arXiv subjects

Pinaki Chaudhuri

Publications and source records attributed to Pinaki Chaudhuri.

At least 37 records · Page 2Linked to original sources

arXiv2022

Turbulent route to two-dimensional soft crystals

We investigate the effects of a two-dimensional, incompressible, turbulent flow on mono-disperse soft granular particles and show the emergence of a crystalline phase due to the interplay of Stokesian drag (measured through the Stokes number) and short-range inter-particle interactions. We quantify this phase through the bond order parameter and local density fluctuations and find a sharp transition between the crystalline and non-crystalline phase as a function of the Stokes number. Furthermore, the nature of preferential concentration, as characterised by the radial distribution function and the correlation dimension $D_2$, is significantly different from that of particle-laden flows in the absence of repulsive potentials.

physics.flu-dyn↗

arXiv2022

Relaxation dynamics in the energy landscape of glass-forming liquids

We numerically study the zero-temperature relaxation dynamics of several glass-forming models to their inherent structures, following quenches from equilibrium configurations sampled across a wide range of initial temperatures. In a mean-field Mari-Kurchan model, we find that relaxation changes from a power-law to an exponential decay below a well-defined temperature, consistent with recent findings in mean-field $p$-spin models. By contrast, for finite-dimensional systems, the relaxation is always algebraic, with a non-trivial universal exponent at high temperatures crossing over to a harmonic value at low temperatures. We demonstrate that this apparent evolution is controlled by a temperature-dependent population of localised glassy excitations. Our work unifies several recent lines of studies aiming at a detailed characterisation of the complex potential energy landscape of glass-formers, and challenges both mean-field and real space descriptions of glasses.

cond-mat.stat-mech↗

arXiv2022

Effect of ring stiffness and ambient pressure on the dynamical slowdown in ring polymers

Using extensive molecular dynamics simulations, we investigate the slowing down of dynamics in a 3D system of ring polymers by varying the ambient pressure and the stiffness of the rings. Our study demonstrates that the stiffness of the rings determines the dynamics of the ring polymers, leading to glassiness at lower pressures for stiffer rings. The threading of the ring polymers, a unique feature that emerges only due to the topological nature of such polymers in three dimensions, is shown to be the determinant feature of dynamical slowing down, albeit only in a certain stiffness range. Our results suggest a possible framework of exploring the phase space spanned by ring stiffness and pressure to obtain spontaneously emerging topologically constrained polymer glasses.

cond-mat.soft↗

arXiv2022

Rheological response of a glass-forming liquid having large bidispersity

Using extensive numerical simulations, we investigate the flow behaviour of a model glass-forming binary mixture whose constituent particles have a large size ratio. The rheological response to applied shear is studied in the regime where the larger species are spatially predominant. We demonstrate that the macroscopic rigidity that emerges with increasing density occurs in the regime where the larger species undergo a glass transition while the smaller species continue to be highly diffusive. We analyse the interplay between the timescale imposed by the shear and the quiescent relaxation dynamics of the two species to provide a microscopic insight into the observed rheological response. Finally, by tuning the composition of the mixture, we illustrate that the systematic insertion of the smaller particles affects the rheology by lowering of viscosity of the system.

cond-mat.soft↗

arXiv2022

Finite-size effects in the diffusion dynamics of a glass-forming binary mixture with large size ratio

Extensive molecular dynamics (MD) computer simulations of an equimolar glass-forming AB mixture with large size ratio are presented. While the large A particles show a glass transition around the critical density of mode coupling theory $ρ_c$, the small B particles remain mobile with a relatively weak decrease of their self diffusion coefficient $D_{\rm B}$ with increasing density. Surprisingly, around $ρ_c$, the self-diffusion coefficient of the A particles, $D_{\rm A}$, also starts to follow a rather weak dependence on density. We show that this is due to finite-size effects that can be understood from the analysis of the collective interdiffusion dynamics.

cond-mat.soft↗

arXiv2021

Universal mechanical instabilities in the energy landscape of amorphous solids: evidence from athermal quasistatic expansion

Using numerical simulations, we study the failure of an amorphous solid under quasi-static expansion starting from a homogeneous high-density state. During the volume expansion, we demonstrate the existence of instabilities manifesting via saddle-node bifurcation in which a minimum meets a saddle. During all such events, the smallest eigenvalue of the Hessian matrix vanishes as a square-root singularity. The plastic instabilities are manifested via sudden jumps in pressure and energy, with the largest event happening when a cavity appears, leading to the yielding of the material. We show that during cavitation and prior to complete fracture, the statistics of pressure or energy jumps corresponding to the plastic events show sub-extensive finite-size scaling, similar to the case of simple shear but with different exponents. Thus, overall, our study reveals universality in the fundamental characteristics during mechanical failure in amorphous solids under any quasi-static deformation protocol.

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↗

arXiv2021

An elasto-plastic approach based on microscopic insights for the steady state and transient dynamics of sheared disordered solids

In this letter, we develop a framework to study the mechanical response of athermal amorphous solids via a coupling of mesoscale and microscopic models. Using measurements of coarse grained quantities from simulations of dense disordered particulate systems, we present a coherent elasto-plastic model approach for deformation and flow of yield stress materials. For a given set of parameters, this model allows to match consistently transient and steady state features of driven disordered systems under both applied shear-rate and creep protocols.

cond-mat.soft↗

arXiv2020

Flow heterogeneities in supercooled liquids and glasses under shear

Using extensive non-equilibrium molecular dynamics simulations, we investigate a glassforming binary Lennard-Jones mixture under shear. Both supercooled liquids and glasses are considered. Our focus is on the characterization of inhomogeneous flow patterns such as shear bands that appear as a transient response to the external shear. For the supercooled liquids, we analyze the crossover from Newtonian to non-Newtonian behavior with increasing shear rate $\dotγ$. Above a critical shear rate $\dotγ_c$ where a non-Newtonian response sets in, the transient dynamics are associated with the occurrence of short-lived vertical shear bands, i.e. bands of high mobility that form perpendicular to the flow direction. In the glass states, long-lived horizontal shear bands, i.e. bands of high mobility parallel to the flow direction, are observed in addition to vertical ones. The systems with shear bands are characterized in terms of mobility maps, stress-strain relations, mean-squared displacements, and (local) potential energies. The initial formation of a horizontal shear band provides an efficient stress release, corresponds to a local minimum of the potential energy, and is followed by a slow broadening of the band towards the homogeneously flowing fluid in the steady-state. Whether a horizontal or a vertical shear band forms cannot be predicted from the initial undeformed sample. Furthermore, we show that with increasing system size the probability for the occurrence of horizontal shear bands increases.

cond-mat.soft↗

arXiv2020

Poiseuille flow of soft polycrystals in 2D rough channels

Polycrystals are partially ordered solids where crystalline order extends over mesoscopic length scales, namely, the grain size. We study the Poisuielle flow of such materials in a rough channel. In general, similar to yield stress fluids, three distinct dynamical states, namely, flowing, stick-slip and jammed can be observed, with a yield threshold dependent on channel width. Importantly, the interplay between the finite channel width, and the intrinsic ordering scale (the grain size) leads to new type of spatiotemporal heterogeneity. In wide channels, although the average flow profile remains plug like, at the underlying granular level, there is vigorous grain remodelling activity resulting from the velocity heterogeneity among the grains. As the channel width approaches typical grain size, the flowing polycrystalline state breaks up into a spatially heterogeneous mixture of flowing liquid like patches and chunks of nearly static grains. Despite these static grains, the average velocity still shows a parabolic profile, dominated by the moving liquid like patches. However, the solid-liquid front moves at nearly constant speed in the opposite direction of the external drive.

cond-mat.soft↗

arXiv2019

Extreme active matter at high densities

Extreme active matter, an assembly of self-propelled particles with large persistence time $τ_p$ and high Péclet number, exhibits remarkable behaviour at high densities. As $τ_p\to 0$, the assembly undergoes a gradual slowing down of density relaxations, as one reduces the active propulsion force $f$, until at the glass transition, the relaxation times diverge. In the other limit, $τ_p \to \infty$, the fluid jams on lowering $f$, at a critical threshold $f^*(\infty)$, with stresses concentrated along force-chains. As one moves away from this jamming threshold, the force-chains dynamically remodel, and the lifetime of the force-balanced configurations diverges as one approaches $f^*(\infty)$, by tuning $τ_p$. In between these limits, the approach to dynamical arrest at low $f$, goes through a phase characterised by intermittency in the kinetic energy. This intermittency is a consequence of long periods of jamming followed by bursts of plastic yielding associated with Eshelby deformations, akin to the response of dense amorphous solids to an externally imposed shear. The frequency of these plastic bursts increases as one moves towards the intermittent phase-fluid boundary, where the correlated plastic events result in large scale vorticity and turbulence. Dense extreme active matter brings together the physics of glass, jamming, plasticity and turbulence, in a new state of driven classical matter.

cond-mat.soft↗

arXiv2018

Discontinuous shear-thinning in adhesive dispersions

We present simulations for the steady-shear rheology of a model adhesive dispersion. We vary the range of the attractive forces $u$ as well as the strength of the dissipation $b$. For large dissipative forces, the rheology is governed by the Weisenberg number $ \text{Wi}\sim b\dotγ/u$ and displays Herschel-Bulkley form $σ= σ_y+c\text{Wi}^ν$ with exponent $ν=0.45$. Decreasing the strength of dissipation, the scaling with $\text{Wi}$ breaks down and inertial effects show up. The stress decreases via the Johnson-Samwer law $Δσ\sim T_s^{2/3}$, where temperature $T_s$ is exclusively due to shear-induced vibrations. During flow particles prefer to rotate around each other such that the dominant velocities are directed tangentially to the particle surfaces. This tangential channel of energy dissipation and its suppression leads to a discontinuity in the flow curve, and an associated discontinuous shear thinning transition. We set up an analogy with frictional systems, where the phenomenon of discontinuous shear thickening occurs. In both cases tangential forces, frictional or viscous, mediate a transition from one branch of the flowcurve with low tangential dissipation to one with large tangential dissipation.

cond-mat.soft↗

arXiv2018

Effect of pinning on the yielding transition of amorphous solids

Using numerical simulations, we have studied the yielding response, in the athermal quasi static limit, of a model amorphous material having inclusions in the form of randomly pinned particles. We show that, with increasing pinning concentration, the plastic activity becomes more spatially localized, resulting in smaller stress drops, and corresponding increase in the magnitude of strain where yielding occurs. We demonstrate that, unlike the spatially heterogeneous and avalanche led yielding in the case of the unpinned glass, for the case of large pinning concentration, yielding takes place via a spatially homogeneous proliferation of localized events.

cond-mat.soft↗

arXiv2018

Precursors of fluidisation in the creep response of a soft glass

Using extensive numerical simulations, we study the fluidisation process of dense amorphous materials subjected to an external shear stress, using a three-dimensional colloidal glass model. In order to disentangle possible boundary effects from finite size effects in the process of fluidisation, we implement a novel geometry-constrained protocol with periodic boundary conditions. We show that this protocol is well controlled and that the longtime fluidisation dynamics is, to a great extent, independent of the details of the protocol parameters. Our protocol, therefore, provides an ideal tool to investigate the bulk dynamics prior to yielding and to study finite size effects regarding the fluidisation process. Our study reveals the existence of precursors to fluidisation observed as a peak in the strain-rate fluctuations, that allows for a robust definition of a fluidisation time. Although the exponents in the power-law creep dynamics seem not to depend significantly on the system size, we reveal strong finite size effects for the onset of fluidisation.

physics.comp-ph↗

arXiv2017

Glassy swirls of active dumbbells

The dynamics of a dense binary mixture of soft dumbbells, each subject to an active propulsion force and thermal fluctuations, shows a sudden arrest, first to a translational then to a rotational glass, as one reduces temperature $T$ or the self-propulsion force $f$. Is the temperature-induced glass different from the activity-induced glass? To address this question, we monitor the dynamics along an iso-relaxation-time contour in the $(T-f)$ plane. We find dramatic differences both in the fragility and in the nature of dynamical heterogeneity which characterise the onset of glass formation - the activity-induced glass exhibits large swirls or vortices, whose scale is set by activity, and appears to diverge as one approaches the glass transition. This large collective swirling movement should have implications for collective cell migration in epithelial layers.

cond-mat.soft↗

arXiv2017

Thermal Conductivity of Glass-Forming Liquids

Thermal conductivity of a model glass-forming system in the liquid and glass states is studied using extensive numerical simulations. We show that near the glass transition temperture, where the structural relaxation time becomes very long, the measured thermal conductivity decreases with increasing age. Secondly the thermal conductivity of the disordered solid obtained at low temperatures depends on the cooling rate with which it was prepared, with lower cooling rates leading to lower thermal conductivity. Our analysis links this decrease of the thermal conductivity with increased exploration of lower-energy inherent structures of the underlying potential energy landscape. Further we show that the lowering of conductivity for lower-energy inherent structures is related to the high frequency harmonic modes associated with the inherent structure being less extended.

cond-mat.dis-nn↗

arXiv2016

Heterogeneous dynamics during yielding of glasses: effect of aging

Molecular dynamics computer simulations of a binary Lennard-Jones glass under shear are presented. The mechanical response of glassy states having different thermal histories is investigated by imposing a wide range of external shear rates, at different temperatures. The stress-strain relations exhibit an overshoot at a strain of around 0.1, marking the yielding of the glass sample and the onset of plastic flow. The amplitude of the overshoot shows a logarithmic behavior with respect to a dimensionless variable, given by the age of the sample times the shear rate. Dynamical heterogeneities having finite lifetimes, in the form of shear bands, are observed as the glass deforms under shear. By quantifying the spatial fluctuations of particle mobility, we demonstrate that such shearbanding occurs only under specific combinations of imposed shear-rate, age of glass and ambient temperature.

cond-mat.soft↗

arXiv2016

Athermal rheology of weakly attractive soft particles

We study the rheology of a soft particulate system where the inter-particle interactions are weakly attractive. Using extensive molecular dynamics simulations, we scan across a wide range of packing fractions ($ϕ$), attraction strengths ($u$) and imposed shear-rates ($\dotγ$). In striking contrast to repulsive systems, we find that at small shear-rates generically a fragile isostatic solid is formed even if we go to $ϕ\ll ϕ_J$. Further, with increasing shear-rates, even at these low $ϕ$, non-monotonic flow curves occur which lead to the formation of persistent shear-bands in large enough systems. By tuning the damping parameter, we also show that inertia plays an important role in this process. Furthermore, we observe enhanced particle dynamics in the attraction-dominated regime as well as a pronounced anisotropy of velocity and diffusion constant, which we take as precursors to the formation of shear bands. At low enough $ϕ$, we also observe structural changes via the interplay of low shear-rates and attraction with the formation of micro-clusters and voids. Finally, we characterize the properties of the emergent shear bands and thereby, we find surprisingly small mobility of these bands, leading to prohibitely long time-scales and extensive history effects in ramping experiments.

cond-mat.soft↗