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Mixing-induced anisotropic correlations in molecular crystalline systems: We investigate the structure of mixed thin films composed of pentacene (PEN)
and diindenoperylene (DIP) using X-ray reflectivity and grazing incidence X-ray
diffraction. For equimolar mixtures we observe vanishing in-plane order
coexisting with an excellent out-of-plane order, a yet unreported disordering
behavior in binary mixtures of organic semiconductors, which are crystalline in
their pure form. One approach to rationalize our findings is to introduce an
anisotropic interaction parameter in the framework of a mean field model. By
comparing the structural properties with those of other mixed systems, we
discuss the effects of sterical compatibility and chemical composition on the
mixing behavior, which adds to the general understanding of interactions in
molecular mixtures. | cond-mat_soft |
Self-replicating segregation patterns in horizontally vibrated binary
mixture of granules: When granular mixtures of different sizes are fluidized, each species
spontaneously separates and condenses to form patterns. Although granular
segregation has been extensively studied, the inability to directly observe the
time evolution of the internal structure hinders the understanding of the
mechanism of segregation dynamics driven by surface flow. In this study, we
report rich band dynamics, including a self-replicating band, in a horizontally
shaken granular mixture in a quasi-two-dimensional container where the granules
formed steady surface waves. Direct observation of surface flow and segregated
internal structure revealed that coupling among segregation, surface flow, and
hysteresis in the fluidity of granules is key to understanding complex band
dynamics. | cond-mat_soft |
Quantum Field Theory of Treasury Bonds: The Heath-Jarrow-Morton (HJM) formulation of treasury bonds in terms of
forward rates is recast as a problem in path integration. The HJM-model is
generalized to the case where all the forward rates are allowed to fluctuate
independently. The resulting theory is shown to be a two-dimensional Gaussian
quantum field theory. The no arbitrage condition is obtained and a functional
integral derivation is given for the price of a futures and an options
contract. | cond-mat_soft |
Mechanical equilibrium of aggregates of dielectric spheres: Industrial as well as natural aggregation of fine particles is believed to be
associated with electrostatics. Yet like charges repel, so it is unclear how
similarly treated particles aggregate. To resolve this apparent contradiction,
we analyze conditions necessary to hold aggregates together with electrostatic
forces. We find that aggregates of particles charged with the same sign can be
held together due to dielectric polarization, we evaluate the effect of
aggregate size, and we briefly summarize consequences for practical
aggregation. | cond-mat_soft |
Towards the description of water adsorption in slit-like nanochannels
with grafted molecular brushes. Density functional theory: We have explored a model for adsorption of water into slit-like nanochannels
with two walls chemically modified by grafted polymer layers forming brushes. A
version of density functional method is used as theoretical tools. The
water-like fluid model adopted from the work of Clark et al. [Mol. Phys., 2006,
104, 3561] adequately reproduces the bulk vapour-liquid coexistence envelope.
The polymer layer consists of chain molecules in the framework of
pearl-necklace model. Each chain molecule is chemically bonded to the pore
walls by a single terminating segment. Our principal focus is in the study of
the dependence of polymer layer height on grafting density and in the
microscopic structure of the interface between adsorbed fluid and brushes.
Thermal response of these properties upon adsorption is investigated in detail.
The results are of importance to understand shrinking and swelling of the
molecular brushes in the nanochannels. | cond-mat_soft |
Analytical classical density functionals from an equation learning
network: We explore the feasibility of using machine learning methods to obtain an
analytic form of the classical free energy functional for two model fluids,
hard rods and Lennard--Jones, in one dimension . The Equation Learning Network
proposed in Ref. 1 is suitably modified to construct free energy densities
which are functions of a set of weighted densities and which are built from a
small number of basis functions with flexible combination rules. This setup
considerably enlarges the functional space used in the machine learning
optimization as compared to previous work 2 where the functional is limited to
a simple polynomial form. As a result, we find a good approximation for the
exact hard rod functional and its direct correlation function. For the
Lennard--Jones fluid, we let the network learn (i) the full excess free energy
functional and (ii) the excess free energy functional related to interparticle
attractions. Both functionals show a good agreement with simulated density
profiles for thermodynamic parameters inside and outside the training region. | cond-mat_soft |
Nematic-Isotropic Transition with Quenched Disorder: Nematic elastomers do not show the discontinuous, first-order, phase
transition that the Landau-De Gennes mean field theory predicts for a
quadrupolar ordering in 3D. We attribute this behavior to the presence of
network crosslinks, which act as sources of quenched orientational disorder. We
show that the addition of weak random anisotropy results in a singular
renormalization of the Landau-De Gennes expression, adding an energy term
proportional to the inverse quartic power of order parameter Q. This reduces
the first-order discontinuity in Q. For sufficiently high disorder strength the
jump disappears altogether and the phase transition becomes continuous, in some
ways resembling the supercritical transitions in external field. | cond-mat_soft |
Wetting, roughness and hydrodynamic slip: The hydrodynamic slippage at a solid-liquid interface is currently at the
center of our understanding of fluid mechanics. For hundreds of years this
science has relied upon no-slip boundary conditions at the solid-liquid
interface that has been applied successfully to model many macroscopic
experiments, and the state of this interface has played a minor role in
determining the flow. However, the problem is not that simple and has been
revisited recently. Due to the change in the properties of the interface, such
as wettability and roughness, this classical boundary condition could be
violated, leading to a hydrodynamic slip. In this chapter, we review recent
advances in the understanding and expectations for the hydrodynamic boundary
conditions in different situations, by focussing mostly on key papers from past
decade. We highlight mostly the impact of hydrophobicity, roughness, and
especially their combination on the flow properties. In particular, we show
that hydrophobic slippage can be dramatically affected by the presence of
roughness, by inducing novel hydrodynamic phenomena, such as giant interfacial
slip, superfluidity, mixing, and low hydrodynamic drag. Promising directions
for further research are also discussed. | 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 |
Oscillatory decay of a two-component Bose-Einstein condensate: We study the decay of a two-component Bose-Einstein condensate with negative
effective interaction energy. With a decreasing atom number due to losses, the
atom-atom interaction becomes less important and the system undergoes a
transition from a bistable Josephson regime to the monostable Rabi regime,
displaying oscillations in phase and number. We study the equations of motion
and derive an analytical expression for the oscillation amplitude. A quantum
trajectory simulation reveals that the classical description fails for low
emission rates, as expected from analytical considerations. Observation of the
proposed effect will provide evidence for negative effective interaction. | cond-mat_soft |
GCIceNet: A Graph Convolutional Network for Accurate Classification of
Water Phases: Understanding phases of water molecules based on local structure is essential
for understanding their anomalous properties. However, due to complicated
structural motifs formed via hydrogen bonds, conventional order parameters
represent the water molecules incompletely. In this paper, we develop a
GCIceNet, which automatically generates machine-based order parameters for
classifying the phases of the water molecules via supervised and unsupervised
learning. Multiple graph convolutional layers in the GCIceNet can learn
topological informations of the complex hydrogen bond networks. It shows a
substantial improvement of accuracy for predicting the phase of water molecules
in the bulk system and the ice/vapor interface system. A relative importance
analysis shows that the GCIceNet can capture the structural features of the
given system hidden in the input data. Augmented with the vast amount of data
provided by molecular dynamics simulations, the GCIceNet is expected to serve
as a powerful tool for the fields of glassy liquids and hydration layers around
biomolecules. | cond-mat_soft |
Sliding droplets of Xanthan solutions: a joint experimental and
numerical study: We have investigated the sliding of droplets made of solutions of Xanthan, a
stiff rodlike polysaccharide exhibiting a non-newtonian behavior, notably
characterized by a shear-rate dependence of the viscosity. These experimental
results are quantitatively compared with those of newtonian fluids (water). The
impact of the non-newtonian behavior on the sliding process was shown through
the relation between the average dimensionless velocity (i.e. the Capillary
number) and the dimensionless volume forces (i.e. the Bond number). To this
aim, it is needed to define operative strategies to compute the Capillary
number for the shear thinning fluids and compare with the corresponding
newtonian case. Results from experiments were complemented with lattice
Boltzmann numerical simulations of sliding droplets, aimed to disentangle the
influence that non-newtonian flow properties have on the sliding. | cond-mat_soft |
An improved integration scheme for Mode-coupling-theory equations: Within the mode-coupling theory (MCT) of the glass transition, we reconsider
the numerical schemes to evaluate the MCT functional. Here we propose
nonuniform discretizations of the wave number, in contrast to the standard
equidistant grid, in order to decrease the number of grid points without losing
accuracy. We discuss in detail how the integration scheme on the new grids has
to be modified from standard Riemann integration. We benchmark our approach by
solving the MCT equations numerically for mono-disperse hard disks and hard
spheres and by computing the critical packing fraction and the nonergodicity
parameters. Our results show that significant improvements in performance can
be obtained by employing a nonuniform grid. | cond-mat_soft |
On the relaxation dynamics of glass-forming systems: Insights from
computer simulations: We discuss the relaxation dynamics of a simple lattice gas model for
glass-forming systems and show that with increasing density of particles this
dynamics slows down very quickly. By monitoring the trajectory of tagged
particles we find that their motion is very heterogeneous in space and time,
leading to regions in space in which there is a fast dynamics and others in
which it is slow. We determine how the geometric properties of these quickly
relaxing regions depend on density and time. Motivated by this heterogeneous
hopping dynamics, we use a simple model, a variant of a continuous time random
walk, to characterize the relaxation dynamics. In particular we find from this
model that for large displacements the self part of the van Hove function shows
an exponential tail, in agreement with recent findings from experiments and
simulations of glass-forming systems. | cond-mat_soft |
Motion of nanodroplets near edges and wedges: Nanodroplets residing near wedges or edges of solid substrates exhibit a
disjoining pressure induced dynamics. Our nanoscale hydrodynamic calculations
reveal that non-volatile droplets are attracted or repelled from edges or
wedges depending on details of the corresponding laterally varying disjoining
pressure generated, e.g., by a possible surface coating. | cond-mat_soft |
Effective interaction between star polymers: The distance-resolved effective interaction between two star polymers in a
good solvent is calculated by Molecular Dynamics computer simulations. The
results are compared with a pair potential proposed recently by Likos et al.
[Phys. Rev. Lett. 1998, 80, 4450] which is exponentially decaying for large
distances and crosses over, at the corona diameter of the star, to an ultrasoft
logarithmic repulsion for small distances. Excellent agreement is found in a
broad range of star arm numbers. | cond-mat_soft |
The Föppl-von Kármán equations of elastic plates with initial
stress: Initially stressed plates are widely used in modern fabrication techniques,
such as additive manufacturing and UV lithography, for their tunable morphology
by application of external stimuli. In this work, we propose a formal
asymptotic derivation of the F\"{o}ppl-von K\'{a}rm\'{a}n equations for an
elastic plate with initial stresses, using the constitutive theory of nonlinear
elastic solids with initial stresses under the assumptions of incompressibility
and material isotropy. Compared to existing works, our approach allows to
determine the morphological transitions of the elastic plate without
prescribing the underlying target metric of the unstressed state of the elastic
body. We explicitly solve the derived FvK equations in some physical problems
of engineering interest, discussing how the initial stress distribution drives
the emergence of spontaneous curvatures within the deformed plate. The proposed
mathematical framework can be used to tailor shape on demand, with applications
in several engineering fields ranging from soft robotics to 4D printing. | cond-mat_soft |
Normal modes analysis of the microscopic dynamics in hard discs: We estimate numerically the normal modes of the free energy in a glass of
hard discs. We observe that, near the glass transition or after a rapid quench
deep in the glass phase, the density of states (i) is characteristic of a
marginally stable structure, in particular it di splays a frequency scale
$\omega^*\sim p^{1/2}$, where $p$ is the pressure and (ii) gives a faithful
representation of the short-time dyn amics. This brings further evidences that
the boson peak near the glass transition corresponds to the relaxation of
marginal modes of a we akly-coordinated structure, and implies that the mean
square displacement in the glass phase is anomalously large and goes as $<
\delta R^2 > \sim p^{-3/2}$, a prediction that we check numerically. | cond-mat_soft |
Local thermal energy as a structural indicator in glasses: Identifying heterogeneous structures in glasses --- such as localized soft
spots --- and understanding structure-dynamics relations in these systems
remain major scientific challenges. Here we derive an exact expression for the
local thermal energy of interacting particles (the mean local potential energy
change due to thermal fluctuations) in glassy systems by a systematic
low-temperature expansion. We show that the local thermal energy can attain
anomalously large values, inversely related to the degree of softness of
localized structures in a glass, determined by a coupling between internal
stresses --- an intrinsic signature of glassy frustration ---, anharmonicity
and low-frequency vibrational modes. These anomalously large values follow a
fat-tailed distribution, with a universal exponent related to the recently
observed universal $\omega^4$ density of states of quasi-localized
low-frequency vibrational modes. When the spatial thermal energy field --- a
`softness field' --- is considered, this power-law tail manifests itself by
highly localized spots which are significantly softer than their surroundings.
These soft spots are shown to be susceptible to plastic rearrangements under
external driving forces, having predictive powers that surpass those of the
normal-modes-based approach. These results offer a general,
system/model-independent, physical-observable-based approach to identify
structural properties of quiescent glasses and to relate them to glassy
dynamics. | cond-mat_soft |
Symmetry breaking and coarsening of clusters in a prototypical driven
granular gas: Granular hydrodynamics predicts symmetry-breaking instability in a
two-dimensional (2D) ensemble of nearly elastically colliding smooth hard
spheres driven, at zero gravity, by a rapidly vibrating sidewall. Super- and
subcritical symmetry-breaking bifurcations of the simple clustered state are
identified, and the supercritical bifurcation curve is computed. The cluster
dynamics proceed as a coarsening process mediated by the gas phase. Far above
the bifurcation point the final steady state, selected by coarsening,
represents a single strongly localized densely packed 2D cluster. | cond-mat_soft |
Drop dynamics on Liquid Infused Surfaces: The Role of the Wetting Ridge: We employ a free energy lattice Boltzmann method to study the dynamics of a
ternary fluid system consisting of a liquid drop driven by a body force across
a regularly textured substrate, infused by a lubricating liquid. We focus on
the case of partial wetting lubricants and observe a rich interplay between
contact line pinning and viscous dissipation at the lubricant ridge, which
become dominant at large and small apparent angles respectively. Our numerical
investigations further demonstrate that the relative importance of viscous
dissipation at the lubricant ridge depends on the drop to lubricant viscosity
ratio, as well as on the shape of the wetting ridge. | cond-mat_soft |
Self-propelled particle in a nonconvex external potential: Persistent
limit in one dimension: Equilibrium mapping techniques for nonaligning self-propelled particles have
made it possible to predict the density profile of an active ideal gas in a
wide variety of external potentials, however they fail when the self-propulsion
is very persistent and the potential is nonconvex, which is precisely when the
most uniquely active phenomena occur. Here we show how to predict the density
profile of a 1D active Ornstein-Uhlenbeck particle in an arbitrary external
potential in the persistent limit and discuss the consequences of the
potential's nonconvexity on the structure of the solution, including the
central role of the potential's inflection points and the nonlocal dependence
of the density profile on the potential. | cond-mat_soft |
Impact of boundaries on velocity profiles in bubble rafts: Under conditions of sufficiently slow flow, foams, colloids, granular matter,
and various pastes have been observed to exhibit shear localization, i.e.
regions of flow coexisting with regions of solid-like behavior. The details of
such shear localization can vary depending on the system being studied. A
number of the systems of interest are confined so as to be quasi-two
dimensional, and an important issue in these systems is the role of the
confining boundaries. For foams, three basic systems have been studied with
very different boundary conditions: Hele-Shaw cells (bubbles confined between
two solid plates); bubble rafts (a single layer of bubbles freely floating on a
surface of water); and confined bubble rafts (bubbles confined between the
surface of water below and a glass plate on top). Often, it is assumed that the
impact of the boundaries is not significant in the ``quasi-static limit'', i.e.
when externally imposed rates of strain are sufficiently smaller than internal
kinematic relaxation times. In this paper, we directly test this assumption for
rates of strain ranging from $10^{-3}$ to $10^{-2} {\rm s^{-1}}$. This
corresponds to the quoted quasi-static limit in a number of previous
experiments. It is found that the top plate dramatically alters both the
velocity profile and the distribution of nonlinear rearrangements, even at
these slow rates of strain. | cond-mat_soft |
Generalized Einstein's and Brinkman's solutions for the effective
viscosity of nanofluids: In this paper, we derive the closed form analytical solutions for the
effective viscosity of the suspensions of solid spheres that take into account
the size effects. This result is obtained using the solution for the effective
shear modulus of particulate composites developed in the framework of the
strain gradient elasticity theory. Assuming incompressibility of matrix and
rigid behavior of particles and using a mathematical analogy between the theory
of elasticity and the theory of viscous fluids we derive the generalized
Einstein's formula for the effective viscosity. Generalized Brinkman's solution
for the concentrated suspensions is derived then using differential method.
Obtained solutions contain single additional length scale parameter, which can
be related to the interactions between base liquid and solid particles in the
suspensions. In the case of the large ratio the between diameter of particles
and the length scale parameter, developed solutions reduce to the classical
solutions, however for the small relative diameter of particles an increase of
the effective viscosity is predicted. It is shown that developed models agree
well with known experimental data. Solutions for the fibrous suspensions are
also derived and validated. | cond-mat_soft |
Emergent Stereoselective Interactions and Self-recognition in Polar
Chiral Active Ellipsoids: In many active matter systems, particle trajectories have a well-defined
handedness or chirality. Whether such chiral activity can introduce
stereoselective interactions between particles is not known. Here we developed
a strategy to tune the nature of chiral activity of 3D-printed granular
ellipsoids without altering their shape or size. In vertically agitated
monolayers of these particles, we observed two types of dimers form depending
on the chirality of the pairing monomers. Heterochiral dimers moved
collectively as a single achiral active unit, while homochiral ones formed a
translationally immobile spinner. In active racemic mixtures, the former was
more abundant than the latter indicating stereoselectivity. Through dimer
lifetime measurements, we provide compelling evidence for chiral
self-recognition in mixtures of particles with different chiral activities. We
finally show that changing only the net chirality of a dense active liquid from
a racemic mixture to an enantiopure liquid fundamentally alters its nature of
collective relaxation. | cond-mat_soft |
Universal origin of boson peak vibrational anomalies in ordered crystals
and in amorphous materials: The vibrational spectra of solids, both ordered and amorphous, in the
low-energy regime, control the thermal and transport properties of materials,
from heat capacity to heat conduction, electron-phonon couplings, conventional
superconductivity etc. The old Debye model of vibrational spectra at low energy
gives the vibrational density of states (VDOS) as proportional to the frequency
squared, but in many materials the spectrum departs from this law which results
in a peak upon normalizing the VDOS by frequency squared, which is known as the
"boson peak". A description of the VDOS of solids (both crystals and glasses)
is presented starting from first principles. Without using any assumptions
whatsoever about the existence and nature of "disorder" in the material, it is
shown that the boson peak in the VDOS of both ordered crystals and glasses
arises naturally from the competition between elastic mode propagation and
viscous damping. The theory explains the recent experimental observations of
boson peak in perfectly ordered crystals, which cannot be explained based on
previous theoretical frameworks. The theory also explains, for the first time,
how the vibrational spectrum changes with the atomic density of the solid, and
explains recent experimental observations of this effect. | cond-mat_soft |
Effective pair interactions between colloidal particles at a
nematic-isotropic interface: The Landau-de Gennes free energy is used to study theoretically the
interaction of parallel cylindrical colloidal particles trapped at a
nematic-isotropic interface. We find that the effective interaction potential
is non-monotonic. The corresponding force-distance curves exhibit jumps and
hysteresis upon approach/separation due to the creation/annihilation of
topological defects. Minimization results suggest a simple empirical pair
potential for the effective colloid-colloid interaction at the interface. We
propose that the interface-mediated interaction can play an important role in
self-organization and clustering of colloidal particles at such interfaces. | cond-mat_soft |
Polymer Glass Formation: Role of Activation Free Energy, Configurational
Entropy, and Collective Motion: We provide a perspective on polymer glass formation, with an emphasis on
models in which the fluid entropy and collective particle motion dominate the
theoretical description and data analysis. We first discuss the dynamics of
liquids in the high temperature Arrhenius regime, where transition state theory
is formally applicable. We then summarize the evolution of the entropy theory
from a qualitative framework for organizing and interpreting
temperature-dependent viscosity data by Kauzmann to the formulation of a
hypothetical `ideal thermodynamic glass transition' by Gibbs and DiMarzio,
followed by seminal measurements linking entropy and relaxation by Bestul and
Chang and the Adam-Gibbs (AG) model of glass formation rationalizing the
observations of Bestul and Chang. These developments laid the groundwork for
the generalized entropy theory (GET), which merges an improved lattice model of
polymer thermodynamics accounting for molecular structural details and enabling
the analytic calculation of the configurational entropy with the AG model,
giving rise to a highly predictive model of the segmental structural relaxation
time of polymeric glass-forming liquids. The development of the GET has
occurred in parallel with the string model of glass formation in which concrete
realizations of the cooperatively rearranging regions are identified and
quantified for a wide range of polymeric and other glass-forming materials. The
string model has shown that many of the assumptions of AG are well supported by
simulations, while others are certainly not, giving rise to an entropy theory
of glass formation that is largely in accord with the GET. As the GET and
string models continue to be refined, these models progressively grow into a
more unified framework, and this Perspective reviews the present status of
development of this promising approach to the dynamics of polymeric
glass-forming liquids. | cond-mat_soft |
Non-Newtonian viscosity of E-coli suspensions: The viscosity of an active suspension of E-Coli bacteria is determined
experimentally in the dilute and semi dilute regime using a Y shaped
micro-fluidic channel. From the position of the interface between the pure
suspending fluid and the suspension, we identify rheo-thickening and
rheo-thinning regimes as well as situations at low shear rate where the
viscosity of the bacteria suspension can be lower than the viscosity of the
suspending fluid. In addition, bacteria concentration and velocity profiles in
the bulk are directly measured in the micro-channel. | cond-mat_soft |
Extraterrestrial sink dynamics in granular matter: A loosely packed bed of sand sits precariously on the fence between
mechanically stable and flowing states. This has especially strong implications
for animals or vehicles needing to navigate sandy environments, which can sink
and become stuck in a "dry quicksand" if their weight exceeds the yield stress
of this fragile matter. While it is known that the contact stresses in these
systems are loaded by gravity, very little is known about the sinking dynamics
of objects into loose granular systems under gravitational accelerations
different from the Earth's (g). A fundamental understanding of how objects sink
in different gravitational environments is not only necessary for successful
planetary navigation and engineering, but it can also improve our understanding
of celestial impact dynamics and crater geomorphology. Here we perform and
explain the first systematic experiments of the sink dynamics of objects into
granular media in different gravitational accelerations. By using an
accelerating experimental apparatus, we explore gravitational conditions
ranging from 0.4g to 1.2g. With the aid of discrete element modeling
simulations, we reproduce these results and extend this range to include
objects as small as asteroids and as large as Jupiter. Surprisingly, we find
that the final sink depth is independent of the gravitational acceleration, an
observation with immediate relevance to the design of future extraterrestrial
structures land-roving spacecraft. Using a phenomenological equation of motion
that includes a gravity-loaded frictional term, we are able to quantitatively
explain the experimental and simulation results. | cond-mat_soft |
Holographic interferometry study of the dissolution and diffusion of
gypsum in water: We have performed holographic interferometry measurements of the dissolution
of the (010) plane of a cleaved gypsum single crystal in pure water. These
experiments have provided the value of the dissolution rate constant k of
gypsum in water and the value of the interdiffusion coefficient D of its
aqueous species in water. D is 1.0 x 10^-9 m2 s^-1, a value close to the
theoretical value generally used in dissolution studies. k is 4 x 10^-5 mol
m^-2 s^-1. It directly characterizes the microscopic transfer rate at the
solid-liquid interface, and is not an averaged value deduced from quantities
measured far from the surface as in macroscopic dissolution experiments. It is
found to be two times lower than the value obtained from macroscopic
experiments. | cond-mat_soft |
Heat Transfer between Graphene and Amorphous SiO2: We study the heat transfer between graphene and amorphous SiO2. We include
both the heat transfer from the area of real contact, and between the surfaces
in the non-contact region. We consider the radiative heat transfer associated
with the evanescent electromagnetic waves which exist outside of all bodies,
and the heat transfer by the gas in the non-contact region. We find that the
dominant contribution to the heat transfer result from the area of real
contact, and the calculated value of the heat transfer coefficient is in good
agreement with the value deduced from experimental data. | cond-mat_soft |
Scaling theory for the jamming transition: We propose a scaling ansatz for the elastic energy of a system near the
critical jamming transition in terms of three relevant fields: the compressive
strain $\Delta \phi$ relative to the critical jammed state, the shear strain
$\epsilon$, and the inverse system size $1/N$. We also use $\Delta Z$, the
number of contacts relative to the minimum required at jamming, as an
underlying control parameter. Our scaling theory predicts new exponents,
exponent equalities and scaling collapses for energy, pressure and shear stress
that we verify with numerical simulations of jammed packings of soft spheres.
It also yields new insight into why the shear and bulk moduli exhibit different
scalings; the difference arises because the shear stress vanishes as
$1/\sqrt{N}$ while the pressure approaches a constant in the thermodynamic
limit. The success of the scaling ansatz implies that the jamming transition
exhibits an emergent scale invariance, and that it should be possible to
develop a renormalization-group theory for jamming. | cond-mat_soft |
Active Motion of Janus Particle by Self-thermophoresis in Defocused
Laser Beam: We study self-propulsion of a half-metal coated colloidal particle under
laser irradiation. The motion is caused by self-thermophoresis: i.e. absorption
of laser at the metal-coated side of the particle creates local temperature
gradient which in turn drives the particle by thermophoresis. To clarify the
mechanism, temperature distribution and a thermal slip flow field around a
micro-scale Janus particle are measured for the first time. With measured
temperature drop across the particle, the speed of self-propulsion is
corroborated with the prediction based on accessible parameters. As an
application for driving micro-machine, a micro-rotor heat engine is
demonstrated. | cond-mat_soft |
Minimal Surfaces, Screw Dislocations and Twist Grain Boundaries: Large twist-angle grain boundaries in layered structures are often described
by Scherk's first surface whereas small twist-angle grain boundaries are
usually described in terms of an array of screw dislocations. We show that
there is no essential distinction between these two descriptions and that, in
particular, their comparative energetics depends crucially on the core
structure of their screw-dislocation topological defects. | cond-mat_soft |
Phase nucleation in curved space: Nucleation and growth is the dominant relaxation mechanism driving first
order phase transitions. In two-dimensional at systems nucleation has been
applied to a wide range of problems in physics, chemistry and biology. Here we
study nucleation and growth of two-dimensional phases lying on curved surfaces
and show that curvature modify both, critical sizes of nuclei and paths towards
the equilibrium phase. In curved space nucleation and growth becomes inherently
inhomogeneous and critical nuclei form faster on regions of positive Gaussian
curvature. Substrates of varying shape display complex energy landscapes with
several geometry-induced local minima, where initially propagating nuclei
become stabilized and trapped by the underlying curvature. | cond-mat_soft |
Relaxation dynamics of functionalized colloids on attractive substrates: Particle-based simulations are performed to study the post-relaxation
dynamics of functionalized (patchy) colloids adsorbed on an attractive
substrate. Kinetically arrested structures that depend on the number of
adsorbed particles and the strength of the particle-particle and
particle-substrate interactions are identified. The radial distribution
function is characterized by a sequence of peaks, with relative intensities
that depend on the number of adsorbed particles. The first-layer coverage is a
non-monotonic function of the number of particles, with an optimal value around
one layer of adsorbed particles. The initial relaxation towards these
structures is characterized by a fast (exponential) and a slow (power-law)
dynamics. The fast relaxation timescale is a linearly increasing function of
the number of adsorbed particles in the submonolayer regime, but it saturates
for more than one adsorbed layer. The slow dynamics exhibits two characteristic
exponents, depending on the surface coverage. | cond-mat_soft |
Effects of strongly selective additives on volume phase transition in
gels: We investigate volume phase transition in gels immersed in mixture solvents,
on the basis of a three-component Flory-Rehner theory. When the selectivity of
the minority solvent component to the polymer network is strong, the gel tends
to shrink with an increasing concentration of the additive, regardless of
whether it is good or poor. This behavior originates from the difference of the
additive concentration between inside and outside the gel. We also found the
gap of the gel volume at the transition point can be controlled by adding the
strongly selective solutes. By dissolving a strongly poor additive, for
instance, the discontinuous volume phase transition can be extinguished.
Furthermore, we observed that another volume phase trasition occurs far from
the original transition point. These behaviors can be well explained by a
simplified theory neglecting the non-linearity of the additive concentration. | cond-mat_soft |
Nanofluidic logic with mechano-ionic memristive switches: While most neuromorphic systems are based on nanoscale electronic devices,
nature relies on ions for energy-efficient information processing. Therefore,
finding memristive nanofluidic devices is a milestone toward realizing
electrolytic computers mimicking the brain down to its basic principles of
operation. Here, we present a nanofluidic device designed for circuit scale
in-memory processing that combines single-digit nanometric confinement and
large entrance asymmetry. Our fabrication process is scalable while the device
operates at the second timescale with a conductance ratio in the range 10-60.
In-operando optical microscopy unveils the origin of memory, arising from the
reversible formation of liquid blisters modulating the device conductance. The
combination of features of these mechano-ionic memristive switches permits
assembling logic circuits composed of two interactive devices and an ohmic
resistor. These results open the way to design multi-component ionic machinery,
such as nanofluidic neural networks, and implementing brain-inspired ionic
computations. | cond-mat_soft |
Surface patterns in drying films of silica colloidal dispersions: We report an experimental study on the drying of silica colloidal
dispersions. Here we focus on a surface instability occurring in a drying paste
phase before crack formation which affects the final film quality. Observations
at macroscopic and microscopic scales reveal the occurrence of the instability,
and the morphology of the film surface. Furthermore, we show that the addition
of adsorbing polymers on silica particles can be used to suppress the
instability under particular conditions of molecular weight and concentration.
We relate this suppression to the increase of the paste elastic modulus. | cond-mat_soft |
Higher-order moment theories for dilute granular gases of smooth
hard-spheres: Grad's method of moments is employed to develop higher-order Grad's moment
equations---up to first 26-moments---for granular gases within the framework of
the (inelastic) Boltzmann equation. The homogeneous cooling state of a freely
cooling granular gas is investigated with the Grad's 26-moment equations in a
semi-linearized setting and it is shown that the granular temperature in the
homogeneous cooling state still decays according to Haff's law while the other
higher-order moments decay on a faster time scale. The constitutive relations
for stress and heat flux (the Navier--Stokes and Fourier relations) are
obtained by performing a Chapman--Enskog-like expansion on the Grad's 26-moment
equations and compared with those existing in the literature. The linear
stability of the homogeneous cooling state is analyzed through the Grad's
26-moment system and various sub-systems by decomposing them into longitudinal
and transverse systems. It is found that one eigenmode in both longitudinal and
transverse systems in case of inelastic gases is unstable. By comparing the
eigenmodes from various theories, it is established that the 13-moment
eigenmode theory predicts that the unstable eigenmode remains unstable for all
wavenumbers below a certain coefficient of restitution while any other
higher-order moment theory shows that this mode becomes stable above some
critical wavenumber for all values of coefficient of restitution. In
particular, the Grad's 26-moment theory leads to a smooth profile for the
critical wavenumber in contrast to the other considered theories. Furthermore,
the critical system size obtained through the Grad 26-moment and existing
theories are also in excellent agreement. | cond-mat_soft |
Coordinated Stress-Structure Self-Organization in Granular Packing: It is accepted that stress and structure self-organize cooperatively during
quasi-static dynamics of granular systems, but the consequences of this
self-organization are not fully understood. Such an understanding is essential
because local structural properties of the settled material are then correlated
with the local stress, which calls into question existing linear theories of
stress transmission in granular media. A method to quantify the local
stress-structure correlations is necessary for addressing this issue and we
present here such a method for planar systems. We then use it to analyze
numerically several different systems, compressed quasi-statically by two
different procedures. We define cells, cell orders, cell orientations, and cell
stresses and report the following. 1. The mean ratio of cell principal stresses
decreases with cell order and increases with friction. 2. The ratio
distributions collapse onto a single curve under a simple scaling, for all
packing protocols and friction coefficients. 3. Cells orient along the local
stress major principal axes. 4. A simple first-principles model explains the
correlations between the local cell and stress principal axis orientations. Our
results quantify the cooperative stress-structure self-organization and provide
a way to relate quantitatively the stress-structure coupling to different
process parameters and particle characteristics. Significantly, the strong
stress-structure correlation, driven by structural re-organization upon
application of external stress, suggests that current stress theories of
granular matter need to be revisited. | cond-mat_soft |
Bogoliubov approach to superfluidity of atoms in an optical lattice: We use the Bogoliubov theory of atoms in an optical lattice to study the
approach to the Mott-insulator transition. We derive an explicit expression for
the superfluid density based on the rigidity of the system under phase
variations. This enables us to explore the connection between the quantum
depletion of the condensate and the quasi-momentum distribution on the one hand
and the superfluid fraction on the other. The approach to the insulator phase
may be characterized through the filling of the band by quantum depletion,
which should be directly observable via the matter wave interference patterns.
We complement these findings by self-consistent Hartree-Fock-Bogoliubov-Popov
calculations for one-dimensional lattices including the effects of a parabolic
trapping potential. | cond-mat_soft |
Stability and interactions of nanocolloids at fluid interfaces: effects
of capillary waves and line tensions: We analyze the effective potential for nanoparticles trapped at a fluid
interface within a simple model which incorporates surface and line tensions as
well as a thermal average over interface fluctuations (capillary waves). For a
single colloid, a reduced steepness of the potential well hindering movements
out of the interface plane compared to rigid interface models is observed, and
an instability of the capillary wave partition sum in case of negative line
tensions is pointed out. For two colloids, averaging over the capillary waves
leads to an effective Casimir--type interaction which is long--ranged,
power-like in the inverse distance but whose power sensitively depends on
possible restrictions of the colloid degress of freedom. A nonzero line tension
leads to changes in the magnitude but not in the functional form of the
effective potential asymptotics. | cond-mat_soft |
Reversible magnetomechanical collapse: virtual touching and detachment
of rigid inclusions in a soft elastic matrix: Soft elastic composite materials containing particulate rigid inclusions in a
soft elastic matrix are candidates for developing soft actuators or tunable
damping devices. The possibility to reversibly drive the rigid inclusions
within such a composite together to a close-to-touching state by an external
stimulus would offer important benefits. Then, a significant tuning of the
mechanical properties could be achieved due to the resulting mechanical
hardening. For a long time, it has been argued whether a virtual touching of
the embedded magnetic particles with subsequent detachment can actually be
observed in real materials, and if so, whether the process is reversible. Here,
we present experimental results that demonstrate this phenomenon in reality.
Our system consists of two paramagnetic nickel particles embedded at finite
initial distance in a soft elastic polymeric gel matrix. Magnetization in an
external magnetic field tunes the magnetic attraction between the particles and
drives the process. We quantify the scenario by different theoretical tools,
i.e., explicit analytical calculations in the framework of linear elasticity
theory, a projection onto simplified dipole-spring models, as well as detailed
finite-element simulations. From these different approaches, we conclude that
in our case the cycle of virtual touching and detachment shows hysteretic
behavior due to the mutual magnetization between the paramagnetic particles.
Our results are important for the design and construction of reversibly tunable
mechanical damping devices. Moreover, our projection on dipole-spring models
allows the formal connection of our description to various related systems,
e.g., magnetosome filaments in magnetotactic bacteria. | cond-mat_soft |
Scaling regimes for wormlike chains confined to cylindrical surfaces
under tension: We compute the free energy of confinement ${\cal{F}}$ for a wormlike chain
(WLC), with persistence length $l_p$, that is confined to the surface of a
cylinder of radius $R$ under an external tension $f$ using a mean field
variational approach. For long chains, we analytically determine the behavior
of the chain in a variety of regimes, which are demarcated by the interplay of
$l_p$, the Odijk deflection length ($l_d=(R^2l_p)^{1/3}$), and the Pincus
length ($l_f = {k_BT}/{f}$, with $k_BT$ being the thermal energy). The theory
accurately reproduces the Odijk scaling for strongly confined chains at $f=0$,
with ${\cal{F}}\sim Ll_p^{-1/3}R^{-2/3}$. For moderate values of $f$, the Odijk
scaling is discernible only when ${l_p}\gg R$ for strongly confined chains.
Confinement does not significantly alter the scaling of the mean extension for
sufficiently high tension. The theory is used to estimate unwrapping forces for
DNA from nucleosomes. | cond-mat_soft |
Improved general-purpose five-point model for water: TIP5P/2018: A new five point potential for liquid water, TIP5P/2018, is presented along
with the techniques used to derive its charges from ab initio per-molecule
electrostatic potentials in the liquid phase using the split charge
equilibration (SQE) of Nistor et al. [J. Chem. Phys. 125, 094108 (2006)]. By
taking the density and diffusion dependence on temperature as target
properties, significant improvements to the behavior of isothermal
compressibility were achieved along with improvements to other thermodynamic
and rotational properties. While exhibiting a dipole moment close to ab initio
values, TIP5P/2018 suffers from a too small quadrupole moment due to the charge
assignment procedure and results in an overestimation of the dielectric
constant. | cond-mat_soft |
Analysis of the shape of x-ray diffraction peaks originating from the
hexatic phase of liquid crystal films: X-ray diffraction studies of the bond-orientational order in the hexatic-B
phase of 75OBC and 3(10)OBC compounds are presented. The temperature evolution
of an angular profile of a single diffraction peak is analyzed. Close to the
hexatic-B-smectic-A transition these profiles can be approximated by the
Gaussian function. At lower temperatures in the hexatic-B phase the profiles
are better fitted by the Voigt function. Theoretical analysis of the width of
diffraction peaks in three-dimentional (3D) hexatics is performed on the basis
of the effective Hamiltonian introduced by Aharony and Kardar. Theoretical
estimations are in good agreement with the results of x-ray experiments. | cond-mat_soft |
Microscopic Description of Entanglements in Polyethylene Networks and
Melts: Strong, Weak, Pairwise, and Collective Attributes: We present atomistic molecular dynamics simulations of two Polyethylene
systems where all entanglements are trapped: a perfect network, and a melt with
grafted chain ends. We examine microscopically at what level topological
constraints can be considered as a collective entanglement effect, as in tube
model theories, or as certain pairwise uncrossability interactions, as in
slip-link models. A pairwise parameter, which varies between these limiting
cases, shows that, for the systems studied, the character of the entanglement
environment is more pairwise than collective.
We employ a novel methodology, which analyzes entanglement constraints into a
complete set of pairwise interactions, similar to slip links. Entanglement
confinement is assembled by a plethora of links, with a spectrum of confinement
strengths, from strong to weak. The strength of interactions is quantified
through a link `persistence', which is the fraction of time for which the links
are active. By weighting links according to their strength, we show that
confinement is imposed mainly by the strong ones, and that the weak, trapped,
uncrossability interactions cannot contribute to the low frequency modulus of
an elastomer, or the plateau modulus of a melt.
A self-consistent scheme for mapping topological constraints to specific,
strong binary links, according to a given entanglement density, is proposed and
validated. Our results demonstrate that slip links can be viewed as the
strongest pairwise interactions of a collective entanglement environment. The
methodology developed provides a basis for bridging the gap between atomistic
simulations and mesoscopic slip link models. | cond-mat_soft |
Mechanics of invagination and folding: hybridized instabilities when one
soft tissue grows on another: We address the folding induced by differential growth in soft layered solids
via an elementary model that consists of a soft growing neo-Hookean elastic
layer adhered to a deep elastic substrate. As the layer/substrate modulus ratio
is varied from above unity towards zero we find a first transition from
supercritical smooth folding followed by cusping of the valleys to direct
subcritical cusped folding, then another to supercritical cusped folding.
Beyond threshold the high amplitude fold spacing converges to about four layer
thicknesses for many modulus ratios. In three dimensions the instability gives
rise to a wide variety of morphologies, including almost degenerate zigzag and
triple-junction patterns that can coexist when the layer and substrate are of
comparable softness. Our study unifies these results providing understanding
for the complex and diverse fold morphologies found in biology, including the
zigzag precursors to intestinal villi, and disordered zigzags and
triple-junctions in mammalian cortex. | cond-mat_soft |
Creation of a monopole in a spinor condensate: We propose a method to create a monopole structure in a spin-1 spinor
condensate by applying the basic methods used to create vortices and solitons
experimentally in single-component condensates. We show, however, that by using
a two-component structure for a monopole, we can simplify our proposed
experimental approach and apply it also to ferromagnetic spinor condensates. We
also discuss the observation and dynamics of such a monopole structure, and
note that the dynamics of the two-component monopole differs from the dynamics
of the three-component monopole. | cond-mat_soft |
Snapping Mechanical Metamaterials under Tension: We present a monolithic mechanical metamaterial comprising a periodic
arrangement of snapping units with tunable tensile behavior. Under tension, the
metamaterial undergoes a large extension caused by sequential snap-through
instabilities, and exhibits a pattern switch from an undeformed wavy-shape to a
diamond configuration. By means of experiments performed on 3D printed
prototypes, numerical simulations and theoretical modeling, we demonstrate how
the snapping architecture can be tuned to generate a range of nonlinear
mechanical responses including monotonic, S-shaped, plateau and non-monotonic
snap-through behavior. This work contributes to the development of design
strategies that allow programming nonlinear mechanical responses in solids. | cond-mat_soft |
Insights into DNA-mediated interparticle interactions from a
coarse-grained model: DNA-functionalized particles have great potential for the design of complex
self-assembled materials. The major hurdle in realizing crystal structures from
DNA-functionalized particles is expected to be kinetic barriers that trap the
system in metastable amorphous states. Therefore, it is vital to explore the
molecular details of particle assembly processes in order to understand the
underlying mechanisms. Molecular simulations based on coarse-grained models can
provide a convenient route to explore these details. Most of the currently
available coarse-grained models of DNA-functionalized particles ignore key
chemical and structural details of DNA behavior. These models therefore are
limited in scope for studying experimental phenomena. In this paper, we present
a new coarse-grained model of DNA-functionalized particles which incorporates
some of the desired features of DNA behavior. The coarse-grained DNA model used
here provides explicit DNA representation (at the nucleotide level) and
complementary interactions between Watson-Crick base pairs, which lead to the
formation of single-stranded hairpin and double-stranded DNA. Aggregation
between multiple complementary strands is also prevented in our model. We study
interactions between two DNA- functionalized particles as a function of DNA
grafting density, lengths of the hybridizing and non-hybridizing parts of DNA,
and temperature. The calculated free energies as a function of pair distance
between particles qualitatively resemble experimental measurements of
DNA-mediated pair interactions. | cond-mat_soft |
Salt-induced reentrant stability of polyion-decorated particles with
tunable surface charge density: The electrostatic complexation between DOTAP-DOPC unilamellar liposomes and
an oppositely charged polyelectrolyte (NaPA) has been investigated in a wide
range of the liposome surface charge density. We systematically characterized
the "reentrant condensation" and the charge inversion of
polyelectrolyte-decorated liposomes by means of dynamic light scattering and
electrophoresis. We explored the stability of this model
polyelectrolyte/colloid system by fixing each time the charge of the bare
liposomes and by changing two independent control parameters of the
suspensions: the polyelectrolyte/colloid charge ratio and the ionic strength of
the aqueous suspending medium. The progressive addition of neutral DOPC lipid
within the liposome membrane gave rise to a new intriguing phenomenon: the
stability diagram of the suspensions showed a novel reentrance due to the
crossing of the desorption threshold of the polyelectrolyte. Indeed, at fixed
charge density of the bare DOTAP/DOPC liposomes and for a wide range of polyion
concentrations, we showed that the simple electrolyte addition first (low salt
regime) destabilizes the suspensions because of the enhanced screening of the
residual repulsion between the complexes, and then (high salt regime)
determines the onset of a new stable phase, originated by the absence of
polyelectrolyte adsorption on the particle surfaces. We show that the observed
phenomenology can be rationalized within the Velegol-Thwar model for
heterogeneously charged particles and that the polyelectrolyte desorption fits
well the predictions of the adsorption theory of Winkler and Cherstvy. Our
findings unambiguously support the picture of the reentrant condensation as
driven by the correlated adsorption of the polyelectrolyte chains on the
particle surface, providing interesting insights into possible mechanisms for
tailoring complex colloids via salt-induced effects. | cond-mat_soft |
Topological vortex formation in a Bose-Einstein condensate: Vortices were imprinted in a Bose-Einstein condensate using topological
phases. Sodium condensates held in a Ioffe-Pritchard magnetic trap were
transformed from a non-rotating state to one with quantized circulation by
adiabatically inverting the magnetic bias field along the trap axis. Using
surface wave spectroscopy, the axial angular momentum per particle of the
vortex states was found to be consistent with $2\hbar$ or $4\hbar$, depending
on the hyperfine state of the condensate. | cond-mat_soft |
Mutual diffusion of inclusions in freely-suspended smectic liquid
crystal films: We study experimentally and theoretically the hydrodynamic interaction of
pairs of circular inclusions in two-dimensional, fluid smectic membranes
suspended in air. By analyzing their Brownian motion, we find that the radial
mutual mobilities of identical inclusions are independent of their size but
that the angular coupling becomes strongly size-dependent when their radius
exceeds a characteristic hydrodynamic length. The observed dependence of the
mutual mobilities on inclusion size is described well for arbitrary separations
by a model that generalizes the Levine/MacKintosh theory of point-force
response functions and uses a boundary-element approach to calculate the
mobility matrix. | cond-mat_soft |
Transient deformation of a droplet near a microfluidic constriction : a
quantitative analysis: We report on experiments that consist in deforming a collection of
monodisperse droplets produced by a microfluidic chip through a flow-focusing
device. We show that a proper numerical modelling of the flow is necessary to
access the stress applied by the latter on the droplet along its trajectory
through the chip. This crucial step enables the full integration of the
differential equation governing the dynamical deformation, and consequently the
robust measurement of the interfacial tension by fitting the experiments with
the calculated deformation. Our study thus demonstrates the feasibility of
quantitative in-situ rheology in microfluidic flows involving e.g. droplets,
capsules or cells. | cond-mat_soft |
How do ionic superdiscs self-assemble in nanopores?: Discotic ionic liquid crystals (DILCs) consist of self-assembled superdiscs
of cations and anions that spontaneously stack in linear columns with high
one-dimensional ionic and electronic charge mobility, making them prominent
model systems for functional soft matter. Unfortunately, a homogeneous
alignment of DILCs on the macroscale is often not achievable, which
significantly limits their applicability. Infiltration into nanoporous solid
scaffolds can in principle overcome this drawback. However, due to the extreme
experimental challenges to scrutinise liquid crystalline order in extreme
spatial confinement, little is known about the structures of DILCs in
nanopores. Here, we present temperature-dependent high-resolution optical
birefringence measurement and 3D reciprocal space mapping based on
synchrotron-based X-ray scattering to investigate the thermotropic phase
behaviour of dopamine-based ionic liquid crystals confined in cylindrical
channels of 180~nm diameter in macroscopic anodic aluminum oxide (AAO)
membranes. As a function of the membranes' hydrophilicity and thus the
molecular anchoring to the pore walls (edge-on or face-on) and the variation of
the hydrophilic-hydrophobic balance between the aromatic cores and the alkyl
side chain motifs of the superdiscs by tailored chemical synthesis, we find a
particularly rich phase behaviour, which is not present in the bulk state. It
is governed by a complex interplay of liquid crystalline elastic energies
(bending and splay deformations), polar interactions and pure geometric
confinement, and includes textural transitions between radial and axial
alignment of the columns with respect to the long nanochannel axis. | cond-mat_soft |
Motion of nanodroplets near chemical heterogeneities: We investigate the dynamics of nanoscale droplets in the vicinity of chemical
steps which separate parts of a substrate with different wettabilities. Due to
long-ranged dispersion forces, nanodroplets positioned on one side of the step
perceive the different character of the other side even at some distances from
the step, leading to a dynamic response. The direction of the ensuing motion of
such droplets does not only depend on the difference between the equilibrium
contact angles on these two parts but in particular on the difference between
the corresponding Hamaker constants. Therefore the motion is not necessarily
directed towards the more wettable side and can also be different from that of
droplets which span the step. | cond-mat_soft |
Sedimentation of active colloidal suspensions: In this paper, we investigate experimentally the non-equilibrium steady state
of an active colloidal suspension under gravity field. The active particles are
made of chemically powered colloids, showing self propulsion in the presence of
an added fuel, here hydrogen peroxide. The active suspension is studied in a
dedicated microfluidic device, made of permeable gel microstructures. Both the
microdynamics of individual colloids and the global stationary state of the
suspension under gravity - density profiles, number fluctuations - are measured
with optical microscopy. This allows to connect the sedimentation length to the
individual self-propelled dynamics, suggesting that in the present dilute
regime the active colloids behave as 'hot' particles. Our work is a first step
in the experimental exploration of the out-of-equilibrium properties of
artificial active systems. | cond-mat_soft |
Metastability of lipid necks via geometric triality: "Necks" are features of lipid membranes characterized by an uniquley large
curvature, functioning as bridges between different compartments. These
features are ubiquitous in the life-cycle of the cell and instrumental in
processes such as division, extracellular vesicles uptake and cargo transport
between organelles, but also in life-threatening conditions, as in the
endocytosis of viruses and phages. Yet, the very existence of lipid necks
challenges our understanding of membranes biophysics: their curvature, often
orders of magnitude larger than elsewhere, is energetically prohibitive, even
with the arsenal of molecular machineries and signalling pathways that cells
have at their disposal. Using a geometric triality, namely a correspondence
between three different classes of geometric objects, here we demonstrate that
lipid necks are in fact metastable, thus can exist for finite, but potentially
long times even in the absence of stabilizing mechanisms. This framework allows
us to explicitly calculate the forces a corpuscle must overcome in order to
penetrate cellular membranes, thus paving the way for a predictive theory of
endo/exo-cytic processes. | cond-mat_soft |
Drag coefficient of a liquid domain in a two-dimensional membrane: Using a hydrodynamic theory that incorporates a momentum decay mechanism, we
calculate the drag coefficient of a circular liquid domain of finite viscosity
moving in a two-dimensional membrane. We derive an analytical expression for
the drag coefficient which covers the whole range of domain sizes. Several
limiting expressions are discussed. The obtained drag coefficient decreases as
the domain viscosity becomes smaller with respect to the outer membrane
viscosity. This is because the flow induced in the domain acts to transport the
fluid in the surrounding matrix more efficiently. | cond-mat_soft |
Depletion-induced crystallization of anisotropic triblock colloids: The intricate interplay between colloidal particle shape and precisely
engineered interaction potentials has paved the way for the discovery of
unprecedented crystal structures in both two and three dimensions. Here, we
make use of anisotropic triblock colloidal particles composed of two distinct
materials. The resulting surface charge heterogeneity can be exploited to
generate regioselective depletion interactions and directional bonding. Using
extensive molecular dynamics simulations and a dimensionality reduction
analysis approach, we map out state diagrams for the self-assembly of such
colloids as a function of their aspect ratio and packing fraction for varying
depletant sizes in a quasi two-dimensional set-up. We observe the formation of
a wide variety of crystal structures such as a herringbone, brick-wall, tilted
brick-wall, and (tilted) ladder-like structures. More specifically, we
determine the optimal parameters to enhance crystallization, and investigate
the nucleation process. Additionally, we explore the potential of using crystal
monolayers as templates for deposition, thereby creating complex
three-dimensional structures that hold promise for future applications. | cond-mat_soft |
Structural Relaxation Time and Dynamic Shear Modulus of Glassy Graphene: We theoretically investigate glass transition behaviors of the glassy
graphene in a wide range of temperature, where this amorphous graphene is
described as a hard-sphere fluid. The dynamic arrest of a particle is
assumingly caused by interactions with its nearest neighbors and surrounding
fluid particles. The assumption allows us to analyze roles of local and
collective particle mobility. We calculate the temperature dependence of
structural relaxation time and dynamic shear modulus, the dynamic fragility,
and the glass transition temperature. In addition, correlations between these
physical quantities are comprehensively discussed. Our theoretical calculations
agree quantitatively well with recent simulations and Dyre's shoving model. | cond-mat_soft |
Control and ultrasonic actuation of a gas-liquid interface in a
microfluidic chip: This article describes the design and manufacturing of a microfluidic chip,
allowing for the actuation of a gas-liquid interface and of the neighboring
fluid. A first way to control the interface motion is to apply a pressure
difference across it. In this case, the efficiency of three different
micro-geometries at anchoring the interface is compared. Also, the critical
pressures needed to move the interface are measured and compared to theoretical
result. A second way to control the interface motion is by ultrasonic
excitation. When the excitation is weak, the interface exhibits traveling
waves, which follow a dispersion equation. At stronger ultrasonic levels,
standing waves appear on the interface, with frequencies that are half integer
multiple of the excitation frequency. An associated microstreaming flow field
observed in the vicinity of the interface is characterized. The meniscus and
associated streaming flow have the potential to transport particles and mix
reagents. | cond-mat_soft |
Knotting and weak knotting in confined, open random walks using virtual
knots: We probe the character of knotting in open, confined polymers, assigning knot
types to open curves by identifying their projections as virtual knots. In this
sense, virtual knots are transitional, lying in between classical knot types,
which are useful to classify the ambiguous nature of knotting in open curves.
Modelling confined polymers using both lattice walks and ideal chains, we find
an ensemble of random, tangled open curves whose knotting is not dominated by
any single knot type, a behaviour we call weakly knotted. We compare cubically
confined lattice walks and spherically confined ideal chains, finding the weak
knotting probability in both families is quite similar and growing with length,
despite the overall knotting probability being quite different. In contrast,
the probability of weak knotting in unconfined walks is small at all lengths
investigated. For spherically confined ideal chains, weak knotting is strongly
correlated with the degree of confinement but is almost entirely independent of
length. For ideal chains confined to tubes and slits, weak knotting is
correlated with an adjusted degree of confinement, again with length having
negligible effect. | cond-mat_soft |
A surface force apparatus for nanorheology under large shear strain: We describe a surface force apparatus designed to probe the rheology of a
nanoconfined medium under large shear amplitudes (up to 500 $\mu$m). The
instrument can be operated in closed-loop, controlling either the applied
normal load or the thickness of the medium during shear experiments. Feedback
control allows to greatly extend the range of confinement/shear strain
attainable with the surface force apparatus. The performances of the instrument
are illustrated using hexadecane as the confined medium. | cond-mat_soft |
UV-Manipulation of Order and Macroscopic Shape in Nematic Elastomers: A range of monodomain nematic liquid crystal elastomers containing differing
proportions of photo-isomerisable mesogenic moieties, which turn from a
rod-like to a kinked shape upon ultraviolet (UV) irradiation, was studied.
Depending on the proportion and positional role of the photo-sensitive groups
in the crosslinked polymer network, different types and magnitudes of response
were found. The principle consequence of such photo-isomerisation is the
destabilisation of the nematic phase, whose order parameter depends on
temperature in a near-critical fashion. Accordingly, the effect of
UV-irradiation is dramatically enhanced near the critical temperature, with the
associated reduction in the nematic order parameter manifesting as a change in
the macroscopic shape of the elastomer samples, producing a large uniaxial
contraction. Theoretical analysis of this phenomenon gives a good quantitative
agreement with experiment. | cond-mat_soft |
Topology in non-linear mechanical systems: Many advancements have been made in the field of topological mechanics. The
majority of the works, however, concerns the topological invariant in a linear
theory. We, in this work, present a generic prescription of defining
topological indices which accommodates non-linear effects in mechanical systems
without taking any approximation. Invoking the tools of differential geometry,
a Z-valued quantity in terms of the Poincare-Hopf index, that features the
topological invariant of non-linear zero modes (ZMs), is predicted. We further
identify one type of topologically protected solitons that are robust to
disorders. Our prescription constitutes a new direction of searching for novel
topologically protected non-linear ZMs in the future. | cond-mat_soft |
Localized fluidization in granular materials: Theoretical and numerical
study: We present analytical and numerical results on localized fluidization within
a granular layer subjected to a local injection of fluid. As the injection rate
increases the three different regimes previously reported in the literature are
recovered: homogeneous expansion of the bed, fluidized cavity in which
fluidization starts developing above the injection area, and finally the
chimney of fluidized grains when the fluidization zone reaches the free
surface. The analytical approach is at the continuum scale, based on Darcy's
law and Therzaghi's effective stress principle. It provides a good description
of the phenomenon as long as the porosity of the granular assembly remains
relatively homogeneous, i.e. for small injection rates. The numerical approach
is at the particle scale based on the coupled DEM-PFV method. It tackles the
more heterogeneous situations which occur at larger injection rates. The
results from both methods are in qualitative agreement with data published
independently. A more quantitative agreement is achieved by the numerical
model. A direct link is evidenced between the occurrence of the different
regimes of fluidization and the injection aperture. While narrow apertures let
the three different regimes be distinguished clearly, larger apertures tend to
produce a single homogeneous fluidization regime. In the former case, it is
found that the transition between the cavity regime and the chimney regime for
an increasing injection rate coincides with a peak in the evolution of inlet
pressure. Finally, the occurrence of the different regimes is defined in terms
of the normalized flux and aperture. | cond-mat_soft |
Continuous rotation of achiral nematic liquid crystal droplets driven by
heat flux: Suspended droplets of cholesteric (chiral nematic) liquid crystals
spontaneously rotate in the presence of a heat flux due to a temperature
gradient, a phenomenon known as Lehmann effect. So far, it is not clear whether
this effect is due to the chirality of the phase and the molecules or only to
the chirality of the director field. Here, we report the continuous rotation in
a temperature gradient of nematic droplets of a lyotropic chromonic liquid
crystal featuring a twisted bipolar configuration. The achiral nature of the
molecular components leads to a random handedness of the spontaneous twist,
resulting in the coexistence of droplets rotating in the two senses, with
speeds proportional to the temperature gradient and inversely proportional to
the droplet radius. This result shows that a macroscopic twist of the director
field is sufficient to induce a rotation of the droplets, and that the phase
and the molecules do not need to be chiral. This suggests that one can also
explain the Lehmann rotation in cholesteric liquid crystals without introducing
the Leslie thermomechanical coupling -- only present in chiral mesophases. An
explanation based on the Akopyan and Zeldovich theory of thermomechanical
effects in nematics is proposed and discussed. | cond-mat_soft |
Entanglement and weak interaction driven mobility of small molecules in
polymer networks: Diffusive transport of small molecules within the internal structures of
biological and synthetic material systems is complex because the crowded
environment presents chemical and physical barriers to mobility. We explored
this mobility using a synthetic experimental system of small dye molecules
diffusing within a polymer network at short time scales. We find that the
diffusion of inert molecules is inhibited by the presence of the polymers.
Counter-intuitively, small, hydrophobic molecules display smaller reduction in
mobility and also able to diffuse faster through the system by leveraging
crowding specific parameters. We explained this phenomenon by developing a de
novo model and using these results, we hypothesized that non-specific
hydrophobic interactions between the molecules and polymer chains could
localize the molecules into compartments of overlapped and entangled chains
where they experience microviscosity, rather than macroviscosity. We introduced
a characteristic interaction time parameter to quantitatively explain
experimental results in the light of frictional effects and molecular
interactions. Our model is in good agreement with the experimental results and
allowed us to classify molecules into two different mobility categories solely
based on interaction. By changing the surface group, polymer molecular weight,
and by adding salt to the medium, we could further modulate the mobility and
mean square displacements of interacting molecules. Our work has implications
in understanding intracellular diffusive transport in microtubule networks and
other systems with macromolecular crowding and could lead to transport
enhancement in synthetic polymer systems. | cond-mat_soft |
Structure factor and thermodynamics of rigid dendrimers in solution: The ''polymer reference interaction site model'' (PRISM) integral equation
theory is used to determine the structure factor of rigid dendrimers in
solution. The theory is quite successful in reproducing experimental structure
factors for various dendrimer concentrations. In addition, the structure factor
at vanishing scattering vector is calculated via the compressibility equation
using scaled particle theory and fundamental measure theory. The results as
predicted by both theories are systematically smaller than the experimental and
PRISM data for platelike dendrimers. | cond-mat_soft |
Modeling growing confluent tissues using a lattice Boltzmann method:
interface stability and fluctuations: Tissue growth underpins a wide array of biological and developmental
processes, and numerical modeling of growing systems has been shown to be a
useful tool for understanding these processes. However, the phenomena that can
be captured are often limited by the size of systems that can be modeled. Here,
we address this limitation by introducing a Lattice-Boltzmann method (LBM) for
a growing system that is able to efficiently model hydrodynamic length-scales.
The model incorporates a novel approach to describing the growing front of a
tissue, which we use to investigate the dynamics of the interface of growing
model tissues. We find that the interface grows with scaling in agreement with
the Kardar-Parisi-Zhang (KPZ) universality class when growth in the system is
bulk driven. Interestingly, we also find the emergence of a previously
unreported hydrodynamic instability when proliferation is restricted to the
tissue edge. We then develop an analytical theory to show that the instability
arises due to a coupling between the number of cells actively proliferating and
the position of the interface. | cond-mat_soft |
Pentagon deposits unpack under gentle tapping: We present results from simulations of regular pentagons arranged in a
rectangular die. The particles are subjected to vertical tapping. We study the
behavior of the packing fraction, number of contacts and arch distributions as
a function of the tapping amplitude. Pentagons show peculiar features as
compared with disks. As a general rule, pentagons tend to form less arches than
disks. Nevertheless, as the tapping amplitude is decreased, the typical size of
the pentagon arches grows significantly. As a consequence, a pentagon packing
reduces its packing fraction when tapped gently in contrast with the behavior
found in rounded particle deposits. | cond-mat_soft |
PECVD and PEALD on polymer substrates (Part II): Understanding and
tuning of barrier and membrane properties of thin films: This feature article presents insights concerning the correlation of PECVD
and PEALD thin film structures with their barrier or membrane properties. While
in principle similar precursor gases and processes can be applied, the
adjustment of deposition parameters for different polymer substrates can lead
to either an effective diffusion barrier or selective permeabilities. In both
cases the understanding of the film growth and the analysis of the pore size
distribution and the pore surface chemistry is of utmost importance for the
understanding of the related transport properties of small molecules. In this
regard the article presents both concepts of thin film engineering and
analytical as well as theoretical approaches leading to a comprehensive
description of the state of the art in this field. Moreover, based on the
presented correlation of film structure and molecular transport properties
perspectives of future relevant research in this area is presented. | cond-mat_soft |
Spin-lattice NMR relaxation by anomalous translational diffusion: A model-free theoretical framework for a phenomenological description of
spin-lattice relaxation by anomalous translational diffusion in inhomogeneous
systems based on the fractional diffusion equation is developed. The dependence
of the spin-lattice relaxation time on the size of the pores in porous glass
Vycor is experimentally obtained and found to agree well with our theoretical
predictions. We obtain nonmonotonic behavior of the translational spin-lattice
relaxation rate constant (it passes through a maximum) with the variation of
the parameter referring to the extent of inhomogeneity of the system. | cond-mat_soft |
Geometric and Topological Entropies of Sphere Packing: We present a statistical mechanical description of randomly packed spherical
particles, where the average coordination number is treated as a macroscopic
thermodynamic variable. The overall packing entropy is shown to have two
contributions: geometric, reflecting statistical weights of individual
configurations, and topological, which corresponds to the number of
topologically distinct states. Both of them are computed in the thermodynamic
limit for isostatic packings in 2D and 3D, and the result is further expanded
to the case of "floppy" particle clusters. The theory is directly applicable to
sticky colloids, and in addition, generalizes concepts of granular and glassy
configurational entropies for the case of non-jammed systems. | cond-mat_soft |
Physical processes causing the formation of penitentes: Snow penitentes form in sublimation conditions by differential ablation. Here
we investigate the physical processes at the initial stage of penitente growth
and perform the linear stability analysis of a flat surface submitted to the
solar heat flux. We show that these patterns do not simply result from the
self-illumination of the surface --a scale-free process-- but are primarily
controlled by vapor diffusion and heat conduction. The wavelength at which snow
penitentes emerge is derived and discussed. We found that it is controlled by
aerodynamic mixing of vapor above the ice surface. | cond-mat_soft |
Partitioning of energy in highly polydisperse granular gases: A highly polydisperse granular gas is modeled by a continuous distribution of
particle sizes, a, giving rise to a corresponding continuous temperature
profile, T(a), which we compute approximately, generalizing previous results
for binary or multicomponent mixtures. If the system is driven, it evolves
towards a stationary temperature profile, which is discussed for several
driving mechanisms in dependence on the variance of the size distribution. For
a uniform distribution of sizes, the stationary temperature profile is
nonuniform with either hot small particles (constant force driving) or hot
large particles (constant velocity or constant energy driving). Polydispersity
always gives rise to non-Gaussian velocity distributions. Depending on the
driving mechanism the tails can be either overpopulated or underpopulated as
compared to the molecular gas. The deviations are mainly due to small
particles. In the case of free cooling the decay rate depends continuously on
particle size, while all partial temperatures decay according to Haff's law.
The analytical results are supported by event driven simulations for a large,
but discrete number of species. | cond-mat_soft |
Self-assembly of binary solutions to complex structures: Self-assembly in natural and synthetic molecular systems can create complex
aggregates or materials whose properties and functionality rises from their
internal structure and molecular arrangement. The key microscopic features that
control such assemblies remain poorly understood, nevertheless. Using classical
density functional theory we demonstrate how the intrinsic length scales and
their interplay in terms of interspecies molecular interactions can be used to
tune soft matter self-assembly. We apply our strategy to two different soft
binary mixtures to create guidelines for tuning intermolecular interactions
that lead to transitions from fully miscible, liquid-like uniform state to
formation of simple and core-shell aggregates, and mixed aggregate structures.
Furthermore, we demonstrate how the interspecies interactions and system
composition can be used to control concentration gradients of component species
within these assemblies. The insight generated by this work contributes towards
understanding and controlling soft multi-component self-assembly systems.
Additionally, our results aid in understanding complex biological assemblies
and their function and provide tools to engineer molecular interactions in
order to control polymeric and protein-based materials, pharmaceutical
formulations, and nanoparticle assemblies. | cond-mat_soft |
On the Granular Stress-Geometry Equation: Using discrete calculus, we derive the missing stress-geometry equation for
rigid granular materials in two dimensions, in the mean-field approximation. We
show that (i) the equation imposes that the voids cannot carry stress, (ii)
stress transmission is generically elliptic and has a quantitative relation to
anisotropic elasticity, and (iii) the packing fabric plays an essential role. | cond-mat_soft |
Spontaneous stable rotation of flocking flexible active matter: In nature, active matter, such as worms or dogs, tend to spontaneously form a
stable rotational cluster when they flock to the same food source on an
unregulated and unconfined surface. {In this paper we present an $n$-node
flexible active matter model to study the collective motion due to the flocking
of individual agents on a two-dimensional surface, and confirm that there
exists a spontaneous stable cluster rotation synchronizing with a chirality
produced by the alignment of their bodies under the impetus of the active
force.} A prefactor of 1.86 is obtained for the linear relationship between
normalized angular velocity and chirality. The angular velocity of such a
rotation is found to be dependent on the individual flexibility, the number of
nodes in each individual, and the magnitude of the active force. The
conclusions well explain the spontaneous stable rotation of clusters that
exists in many flexible active matter, like worms or {dogs}, when they flock to
the same single source. | cond-mat_soft |
An Ising model for the thermal and dynamic properties of supercooled
liquids and the glass transition: We describe the behavior of an Ising model with orthogonal dynamics, where
changes in energy and changes in alignment never occur during the same Monte
Carlo (MC) step. This orthogonal Ising model (OIM) allows conservation of
energy and conservation of momentum to proceed independently, on their own
preferred time scales. MC simulations of the OIM mimic more than twenty
distinctive characteristics that are commonly found above and below the glass
temperature, Tg. Examples include a specific heat that has hysteresis around
Tg, out-of-phase loss that exhibits primary and secondary peaks,
super-Arrhenius T dependence for the alpha response time, and fragilities that
increase with increasing system size (N). Mean-field theory for energy
fluctuations in the OIM yields a novel expression for the super-Arrhenius
divergence. Because this divergence is reminiscent of the Vogel-Fulcher-Tammann
(VFT) law squared, we call it the VFT2 law. A modified Stickel plot, which
linearizes the VFT2 law, gives qualitatively consistent agreement with
measurements of primary response (from the literature) on five glass-forming
liquids. Such agreement with the OIM suggests that several basic features
govern supercooled liquids. The freezing of a liquid into a glass involves an
underlying 2nd-order transition that is broadened by finite-size effects. The
VFT2 law comes from energy fluctuations that enhance the pathways through an
entropy bottleneck, not activation over an energy barrier. Primary response
times vary exponentially with inverse N, consistent with the distribution of
relaxation times deduced from measurements. System sizes found via the T
dependence of the primary response are similar to sizes of independently
relaxing regions measured by nuclear magnetic resonance for simple-molecule
glass-forming liquids. The OIM provides a broad foundation for more-detailed
models of liquid-glass behavior. | cond-mat_soft |
Entropically Driven Helix Formation: The helix is a ubiquitous motif for biopolymers. We propose a heuristic,
entropically based model that predicts helix formation in a system of hard
spheres and semiflexible tubes. We find that the entropy of the spheres is
maximized when short stretches of the tube form a helix with a geometry close
to that found in natural helices. Our model could be directly tested with
wormlike micelles as the tubes, and the effect could be used to self-assemble
supramolecular helices. | cond-mat_soft |
How to make and trap a pseudo-vesicle with a micropipette: We present a simple method to produce giant lipid pseudo-vesicles (vesicles
with an oily cap on the top), trapped in an agarose gel. The method can be
implemented using only a regular micropipette and relies on the formation of a
water/oil/water double droplet in liquid agarose. We characterize the produced
vesicle with fluorescence imaging and establish the presence and integrity of
the lipid bilayer by the successful insertion of {\alpha}-Hemolysin
transmembrane proteins. Finally, we show that the vesicle can be easily
mechanically deformed, non-intrusively, by indenting the surface of the gel. | cond-mat_soft |
Nucleation-induced transition to collective motion in active systems: While the existence of polar ordered states in active systems is well
established, the dynamics of the self-assembly processes are still elusive. We
study a lattice gas model of self-propelled elongated particles interacting
through excluded volume and alignment interactions, which shows a phase
transition from an isotropic to a polar ordered state. By analyzing the
ordering process we find that the transition is driven by the formation of a
critical nucleation cluster and a subsequent coarsening process. Moreover, the
time to establish a polar ordered state shows a power-law divergence. | cond-mat_soft |
A multi-species exchange model for fully fluctuating polymer field
theory simulations: Field-theoretic models have been used extensively to study the phase behavior
of inhomogeneous polymer melts and solutions, both in self-consistent
mean-field calculations and in numerical simulations of the full theory
capturing composition fluctuations. The models commonly used can be grouped
into two categories, namely {\it species} models and {\it exchange} models.
Species models involve integrations of functionals that explicitly depend on
fields originating both from species density operators and their conjugate
chemical potential fields. In contrast, exchange models retain only linear
combinations of the chemical potential fields. In the two-component case,
development of exchange models has been instrumental in enabling stable complex
Langevin (CL) simulations of the full complex-valued theory. No comparable
stable CL approach has yet been established for field theories of the species
type. Here we introduce an extension of the exchange model to an arbitrary
number of components, namely the multi-species exchange (MSE) model, which
greatly expands the classes of soft material systems that can accessed by the
complex Langevin simulation technique. We demonstrate the stability and
accuracy of the MSE-CL sampling approach using numerical simulations of
triblock and tetrablock terpolymer melts, and tetrablock quaterpolymer melts.
This method should enable studies of a wide range of fluctuation phenomena in
multiblock/multi-species polymer blends and composites. | cond-mat_soft |
Optical responses through dilute anisotropic composites: Numerical
calculations via Green's-function formalism: We investigate the linear and nonlinear optical responses of dilute
anisotropic networks using the Green's-function formalism (GFF)[Gu Y et al.
1999 Phys. Rev. B 59 12847]. For the different applied fields, numerical
calculations indicate that a large third order nonlinear enhancement and a
broad infrared absorption arise from the geometric anisotropy. It is also shown
the overlap and separation between the absorption peak and nonlinear
enhancement peak when the applied field is parallel, perpendicular to the
anisotropy respectively. In terms of the inverse participation ratios (IPR)
with q=2 and spectral distribution of optical responses, the results can be
understood. | cond-mat_soft |
Ordering transitions of weakly anisotropic hard rods in narrow slit-like
pores: The effect of strong confinement on the positional and orientational ordering
is examined in a system of hard rectangular rods with length L and diameter D
(L>D) using the Parsons-Lee modification of the second virial density
functional theory. The rods are nonmesogenic (L/D<3)and confined between two
parallel hard walls, where the width of the pore (H) is chosen in such a way
that both planar (particle's long axis parallel to the walls) and homeotropic
(particle's long axis perpendicular to the walls) orderings are possible and a
maximum of two layers are allowed to form in the pore. In the extreme
confinement limit of ,where only one layer structures appear, we observe a
structural transition from a planar to a homeotropic fluid layer with
increasing density, which becomes sharper as L->H. In wider pores (2D<H<3D)
planar order with two layers, homeotropic order, and even combined bilayer
structures (one layer is homeotropic, while the other is planar) can be
stabilized at high densities. Moreover, first order phase transitions can be
seen between different structures. One of them emerges between a monolayer and
a bilayer with planar orders at relatively low packing fractions. | cond-mat_soft |
Actuation performances of anisotropic gels: We investigated the actuation performances of anisotropic gels driven by
mechanical and chemical stimuli, in terms of both deformation processes and
stroke--curves, and distinguished between the fast response of gels before
diffusion starts and the asymptotic response attained at the steady state. We
also showed as the range of forces that an anisotropic hydrogel can exert when
constrained is especially wide;indeed, changing fiber orientation allows to
induce shear as well as transversely isotropic extensions. | cond-mat_soft |
Membrane stress and torque induced by Frank's nematic textures: A
geometric perspective using surface-based constraints: An elastic membrane with embedded nematic molecules is considered as a model
of anisotropic fluid membrane with internal ordering. By considering the
geometric coupling between director field and membrane curvature, the nematic
texture is shown to induce anisotropic stresses additional to Canham-Helfrich
elasticity. Building upon differential geometry, analytical expressions are
found for the membrane stress and torque induced by splaying, twisting and
bending of the nematic director as described by the Frank energy of liquid
crystals. The forces induced by prototypical nematic textures are visualized on
the sphere and on cylindrical surfaces. | cond-mat_soft |
Hierarchical microphase separation in non-conserved active mixtures: Non-equilibrium phase separating systems with reactions can break
time-reversal symmetry (TRS) in two distinct ways. Firstly, the conservative
and non-conservative sectors of the dynamics can be governed by incompatible
free energies; when both sectors are present, this is the leading-order TRS
violation, captured in its simplest form by 'Model AB'. Second, the diffusive
dynamics can break TRS in its own right. This happens only at higher order in
the gradient expansion (but is the leading behaviour without reactions present)
and is captured by 'Active Model B+' (AMB+). Each of the two mechanisms can
lead to microphase separation, by quite different routes. Here we introduce
Model AB+, for which both mechanisms are simultaneously present, and show that
for slow reaction rates the system can undergo a new type of hierarchical
microphase separation, whereby a continuous phase of fluid 1 contains large
droplets of fluid 2 within which small droplets of fluid 1 are continuously
created and then absorbed into the surrounding fluid-1 phase. In this state of
'bubbly microphase separation' the small-scale 1-in-2 droplets arise by the
conservative diffusive dynamics with the larger scale 2-in-1 structure governed
by the nonconservative reactions. | cond-mat_soft |
2D to 3D transition in soap films demonstrated by microrheology: We follow the diffusive motion of colloidal particles of diameter $d$ in soap
films of varying thickness $h$ with fluorescence microscopy. Diffusion
constants are obtained both from one- and two-particle microrheological
measurements of particle motion in these films. These diffusion constants are
related to the surface viscosity of the interfaces comprising the soap films,
by means of the Trapeznikov approximation [A. A. Trapeznikov, \emph{PICSA}
(1957)] and Saffman's equation for diffusion in a 2D fluid. Unphysical values
of the surface viscosity are found for thick soap films ($h/d > 7$), indicating
a transition from 2D to 3D behavior. | cond-mat_soft |
Stopping and reversing sound via dynamic dispersion tuning in a phononic
metamaterial: Slowing down, stopping, and reversing a signal is a core functionality for
information processing. Here, we show that this functionality can be realized
by tuning the dispersion of a periodic system through a dispersionless, or
flat, band. Specifically, we propose a phononic metamaterial based on plate
resonators, in which the phonon band dispersion can be modified from an
acoustic-like to an optical character by modulating a uniform prestress. The
switch is enabled by the change in sign of an effective coupling between
fundamental modes, which generically leads to a nearly dispersion-free band at
the transition point. We demonstrate how adiabatic tuning of the band
dispersion can immobilize and reverse the propagation of a sound pulse in
simulations of a one-dimensional resonator chain. Our study relies on the basic
principles of thin-plate elasticity independently of any specific material,
making our results applicable across varied length scales and experimental
platforms. More broadly, our approach could be metamaterials and electronic
heterostructures. | cond-mat_soft |
Exceptional Anti-Icing Performance of Self-Impregnating Slippery
Surfaces: A heat exchange interface at subzero temperature in a water vapor
environment, exhibits high probability of frost formation due to freezing
condensation, a factor that markedly decreases the heat transfer efficacy due
to the considerable thermal resistance of ice. Here we report a novel strategy
to delay ice nucleation on these types of solid-water vapor interfaces. With a
process-driven mechanism, a self-generated liquid intervening layer immiscible
to water, is deposited on a textured superhydrophobic surface and acts as a
barrier between the water vapor and the solid substrate. This liquid layer
imparts remarkable slippery conditions resulting in high mobility of condensing
water droplets. A large increase of the ensuing ice coverage time is shown
compared to the cases of standard smooth hydrophilic or textured
superhydrophobic surfaces. During deicing of these self-impregnating surfaces
we show an impressive tendency of ice fragments to skate expediting defrosting.
Robustness of such surfaces is also demonstrated by operating them under
subcooling for at least 490hr without a marked degradation. This is attributed
to the presence of the liquid intervening layer, which protects the substrate
from hydrolyzation enhancing longevity and sustaining heat transfer efficiency. | cond-mat_soft |
Surface sulci in squeezed soft solids: The squeezing of soft solids, the constrained growth of biological tissues,
and the swelling of soft elastic solids such as gels can generate large
compressive stresses at their surfaces. This causes the otherwise smooth
surface of such a solid to becomes unstable when its stress exceeds a critical
value. Previous analyses of the surface instability have assumed
two-dimensional plane-strain conditions, but in experiments isotropic stresses
often lead to complex three-dimensional sulcification patterns. Here we show
how such diverse morphologies arise by numerically modeling the lateral
compression of a rigidly clamped elastic layer. For incompressible solids,
close to the instability threshold, sulci appear as I-shaped lines aligned
orthogonally with their neighbors; at higher compressions they are Y-shaped and
prefer a hexagonal arrangement. In contrast, highly compressible solids when
squeezed show only one sulcified phase characterized by a hexagonal sulcus
network. | cond-mat_soft |
Asymptotic Analysis of Diffuse-Layer Effects on Time-Dependent
Interfacial Kinetics: We investigate the subtle effects of diffuse charge on interfacial kinetics
by solving the governing equations for ion transport (Nernst-Planck) with
realistic boundary conditions representing reaction kinetics (Butler-Volmer)
and compact-layer capacitance (Stern) in the asymptotic limit $\epsilon =
\lambda_D/L \to 0$, where $\lambda_D$ is the Debye screening length and $L$ is
the distance between the working and counter electrodes. Using the methods of
singular perturbation theory, we derive the leading-order steady-state response
to a nonzero applied current in the case of the oxidation of a neutral species
into cations, without any supporting electrolyte. In certain parameter regimes,
the theory predicts a reaction-limited current smaller than the classical
diffusion-limited current. We also analyze the impedance of the electrochemical
cell when a small AC current modulation is added to an applied DC current. At
sufficiently high AC frequencies, the Maxwell displacement current is found to
exceed the Faradaic conduction current, and experimentally observed ``negative
impedances'' (out of phase AC voltage responses) are predicted close to the
reaction-limited current. Overall, we demonstrate that the dynamics of diffuse
charge plays a fundamental role in nonequilibrium surface reactions when the
transport of one of the reacting species is coupled to the total interfacial
reponse of the compact and diffuse layers. | cond-mat_soft |
Bistability in a self-assembling system confined by elastic walls. Exact
results in a one-dimensional lattice model: The impact of confinement on self-assembly of particles interacting with
short-range attraction and long-range repulsion (SALR) potential is studied for
thermodynamic states corresponding to local ordering of clusters or layers in
the bulk. Exact and asymptotic expressions for the local density and for the
effective potential between the confining surfaces are obtained for a
one-dimensional lattice model introduced in [J. P\k{e}kalski et al. $J. Chem.
Phys.$ ${\bf 140}$, 144903 (2013)].The simple asymptotic formulas are shown to
be in good quantitative agreement with exact results for slits containing at
least 5 layers. We observe that the incommensurability of the system size and
the average distance between the clusters or layers in the bulk leads to
structural deformations that are different for different values of the chemical
potential $\mu$. The change of the type of defects is reflected in the
dependence of density on $\mu$ that has a shape characteristic for phase
transitions. Our results may help to avoid misinterpretation of the change of
the type of defects as a phase transition in simulations of inhomogeneous
systems. Finally, we show that a system confined by soft elastic walls may
exhibit bistability such that two system sizes that differ approximately by the
average distance between the clusters or layers are almost equally probable.
This may happen when the equilibrium separation between the soft boundaries of
an empty slit corresponds to the largest stress in the confined self-assembling
system. | cond-mat_soft |
Time resolved viscoelastic properties during structural arrest and aging
of a colloidal glass: Evolution of the energy landscape during physical aging of glassy materials
can be understood from the frequency and strain dependence of the shear modulus
but the non-stationary nature of these systems frustrates investigation of
their instantaneous underlying properties. Using a series of time dependent
measurements we systematically reconstruct the frequency and strain dependence
as a function of age for a repulsive colloidal glass undergoing structural
arrest. In this manner, we are able to unambiguously observe the structural
relaxation time, which increases exponentially with sample age at short times.
The yield stress varies logarithmically with time in the arrested state,
consistent with recent simulation results, whereas the yield strain is nearly
constant in this regime. Strikingly, the frequency dependence at fixed times
can be rescaled onto a master curve, implying a simple connection between the
aging of the system and the change in the frequency dependent modulus. | cond-mat_soft |
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