arXiv subjects

Vishwas V. Vasisht

Publications and source records attributed to Vishwas V. Vasisht.

10 recordsLinked to original sources

arXiv2026

Origin of Persistent Boundary Motion in Confined Active Matter

Active matter systems under confinement display persistent surface motion and a strong boundary affinity. However, despite extensive studies of their positional dynamics, much less attention has been given to the corresponding orientational behavior. Here, using molecular simulations of an active Brownian particle confined within a hard circular boundary and the Fokker-Planck equation, we show that the positional distribution of the particle is directly coupled to orientational fluctuations, as characterized by the conditional orientational distribution. Confinement generates two preferred tangential orientational states connected by stochastic flipping pathways: rapid boundary-localized switching and slower bulk-mediated excursions. Further, the positional distribution exhibits a nontrivial power-law decay with distance from the boundary that is closely linked to curvature-induced bistable orientational states and the variance of the associated conditional distribution. The mean waiting time between flips exhibits power-law dependence on the confinement strength. Our results establish that the interplay between orientational fluctuations, bistability, positional accumulation, and stochastic switching governs the observed dynamics of active particles under confinement, providing a framework for understanding transport, exploration, and escape processes in confined active systems.

cond-mat.soft↗

arXiv2025

Yielding in amorphous solids reveals an age-dependent intrinsic lengthscale

Understanding how amorphous solids yield under shear is central to predicting material failure, yet prescribing reliable local yielding criteria remains a fundamental challenge. Here, through a mesoscale analysis of localized yielding, we reveal an intrinsic length scale (ζ) that governs local failure, and demonstrate that ζgrows with the age of the system. The age dependence shows up not only in the features of the distribution of local yield stress but also in the pseudogap exponent θ, which provides a measure of marginal stability of the amorphous solids. These insights are made possible by a new method, termed the soft matrix approach, that allows local regions of an amorphous solid to yield within a minimally constrained, elastically coupled environment. By overcoming key limitations of earlier techniques, our approach provides a robust platform for probing failure mechanisms, particularly in soft disordered materials and paves the way for improved elastoplastic modeling of disordered solids.

cond-mat.soft↗

arXiv2025

Breakdown of the Stokes-Einstein relation in Stillinger-Weber Silicon

We investigate the dynamical properties of liquid and supercooled liquid silicon, modeled using the Stillinger-Weber (SW) potential, to examine the validity of the Stokes-Einstein (SE) relation. Towards this end, we examine the relationship among various dynamical quantities, including (i) the macroscopic transport coefficients - self diffusion coefficient $D$ and viscosity $η$, (ii) relaxation time $τ_α$ as well as (iii) lengthscale dependent relaxation times $τ_α(q)$ over a broad range of temperature $T$, pressure $P$ and density $ρ$ covering both equilibrium and metastable liquid state points in the phase diagram. Our study shows a weak break down in SE relation involving $D$ and $η$, and the loci of \Revision{the breakdown of the SE relation} (SEB) is found in the high T liquid phase. The $τ_α$, when used as a proxy to $η$, shows distinct breakdown in the SE relation whose loci is found in the supercooled liquid phase. Interestingly, certain parts of the phase diagram shows that loci of onset of slow dynamics lie below the loci of SEB, suggesting a regime that exhibits Arrhenius but non-Fickian behaviour. Computation of $τ_α(q)$ enables us to extract the lengthscale associated with the Fickian to non-Fickian behaviour using which we show that the \Revision{breakdown of the SE relation} occurs only below a specific lengthscale at a given temperature. Further we also compare the SEB loci with other features of the phase behaviour, including the loci of compressiblity maximum, density maximum as well as diffusivity maximum.

cond-mat.mtrl-sci↗

arXiv2023

Memory of shear flow in soft jammed materials

Cessation of flow in simple yield stress fluids results in a complex stress relaxation process that depends on the preceding flow conditions and leads to finite residual stresses. To assess the microscopic origin of this phenomenon, we combine experiments with largescale computer simulations, exploring the behavior of jammed suspensions of soft repulsive particles. A spatio-temporal analysis of microscopic particle motion and local particle configurations reveals two contributions to stress relaxation. One is due to flow induced accumulation of elastic stresses in domains of a given size, which effectively sets the unbalanced stress configurations that trigger correlated dynamics upon flow cessation. This scenario is supported by the observation that the range of spatial correlations of quasi-ballistic displacements obtained upon flow cessation almost exactly mirrors those obtained during flow. The second contribution results from the particle packing that reorganize to minimize the resistance to flow by decreasing the number of locally stiffer configurations. Regaining rigidity upon flow cessation then effectively sets the magnitude of the residual stress. Our findings highlight that flow in yield stress fluids can be seen as a training process during which the material stores information of the flowing state through the development of domains of correlated particle displacements and the reorganization of particle packings optimized to sustain the flow. This encoded memory can then be retrieved in flow cessation experiments.

cond-mat.soft↗

arXiv2021

Liquid-Liquid Phase Transition in Supercooled Silicon

We present a review on the study of metastable silicon, primarily focusing mainly on the aspects of liquid-liquid transition, critical point and phase behaviour, structural and dynamic properties of liquid phase as well as crystal nucleation. We begin with an extensive survey of the investigations of liquid silicon pursued over three decades, with salient experimental, theoretical and simulation results. Following which we present various scenarios put forward to rationalize the density and related anomalies often observed in water and other network forming liquids. After which we present the more recent investigations (both simulation and experimental works) of the phase behavior of Silicon. Since a significant part of metastable silicon work is on a classical empirical potential an important question to address is the reliability of this potential in describing the behavior of silicon. To provide a critical assessment of the applicability of classical simulation results to real silicon we present a comparison of the structural, dynamical, and thermodynamic quantities obtained from the SW potential with those from ab initio simulations and with available experimental data. We also discuss the sensitivity of the thermodynamic properties to model parameters.

cond-mat.mtrl-sci↗

arXiv2021

Thermodynamics and kinetics of crystallisation in deeply supercooled Stillinger-Weber silicon

We study the kinetics of crystallization in deeply supercooled liquid silicon employing computer simulations and the Stillinger-Weber three body potential. The free energy barriers to crystallisation are computed using umbrella sampling Monte Carlo simulations, and for selected low temperature and zero pressure state points, using unconstrained molecular dynamics simulations to reconstruct the free energy from a mean first passage time formulation. We focus on state points that have been described in earlier work [Sastry and Angell, Nature Mater., 2, 739, 2003] as straddling a first order liquid-liquid phase transition (LLPT) between two metastable liquid states. It was argued subsequently [Ricci et al., Mol. Phys., 117, 3254, 2019] that the apparent phase transition is in fact due the loss of metastability of the liquid state with respect to the globally stable crystalline state. The presence or absence of a barrier to crystallization for these state points is therefore of importance to ascertain, with due attention to ambiguities that may arise from the choice of order parameters. We discuss our choice of order parameters and also our choice of methods to calculate the free energy at deep supercooling. We find a well-defined free energy barrier to crystallisation and demonstrate that both umbrella sampling and mean first passage time methods yield results that agree quantitatively. Our results thus provide strong evidence against the possibility that the liquids at state points close to the reported LLPT exhibit slow, spontaneous crystallisation, but they do not address the existence of a LLPT (or lack thereof). We also compute the free energy barriers to crystallisation at other state points over a broad range of temperatures and pressures, and discuss the effect of changes in the microscopic structure of the metastable liquid on the free energy barrier heights.

cond-mat.soft↗

arXiv2021

Residual stress in athermal soft disordered solids: insights from microscopic and mesoscale models

In soft amorphous materials, shear cessation after large shear deformation leads to structures having residual shear stress. The origin of these states and the distribution of the local shear stresses within the material is not well understood, despite its importance for the change in material properties and consequent applications. In this work, we use molecular dynamics simulations of a model dense non-Brownian soft amorphous material to probe the non-trivial relaxation process towards a residual stress state. We find that, similar to thermal glasses, an increase in shear rate prior to the shear cessation leads to lower residual stress states. We rationalise our findings using a mesoscopic elasto-plastic description that explicitly includes a long range elastic response to local shear transformations. We find that after flow cessation the initial stress relaxation indeed depends on the pre-sheared stress state, but the final residual stress is majorly determined by newly activated plastic events occurring during the relaxation process. Our simplified coarse grained description not only allows to capture the phenomenology of residual stress states but also to rationalise the altered material properties that are probed using small and large deformation protocols applied to the relaxed material.

cond-mat.soft↗

arXiv2019

A computational study of transient shear banding in soft jammed solids

We have designed 3D numerical simulations of a soft spheres model, with size polidispersity and in athermal conditions, to study the transient shear banding that occurs during yielding of jammed soft solids. We analyze the effects of different types of drag coefficients used in the simulations and compare the results obtained using Lees-Edwards periodic boundary conditions with the case in which the same model solid is confined between two walls. The specific damping mechanism and the different boundary conditions indeed modify the load curves and the velocity profiles in the transient regime. Nevertheless, we find that the presence of a stress-overshoot and of a related transient banding phenomenon for large enough samples are a robust feature for overdamped systems, where their presence do not depend on the specific drag used and on the different boundary conditions.

cond-mat.soft↗

arXiv2017

Emergence and persistence of flow inhomogeneities in the yielding and fluidization of dense soft solids

The response to shear of the dense soft solids features a stress overshoot and a persistent shear banding before reaching a homogeneously flowing state. In 3D large scale simulations we analyze the time required for the onset of homogeneous flow, the normal stresses and structural signatures at different shear rates and in different flow geometries, finding that the stress overshoot, the shear band formation and its persistence are controlled by the presence of overconstrained microscopic domains in the initially solid samples. Being able to identify such domains in our model by prevalently icosahedrally packed regions, we show that they allow for stress accumulation during the stress overshoot and that their structural reorganization controls the emergence and the persistence of the shear banding.

cond-mat.soft↗

arXiv2016

Making soft solids flow: microscopic bursts and conga lines in jammed emulsions

It is well known that jammed soft materials will flow if sheared above their yield stress - think mayonnaise spread on bread - but a complete microscopic description of this seemingly sim- ple process has yet to emerge. What remains elusive is a microscopic framework that explains the macroscopic flow, derived from a 3-D spatially resolved analysis of the dynamics of the droplets or particles that compose the soft material. By combining confocal-rheology experiments on compressed emulsions and numerical simulations, we unravel that the primary microscopic mechanisms for flow are strongly influenced by the rate of the imposed deformation. When shearing fast, small coordinated clusters of droplets move collectively as in a conga line, while at low rates the flow emerges from bursts of droplet rearrangements, correlated over large domains. These regions exhibit complex spatio-temporal correlation patterns that reflect the long range elasticity embedded in the jammed material. These results identify the three-dimensional structure of microscopic rearrangements within sheared soft solids, revealing that the characteristic shape and dynamics of these structures are strongly determined by the rate of the external shear.

cond-mat.soft↗