36 research outputs found
Nontrivial rheological exponents in sheared yield stress fluids
In this work we discuss possible physical origins for non-trivial exponents
in the athermal rheology of soft materials at low but finite driving rates. A
key ingredient in our scenario is the presence of a self-consistent mechanical
noise that stems from the spatial superposition of long-range elastic responses
to localized plastically deforming regions. We study analytically a mean-field
model, in which this mechanical noise is accounted for by a stress diffusion
term coupled to the plastic activity. Within this description we show how a
dependence of the shear modulus and/or the local relaxation time on the shear
rate introduces corrections to the usual mean-field prediction, concerning the
Herschel-Bulkley-type rheological response of exponent 1/2. This feature of the
mean-field picture is then shown to be robust with respect to structural
disorder and partial relaxation of the local stress. We test this prediction
numerically on a mesoscopic lattice model that implements explicitly the
long-range elastic response to localized shear transformations, and we conclude
on how our scenario might be tested in rheological experiments
Out-of-equilibrium dynamical equations of infinite-dimensional particle systems. I. The isotropic case
We consider the Langevin dynamics of a many-body system of interacting
particles in dimensions, in a very general setting suitable to model
several out-of-equilibrium situations, such as liquid and glass rheology,
active self-propelled particles, and glassy aging dynamics. The pair
interaction potential is generic, and can be chosen to model colloids, atomic
liquids, and granular materials. In the limit , we show that the
dynamics can be exactly reduced to a single one-dimensional effective
stochastic equation, with an effective thermal bath described by kernels that
have to be determined self-consistently. We present two complementary
derivations, via a dynamical cavity method and via a path-integral approach.
From the effective stochastic equation, one can compute dynamical observables
such as pressure, shear stress, particle mean-square displacement, and the
associated response function. As an application of our results, we derive
dynamically the `state-following' equations that describe the response of a
glass to quasistatic perturbations, thus bypassing the use of replicas. The
article is written in a modular way, that allows the reader to skip the details
of the derivations and focus on the physical setting and the main results
Static fluctuations of a thick 1D interface in the 1+1 Directed Polymer formulation
Experimental realizations of a 1D interface always exhibit a finite
microscopic width ; its influence is erased by thermal fluctuations at
sufficiently high temperatures, but turns out to be a crucial ingredient for
the description of the interface fluctuations below a characteristic
temperature . Exploiting the exact mapping between the static 1D
interface and a 1+1 Directed Polymer (DP) growing in a continuous space, we
study analytically both the free-energy and geometrical fluctuations of a DP,
at finite temperature , with a short-range elasticity and submitted to a
quenched random-bond Gaussian disorder of finite correlation length .
We derive the exact `time'-evolution equations of the disorder free-energy
, its derivative , and their respective two-point
correlators and . We compute the exact solution of
its linearized evolution , and we combine its qualitative
behavior and the asymptotic properties known for an uncorrelated disorder
(), to construct a `toymodel' leading to a simple description of the DP.
This model is characterized by Brownian-like free-energy fluctuations,
correlated at small , of amplitude . We
present an extended scaling analysis of the roughness predicting
at high-temperatures and at low-temperatures. We identify the connection between the
temperature-induced crossover and the full replica-symmetry breaking in
previous Gaussian Variational Method computations. Finally we discuss the
consequences of the low-temperature regime for two experimental realizations of
KPZ interfaces, namely the static and quasistatic behavior of magnetic domain
walls and the high-velocity steady-state dynamics of interfaces in liquid
crystals.Comment: 33 pages, 6 figures. The initial preprint arXiv:1209.0567v1 has been
split into two parts upon refereeing process. The first part gathers the
analytical results and is published (see reference below). It corresponds to
the current version of arXiv:1209.0567. The second part gathers the numerical
results and corresponds the other arXiv preprint arXiv:1305.236
On the relevance of disorder in athermal amorphous materials under shear
We show that, at least at a mean-field level, the effect of structural
disorder in sheared amorphous media is very dissimilar depending on the thermal
or athermal nature of their underlying dynamics. We first introduce a toy
model, including explicitly two types of noise (thermal versus athermal).
Within this interpretation framework, we argue that mean-field athermal
dynamics can be accounted for by the so-called H{\'e}braud-Lequeux (HL) model,
in which the mechanical noise stems explicitly from the plastic activity in the
sheared medium. Then, we show that the inclusion of structural disorder, by
means of a distribution of yield energy barriers, has no qualitative effect in
the HL model, while such a disorder is known to be one of the key ingredients
leading kinematically to a finite macroscopic yield stress in other mean-field
descriptions, such as the Soft-Glassy-Rheology model. We conclude that the
statistical mechanisms at play in the emergence of a macroscopic yield stress,
and a complex stationary dynamics at low shear rate, are different in thermal
and athermal amorphous systems
Mean-field dynamics of infinite-dimensional particle systems: global shear versus random local forcing
In infinite dimension, many-body systems of pairwise interacting particles
provide exact analytical benchmarks for features of amorphous materials, such
as the stress-strain curve of glasses under quasistatic shear. Here, instead of
a global shear, we consider an alternative driving protocol as recently
introduced in Ref. [1], which consists of randomly assigning a constant local
displacement on each particle, with a finite spatial correlation length. We
show that, in the infinite-dimension limit, the mean-field dynamics under such
a random forcing is strictly equivalent to that under global shear, upon a
simple rescaling of the accumulated strain. Moreover, the scaling factor is
essentially given by the variance of the relative local displacements on
interacting pairs of particles, which encodes the presence of a finite spatial
correlation. In this framework, global shear is simply a special case of a much
broader family of local forcing, that can be explored by tuning its spatial
correlations. We discuss specifically the implications on the quasistatic
driving of glasses -- initially prepared at a replica-symmetric equilibrium --
and how the corresponding 'stress-strain'-like curves and the elastic moduli
can be rescaled onto their quasistatic-shear counterparts. These results hint
at a unifying framework for establishing rigourous analogies, at the mean-field
level, between different driven disordered systems
Out-of-equilibrium dynamical equations of infinite-dimensional particle systems. II. The anisotropic case under shear strain
As an extension of the isotropic setting presented in the companion paper [J.
Phys. A 52, 144002 (2019)], we consider the Langevin dynamics of a many-body
system of pairwise interacting particles in dimensions, submitted to an
external shear strain. We show that the anisotropy introduced by the shear
strain can be simply addressed by moving into the co-shearing frame, leading to
simple dynamical mean field equations in the limit . The dynamics
is then controlled by a single one-dimensional effective stochastic process
which depends on three distinct strain-dependent kernels - self-consistently
determined by the process itself - encoding the effective restoring force,
friction and noise terms due to the particle interactions. From there one can
compute dynamical observables such as particle mean-square displacements and
shear stress fluctuations, and eventually aim at providing an exact benchmark for liquid and glass rheology. As an application of our
results, we derive dynamically the 'state-following' equations that describe
the static response of a glass to a finite shear strain until it yields.Comment: Typo corrected in Eq. (47
Explaining the effects of non-convergent sampling in the training of Energy-Based Models
In this paper, we quantify the impact of using non-convergent Markov chains
to train Energy-Based models (EBMs). In particular, we show analytically that
EBMs trained with non-persistent short runs to estimate the gradient can
perfectly reproduce a set of empirical statistics of the data, not at the level
of the equilibrium measure, but through a precise dynamical process. Our
results provide a first-principles explanation for the observations of recent
works proposing the strategy of using short runs starting from random initial
conditions as an efficient way to generate high-quality samples in EBMs, and
lay the groundwork for using EBMs as diffusion models. After explaining this
effect in generic EBMs, we analyze two solvable models in which the effect of
the non-convergent sampling in the trained parameters can be described in
detail. Finally, we test these predictions numerically on the Boltzmann
machine.Comment: 13 pages, 3 figure
From bulk descriptions to emergent interfaces: connecting the Ginzburg-Landau and elastic line models
Controlling interfaces is highly relevant from a technological point of view.
However, their rich and complex behavior makes them very difficult to describe
theoretically, and hence to predict. In this work, we establish a procedure to
connect two levels of descriptions of interfaces: for a bulk description, we
consider a two-dimensional Ginzburg-Landau model evolving with a Langevin
equation, and boundary conditions imposing the formation of a rectilinear
domain wall. At this level of description no assumptions need to be done over
the interface, but analytical calculations are almost impossible to handle. On
a different level of description, we consider a one-dimensional elastic line
model evolving according to the Edwards-Wilkinson equation, which only allows
one to study continuous and univalued interfaces, but which was up to now one
of the most successful tools to treat interfaces analytically. To establish the
connection between the bulk description and the interface description, we
propose a simple method that applies both to clean and disordered systems. We
probe the connection by numerical simulations at both levels, and our
simulations, in addition to making contact with experiments, allow us to test
and provide insight to develop new analytical approaches to treat interfaces
Driven interfaces: from flow to creep through model reduction
Accepted for publication in the Journal of Statistical Physics.The response of spatially extended systems to a force leading their steady state out of equilibrium is strongly affected by the presence of disorder. We focus on the mean velocity induced by a constant force applied on one-dimensional interfaces. In the absence of disorder, the velocity is linear in the force. In the presence of disorder, it is widely admitted, as well as experimentally and numerically verified, that the velocity presents a stretched exponential dependence in the force (the so-called 'creep law'), which is out of reach of linear response, or more generically of direct perturbative expansions at small force. In dimension one, there is no exact analytical derivation of such a law, even from a theoretical physical point of view. We propose an effective model with two degrees of freedom, constructed from the full spatially extended model, that captures many aspects of the creep phe-nomenology. It provides a justification of the creep law form of the velocity-force characteristics, in a quasistatic approximation. It allows, moreover, to capture the non-trivial effects of short-range correlations in the disorder, which govern the low-temperature asymptotics. It enables us to establish a phase diagram where the creep law manifests itself in the vicinity of the origin in the force â system-size â temperature coordinates. Conjointly, we characterise the crossover between the creep regime and a linear-response regime that arises due to finite system size