arXiv subjects

Pinaki Chaudhuri

Publications and source records attributed to Pinaki Chaudhuri.

At least 55 records · Page 3Linked to original sources

arXiv2016

Ultra-long-range dynamic correlations in a microscopic model for aging gels

We use large-scale computer simulations to explore the non-equilibrium aging dynamics in a microscopic model for colloidal gels. We find that gelation resulting from a kinetically-arrested phase separation is accompanied by `anomalous' particle dynamics revealed by superdiffusive particle motion and compressed exponential relaxation of time correlation functions. Spatio-temporal analysis of the dynamics reveals intermittent heterogeneities producing spatial correlations over extremely large length scales. Our study is the first microscopically-resolved model reproducing all features of the spontaneous aging dynamics observed experimentally in soft materials.

cond-mat.soft↗

arXiv2015

Exploring structural inhomogeneities in glasses during cavitation

Using large-scale molecular dynamics simulations for a system of $10^6$ particles, the response of a dense amorphous solid to the continuous expansion of its volume is investigated. We find that the spatially uniform glassy state becomes unstable via the formation of cavities, which eventually leads to failure. By scanning through a wide range of densities and temperatures, we determine the state points at which the instability occurs and thereby provide estimates of the co-existence density of the resultant glass phase. Evidence for long-lived, inhomogeneous configurations with a negative pressure is found, where the frozen-in glass structure contains spherical cavities or a network of void space. Furthermore, we demonstrate the occurrence of hysteretic effects when the cavitated solid is compressed to regain the dense glassy state. As a result, a new glass state is obtained, the pressure of which is different from the initial one due to small density inhomogeneities that are generated by the dilation-compression cycle.

cond-mat.stat-mech↗

arXiv2015

Rheology of confined non-Brownian suspensions

We study the rheology of confined suspensions of neutrally buoyant rigid monodisperse spheres in plane-Couette flow using Direct Numerical Simulations. We find that if the width of the channel is a (small) integer multiple of the sphere's diameter, the spheres self-organize into two-dimensional layers that slide on each other and the suspension's effective viscosity is significantly reduced. Each two-dimensional layer is found to be structurally liquid-like but their dynamics is frozen in time.

cond-mat.soft↗

arXiv2015

Formation and growth of shear bands in glasses: existence of an underlying directed percolation transition

The response of glasses to mechanical loading often leads to the formation of inhomogeneous flow patterns that strongly affect materials properties. Among them, shear bands are ubiquitous in a wide variety of materials, ranging from soft matter systems to metallic alloys. Shear banding is associated with strain localization, i.e. the deformation of the sheared glassy solid is localized in space in form of band-like structures. These structures are often precursors to catastrophic failure, implying that a proper understanding of the underlying mechanisms could lead to the design of smarter materials. However, despite its importance in material science, the microscopic origin of shear banding in glassy materials is only poorly understood. Here, the formation of shear banding in glassy systems is revealed by non-equilibrium molecular dynamics simulations (NEMD) of a binary Lennard-Jones mixture, subject to a constant strain rate. In its glass state, this system exhibits for all considered strain rates the formation of a percolating cluster of mobile regions at a critical strain. We show that this percolation transition belongs to the universality class of directed percolation. Only at low shear rates, where the steady-state stress is close to the yielding threshold, the percolating cluster evolves into a transient (but long-lived) shear band with a diffusive growth of its width.

cond-mat.mtrl-sci↗

arXiv2015

Relaxation dynamics in a transient network fluid with competing gel and glass phases

We use computer simulations to study the relaxation dynamics of a model for oil-in-water microemulsion droplets linked with telechelic polymers. This system exhibits both gel and glass phases and we show that the competition between these two arrest mechanisms can result in a complex, three-step decay of the time correlation functions, controlled by two different localization lengthscales. For certain combinations of the parameters, this competition gives rise to an anomalous logarithmic decay of the correlation functions and a subdiffusive particle motion, which can be understood as a simple crossover effect between the two relaxation processes. We establish a simple criterion for this logarithmic decay to be observed. We also find a further logarithmically slow relaxation related to the relaxation of floppy clusters of particles in a crowded environment, in agreement with recent findings in other models for dense chemical gels. Finally, we characterize how the competition of gel and glass arrest mechanisms affects the dynamical heterogeneities and show that for certain combination of parameters these heterogeneities can be unusually large. By measuring the four-point dynamical susceptibility, we probe the cooperativity of the motion and find that with increasing coupling this cooperativity shows a maximum before it decreases again, indicating the change in the nature of the relaxation dynamics. Our results suggest that compressing gels to large densities produces novel arrested phases that have a new and complex dynamics.

cond-mat.soft↗

arXiv2014

Onset of flow in a confined colloidal glass under an imposed shear stress

A confined colloidal glass, under the imposition of a uniform shear stress, is investigated using numerical simulations. Both at macro- and microscales, the consequent dynamics during the onset of flow is studied. When the imposed stress is gradually decreased, the time scale for the onset of steady flow diverges, associated with long-lived spatial heterogeneities. Near this yield-stress regime, persistent creep in the form of shear-banded structures is observed.

cond-mat.soft↗

arXiv2014

Poiseuille flow of soft glasses in narrow channels: From quiescence to steady state

Using numerical simulations, the onset of Poiseuille flow in a confined soft glass is investigated. Starting from the quiescent state, steady flow sets in at a time scale which increases with a decrease in applied forcing. At this onset time scale, a rapid transition occurs via the simultaneous fluidization of regions having different local stresses. In the absence of steady flow at long times, creep is observed even in regions where the local stress is larger than the bulk yielding threshold. Finally, we show that the time scale to attain steady flow depends strongly on the history of the initial state.

cond-mat.soft↗

arXiv2014

Impact of attractive interactions on the rheology of dense athermal particles

Using numerical simulations, the rheological response of an athermal assembly of soft particles with tunable attractive interactions is studied in the vicinity of jamming. At small attractions, a fragile solid develops and a finite yield stress is measured. Moreover, the measured flow curves have unstable regimes, which lead to persistent shearbanding. These features are rationalized by establishing a link between the rheology and the inter-particle connectivity, which also provides a minimal model to describe the flow curves.

cond-mat.soft↗

arXiv2011

Inhomogeneous shear flows in soft jammed materials with tunable attractive forces

We perform molecular dynamics simulations to characterize the occurrence of inhomogeneous shear flows in soft jammed materials. We use rough walls to impose a simple shear flow and study the athermal motion of jammed assemblies of soft particles, both for purely repulsive interactions and in the presence of an additional short-range attraction of varying strength. In steady state, pronounced flow inhomogeneities emerge for all systems when the shear rate becomes small. Deviations from linear flow are stronger in magnitude and become very long-lived when the strength of the attraction increases, but differ from permanent shear-bands. Flow inhomogeneities occur in a stress window bounded by the dynamic and static yield stress values. Attractive forces enhance the flow heterogeneities because they accelerate stress relaxation, thus effectively moving the system closer to the yield stress regime where inhomogeneities are most pronounced. The present scenario for understanding the effect of particle adhesion on shear localization, which is based on detailed molecular dynamics simulations with realistic particle interactions, differs qualitatively from previous qualitative explanations and ad-hoc theoretical modelling.

cond-mat.soft↗

arXiv2011

Suppressed compressibility at large scale in jammed packings of size disperse spheres

We analyze the large scale structure and fluctuations of jammed packings of size disperse spheres, produced in a granular experiment as well as numerically. While the structure factor of the packings reveals no unusual behavior for small wavevectors, the compressibility displays an anomalous linear dependence at low wavectors and vanishes when q -> 0. We show that such behavior occurs because jammed packings of size disperse spheres have no bulk fluctuations of the volume fraction and are thus hyperuniform, a property not observed experimentally before. Our results apply to arbitrary particle size distributions. For continuous distributions, we derive a perturbative expression for the compressibility that is accurate for polydispersity up to about 30%.

cond-mat.stat-mech↗

arXiv2010

Fluctuations and correlations during the shear flow of elastic particles near the jamming transition

We present a numerical study of the flow of an assembly of frictionless soft discs at zero temperature, in the vicinity of and slightly above the jamming density. We find that some of the flow properties, such as the fluctuations in the number of contacts or the shear modulus, display a critical like behaviour that is governed by the proximity to the jamming point. Dynamical correlations during a quasistatic deformation, however, are non critical and dominated by system size. At finite strain rates, these dynamical correlations acquire a finite, strain-rate dependent amplitude, that decreases when approaching the jamming point from above.

cond-mat.mtrl-sci↗

arXiv2009

When gel and glass meet: A mechanism for multistep relaxation

We use computer simulations to study the dynamics of a physical gel at high densities where gelation and the glass transition interfere. We report and provide detailed physical understanding of complex relaxation patterns for time correlation functions which generically decay in a three-step process. For certain combinations of parameters we find logarithmic decays of the correlators and subdiffusive particle motion.

cond-mat.soft↗

arXiv2009

Jamming transitions in amorphous packings of frictionless spheres occur over a continuous range of volume fractions

We numerically produce fully amorphous assemblies of frictionless spheres in three dimensions and study the jamming transition these packings undergo at large volume fractions. We specify four protocols yielding a critical value for the jamming volume fraction which is sharply defined in the limit of large system size, but is different for each protocol. Thus, we directly establish the existence of a continuous range of volume fraction where nonequilibrium jamming transitions occur. However, these jamming transitions share the same critical behaviour. Our results suggest that, even in the absence of partial crystalline ordering, a unique location of a random close packing does not exist, and that volume fraction alone is not sufficient to describe the properties of jammed states.

cond-mat.stat-mech↗

arXiv2008

Static and dynamic properties of a reversible gel

We study a microscopically realistic model of a physical gel and use computer simulations to investigate its static and dynamic properties at thermal equilibrium. The phase diagram comprises a sol phase, a coexistence region ending at a critical point, a gelation line, and an equilibrium gel phase unrelated to phase separation. The global structure of the gel is homogeneous, but the stress is supported by a fractal network. Gelation results in a dramatic slowing down of the dynamics, which can be used to locate the transition, which otherwise shows no structural signatures. Moreover, the equilibrium gel dynamics is highly heterogeneous as a result of the presence of particle families with different mobilities. An analysis of gel dynamics in terms of mobile and arrested particles allows us to elucidate several differences between the dynamics of equilibrium gels and that of glass-formers.

cond-mat.soft↗

arXiv2008

Tracking heterogeneous dynamics during the alpha-relaxation of a simple glass-former

We study the relaxation process in a simple glass-former - the KA lattice gas model. We show that, for this model, structural relaxation is due to slow percolation of regions of co-operatively moving particles, which leads to heterogeneous dynamics of the system. We find that the size distribution of these regions is given by a power-law and that their formation is encoded in the initial structure of the particles, with the memory of initial configuration increasingly retained with increasing density.

cond-mat.stat-mech↗

arXiv2008

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↗

arXiv2008

A random walk description of the heterogeneous glassy dynamics of attracting colloids

We study the heterogeneous dynamics of attractive colloidal particles close to the gel transition using confocal microscopy experiments combined with a theoretical statistical analysis. We focus on single particle dynamics and show that the self part of the van Hove distribution function is not the Gaussian expected for a Fickian process, but that it reflects instead the existence, at any given time, of colloids with widely different mobilities. Our confocal microscopy measurements can be described well by a simple analytical model based on a conventional continuous time random walk picture, as already found in several other glassy materials. In particular, the theory successfully accounts for the presence of broad tails in the van Hove distributions that exhibit exponential, rather than Gaussian, decay at large distance.

cond-mat.soft↗

arXiv2007

Signatures of Dynamical Heterogeneity in the Structure of Glassy Free-energy Minima

From numerical minimization of a model free energy functional for a system of hard spheres, we show that the width of the local peaks of the time-averaged density field at a glassy free-energy minimum exhibits large spatial variation, similar to that of the ``local Debye-Waller factor'' in simulations of dynamical heterogeneity. Molecular dynamics simulations starting from a particle configuration generated from the density distribution at a glassy free-energy minimum show similar spatial heterogeneity in the degree of localization, implying a direct connection between dynamical heterogeneity and the structure of glassy free energy minima.

cond-mat.stat-mech↗