simulations, we investigate the dynamics of self-assembly
of dielectric nanocubes upon application of an electric field. Switching the
field on induces in-particle dipoles and, at sufficiently large field
intensity, leads to stringlike clusters of variable length across a spectrum of
volume fractions. The kinetics of switching from the isotropic to the
string-like state suggests the existence of two mechanisms, the first related
to the nucleation of chains and the second to the competition between further
merging and separation. We characterise the transient unsteady state by
following the chain length distribution and analysing the probability of
transition of nanocubes from one chain to another over time. Additionally, we
employ passive microrheology to gain an insight into the effect of the electric
field on the viscoelastic response of our model fluid. Not only do we observe
that it becomes more viscoelastic in the presence of the field, but also that
its viscoelasticity assumes an anisotropic signature, with both viscous and
elastic moduli in planes perpendicular to the external field being larger than
those along it. | cond-mat_soft |
Surface phase transitions in foams and emulsions: Surface phase transitions in surfactant adsorption layers are known to affect
the dynamic properties of foams and to induce surface nucleation in freezing
emulsion drops. Recently, these transitions were found to play a role in
several other phenomena, opening new opportunities for controlling foam and
emulsion properties. This review presents a brief outlook of the emerging
opportunities in this area. Three topics are emphasized: (1) The use of
surfactant mixtures for inducing phase transitions on bubble surfaces in foams;
(2) The peculiar properties of natural surfactants saponins which form
extremely viscoelastic surface layers; and (3) The main phenomena in emulsions,
for which the surface phase transitions are important. The overall conclusion
from the reviewed literature is that surface phase transitions could be used as
a powerful tool to control many foam and emulsion properties, but we need
deeper understanding of the underlying phenomena to explore fully these
opportunities. | cond-mat_soft |
Hydrodynamic response of a surfactant-laden interface to a radial flow: We study the features of a radial Stokes flow due to a submerged jet directed
toward a liquid-air interface. The presence of surface-active impurities
confers to the interface an in-plane elasticity that resists the incident flow.
Both analytical and numerical calculations show that a minute amount of
surfactants is enough to profoundly alter the morphology of the flow. The
hydrodynamic response of the interface is affected as well, shifting from slip
to no-slip boundary condition as the surface compressibility decreases. We
argue that the competition between the divergent outward flow and the elastic
response of the interface may actually be used as a practical way to detect and
quantify a small amount of impurities. | cond-mat_soft |
Universality and stability phase-diagram of two-dimensional brittle
fracture: The two-dimensional oscillatory crack instability, experimentally observed in
a class of brittle materials under strongly dynamic conditions, has been
recently reproduced by a nonlinear phase-field fracture theory. Here we
highlight the universal character of this instability by showing that it is
present in materials exhibiting widely different near crack tip elastic
nonlinearity, and by demonstrating that the oscillations wavelength follows a
universal master curve in terms of dissipation-related and nonlinear elastic
intrinsic length scales. Moreover, we show that upon increasing the driving
force for fracture, a high-velocity tip-splitting instability emerges, as
experimentally demonstrated. The analysis culminates in a comprehensive
stability phase-diagram of two-dimensional brittle fracture, whose salient
properties and topology are independent of the form of near tip nonlinearity. | cond-mat_soft |
Ion clustering in aqueous salt solutions near the liquid/vapor interface: Molecular dynamics simulations of aqueous NaCl, KCl, NaI, and KI solutions
are used to study the effects of salts on the properties of the liquid/vapor
interface. The simulations use the models which include both charge transfer
and polarization effects. Pairing and the formation of larger ion clusters
occurs both in the bulk and surface region, with a decreased tendency to form
larger clusters near the interface. An analysis of the roughness of the surface
reveals that the chloride salts, which have less tendency to be near the
surface, have a roughness that is less than pure water, while the iodide salts,
which have a greater surface affinity, have a larger roughness. This suggests
that ions away from the surface and ions near the surface affect the interface
in opposite ways. | cond-mat_soft |
A comprehensive continuum theory of structured liquids: We develop a comprehensive continuum model capable of treating both
electrostatic and structural interactions in liquid dielectrics. Starting from
a two-order parameter description in terms of charge density and polarization,
we derive a field-theoretic model generalizing previous theories. Our theory
explicitly includes electrostatic and structural interactions in the bulk of
the liquid and allows for polarization charges within a Drude model. In
particular, we develop a detailed description of the boundary conditions which
include the charge regulation mechanism and surface polarization. The general
features for solving the saddle-point equations of our model are elucidated and
future applications to predict and validate experimental results are outlined. | cond-mat_soft |
Phase diagram and melting scenarios of two-dimensional Hertzian spheres: We present computer simulations of a system of purely repulsive soft
colloidal particles interacting via the Hertz potential and constrained to a
two-dimensional plane. This potential describes the elastic interaction of
weakly deformable bodies and can be a reliable model for qualitative
description of behavior of soft macromolecules, like globular micelles and star
polymers. We find a large number of ordered phases, including the dodecagonal
quasicrystal, and analyze the melting scenarios of low density triangle and
square phases. It is interesting that depending on the position on the phase
diagram the system can melt both through the first order transition and in
accordance with the Berezinskii-Kosterlitz-Thouless-Halperin-Nelson-Young
(BKTHNY) scenario (two continuous transitions with the intermediate hexatic
phase) and also in accordance with recently proposed two-stage melting with the
first order hexatic-isotropic liquid transition and continuous solid-hexatic
transition. We also demonstrate the possibility of the tricritical point on the
melting line. | cond-mat_soft |
Hydrogel menisci: Shape, interaction, and instability: The interface of a soft hydrogel is easily deformed when it is in contact
with particles, droplets or cells. Here we compute the intricate shapes of
hydrogel menisci due to the indentation of point particles. The analysis is
based on a free energy formulation, by which we also assess the interaction
laws between neighbouring particles on hydrogel interfaces, similar to the
"Cheerios effect". It is shown how the meniscus formed around the particles
results from a competition between surface tension, elasticity and hydrostatic
pressure inside the gel. We provide a detailed overview of the various scaling
laws, which are governed by a characteristic shear modulus
$G^*=\sqrt{\gamma\rho g}$ that is based on surface tension $\gamma$ and gravity
$\rho g$. Stiffer materials exhibit a solid-like response while softer
materials are more liquid-like. The importance of $G^*$ is further illustrated
by examining the Rayleigh-Taylor instability of soft hydrogels. | cond-mat_soft |
Hourglass of constant weight: In contrast to a still common belief, a steadily flowing hourglass changes
its weight in the course of time. We will show that, nevertheless, it is
possible to construct hourglasses that do not change their weight. | cond-mat_soft |
Mpemba-like effect protocol for granular gases of inelastic and rough
hard disks: We study the conditions under which a Mpemba-like effect emerges in granular
gases of inelastic and rough hard disks driven by a class of thermostats
characterized by the splitting of the noise intensity into translational and
rotational counterparts. Thus, granular particles are affected by a stochastic
force and a stochastic torque, which inject translational and rotational
energy, respectively. We realize that a certain choice of a thermostat of this
class can be characterized just by the total intensity and the fraction of
noise transferred to the rotational degree of freedom with respect to the
translational ones. Firstly, Mpemba effect is characterized by the appearance
of a crossing between the temperature curves of the considered samples. Later,
an overshoot of the temperature evolution with respect to the steady-state
value is observed and the mechanism of Mpemba effect generation is changed. The
election of parameters allows to design plausible protocols based on these
thermostats for generating the initial states to observe the Mpemba-like effect
in experiments. In order to obtain explicit results, we use a well-founded
Maxwellian approximation for the evolution dynamics and the steady-state
quantities. Finally, theoretical results are compared with direct simulation
Monte Carlo and molecular dynamics results, and a very good agreement is found. | cond-mat_soft |
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