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We study the kinetics of vapor-liquid phase separation in a quasi one-dimensional confined active matter system using molecular dynamics simulations. Activity is invoked via the Vicsek rule, while passive interaction follows the Lennard-Jones potential. With the system density near the vapor branch, the evolution morphology features disconnected liquid clusters. In the passive limit, coarsening begins with nucleation, followed by an evaporation-condensation growth mechanism, leading to a metastable state without complete phase separation. We aim to understand the impact of Vicsek-like self-propulsion on the structure and growth of these clusters. Our key finding is that Vicsek activity results in a distinct growth mechanism, notably rapid cluster growth and the breakdown of the metastable state through ballistic aggregation. Relevant growth laws are analyzed and explained using appropriate theoretical models.

We investigate the effect of annealed disorder on the mechanical properties and plasticity of a modeled amorphous solid by introducing a small fraction of heavy impurities into the material which resembles real experimental systems. The disorder being mobile, the total degrees of freedom and the potential energy landscape of the pure system are preserved in our model. The mechanical failure and the shear band formation in the amorphous solid in the presence of annealed disorder are studied at the microscopic level by employing the finite shear deformation protocol at nonzero temperature. A significant enhancement in the shear modulus and yield stress is observed as a function of the heaviness of the impurity particles. Via the analysis of the non-affine displacement field and the microscopic strain fluctuations and the nature of their spatial correlations we find that the shear band formation in the plastic regime is gradually suppressed with increasing impurity mass. Eventually, a critical mass of the disordered particles is identified above which the plastic events become completely localized. This is marked by a transition from a power law to an exponential decay in the spatial correlations non-affine displacement field. Likewise, a similar change is observed in the strain correlation function, transitioning from a slower $1/r$ decay to a more rapid $1/r^3$ decay. Finally, the effect shear rate on the plastic events in the presence of annealed disorder is explored.

The surface directed spinodal decomposition of a binary liquid confined inside cylindrical pore is investigated using molecular dynamics simulation. One component of the liquid wets the pore surface while the other remains neutral. A variety of wetting conditions are studied. For the partial wetting case, after an initial period of phase separation, the domains organize themselves into plug-like structure and the system enters into a metastable state. Therefore, a complete phase separation is never achieved. Analysis of domain growth and the structure factor suggests an one-dimensional growth dynamics for partial wetting case. As the wetting interaction is increased beyond a critical value, a transition from the plug-like to tube-like domain formation is observed which corresponds to the full wetting morphology. Thus, a complete phase separation is achieved as the wetting species moves towards the pore surface and forms layers enclosing the non wetting species residing around the axis of the cylinder. The coarsening dynamics of both the species are studied separately. The wetting species is found to follow a two-dimensional domain growth dynamics with a growth exponent 1/2 in the viscous hydrodynamic regime. This was substantiated by the Porod tail of the structure factor. On the other hand, the domain grows linearly with time for the non wetting species. This suggests that the non wetting species behaves akin to a three-dimensional bulk system. An appropriate reasoning is presented to justify the given observations.

We study the kinetics of vapor-liquid and vapor-solid phase separation of a hydrodynamics preserving three-dimensional one component Lennard Jones system in the presence of external gravitational field using extensive molecular dynamic simulation. A bicontinuous domain structure is formed when the homogeneous system near the critical density is quenched inside the coexistence region. In the absence of gravity, the domain morphology is statistically self-similar and the length scale grows as per the existing laws. However, the presence of gravity destroys the isotropy of the system and affects the scaling laws. We observe an accelerated domain growth in the direction of the field at late time which resembles sedimentation process. Consequently, a new length scale emerges which strongly depends on the field strength. Similar behavior is observed in the direction perpendicular to the applied field, with a different growth rate. Finally, the validity of Porod's law and Superuniversality in such anisotropic systems is verified in terms of two-point equal time order parameter correlation function and static structure factor.

We employ the state-of-the-art molecular dynamics simulations to study the kinetics of phase separation and aging phenomena of segregating binary fluid mixtures imbibed in porous materials. Different random porous structures are considered to understand the effect of pore morphology on coarsening dynamics. We find the effect of complex geometrical confinement resulting in the dramatic slowing down in the phase separation dynamics. The domain growth follows the power law with an exponent dependent on the porous host structure. After the transient period, a crossover to a slower domain growth is observed when the domain size becomes comparable to the pore size. Due to the geometric confinement, the correlation function and structure factor modify to a non-Porod behavior and violate the superuniversality hypothesis. The role of porous host structure on the nonequilibrium aging dynamics is studied qualitatively by computing the two-time order-parameter autocorrelation function. This quantity exhibits scaling laws with respect to the ratio of the domain length at the observation time and the age of the system. We find the scaling laws hold good for such confined segregating fluid mixtures.

When an amorphous solid is deformed homogeneously, the response exhibits heterogeneous plastic instabilities with localized cooperative rearrangement of cluster of particles. The heterogeneous behavior plays an important role in deciding the mechanical properties of amorphous solids. In this paper, we employ computer simulation to study the characteristics and the spatial correlations of these clusters characterized by the non-affine displacements in amorphous solids under simple shear deformation in the athermal quasistatic limit. The clusters with large displacements are found to be homogeneously distributed in space in the elastic regime, followed by a localization within a system-spanning shear band after yielding. The distributions of the displacement field exhibit power-law nature with exponents strongly dependent on the deformation. The non-affine displacements show strong spatial correlations which become long-ranged with increasing strain. From our results, it is evident that the decay of the correlation functions is exponential in nature in the elastic regime. The yielding transition is marked by an abrupt change in the decay after which it is well described by power-law. These results demonstrate a scale-free character of non-affine correlations in the stead flow regime. These results are found to be robust and independent of the strain window over which the total non-affine displacement is calculated.

We study the shear induced fluidization of amorphous solids subjected to external loading by investigating the relaxation dynamics of the deformed states using computer simulation. A simple shear deformation is employed at constant rate to the thermal glassy materials. The shear localization and the plastic deformation heterogeneity with strain is investigated in terms of the non-affine displacement field. The mean square displacement shows an enhanced mobility of the particles with strain, indicating the fluidization of the material. Using the time correlation function we estimate the relaxation time of the sheared glasses. A significant decrease in the relaxation time is observed up to the yielding point as the material loses its solid nature and eventually becomes liquid-like. Finally, the imprint of memory of the quiescent sample on the rheological properties of the shear melted glass is investigated by computing the relaxation of the shear stress. We find a finite persistent residual stress that outlasts the experimental observation time in our system.

We use state-of-the-art molecular dynamics simulations to study the effects of annealed disorder on the phase separating kinetics and aging phenomena of a segregating binary fluid mixture. In the presence of disorder, we observe a dramatic slowing down in the phase separation dynamics. The domain growth follows the power-law with a disorder-dependent exponent. Due to the energetically favorable positions, the domain boundary roughens which modifies the correlation function and structure factor to a non-Porod behavior. The correlation function and structure factor provide clear evidence that the superuniversality does not hold in our system. The role of annealed disorder on the non-equilibrium aging dynamics is studied qualitatively by computing the two-time order parameter autocorrelation function. The decay of the correlation function slows down significantly with the disorder. This quantity exhibits scaling laws with respect to the ratio of the domain length at the observation time and the age of the system. We find the scaling laws hold good for the disordered system and therefore, robust and generic to such segregating fluid mixtures., Comment: arXiv admin note: substantial text overlap with arXiv:2104.06477

In this paper we study the mechanical properties and pore structure in a three-dimensional molecular dynamics model of porous glass under athermal quasistatic shear. The vitreous samples are prepared by rapid thermal quench from a high temperature molten state. The pore structures form via solid-gas phase separation. The quiescent samples exhibit a wide range of pore topography, from inter-connected pore network to randomly distributed compact pores depending on the material density. We find the shear modulus strongly depends on the density and porosity. Under mechanical loading, the pore structure rearranges which is reflected in the pore size distribution function. Our results show that with increase in strain the distribution widens as the adjacent pores coalesce and form larger pores. We also propose a universal scaling law for the pore size distribution function which offers excellent data collapse for highly porous materials in the undeformed case. From the data scaling we identify a critical density which can be attributed to the transition point from a porous-type to bulk-type material. The validity of the scaling law under finite deformation is also analyzed.

Quenched or frozen-in structural disorder is ubiquitous in real experimental systems. Much of the progress is achieved in understanding the phase separation of such systems using the diffusion-driven coarsening in Ising model with quenched disorder. But there is a paucity of research in the phase separation kinetics in fluids with quenched disorder. In this paper, we present results from a detailed Molecular dynamics simulation, the effects of randomly placed localized impurities on the phase separating kinetics of binary fluid mixture. Two different models are offered for representing the impurities. We observe a dramatic slowing down in the pattern formation with increasing impurity concentration. This sluggish domain growth kinetics follows power-law with a disorder-dependent exponent. The correlation function and structure factor show a non-Porod behavior, indicating the roughening of domain interfaces. We have also studied the effect of quenched disorder on the aging dynamics by calculating the two-time order parameter auto correlation function and find that the Fisher and Huse scaling law holds good in presence of quenched disorder.

Long-ranged dipole-dipole interactions in magnetic glasses give rise to magnetic domains having labyrinthine patterns. Barkhausen Noise is then expected to result from the movement of domain boundaries which is supposed to be modeled by the motion of elastic membranes with random pinning. We propose an atomistic model of such magnetic glasses in which we measure the Barkhausen Noise which indeed results from the movement of domain boundaries. Nevertheless the statistics of the Barkhausen Noise is found in striking disagreement with the expectations in the literature. In fact we find exponential statistics without any power law, stressing the fact that Barkhausen Noise can belong to very different universality classes. In this glassy system the essence of the phenomenon is the ability of spin-carrying particles to move and minimize the energy without any spin flip. A theory is offered in excellent agreement with the measured data without any free parameter., Comment: 5 Pages, 5 Figures, Submitted to EPL

Plastic events in amorphous solids can be much more than just "shear transformation zones" when the positional degrees of freedom are coupled non-trivially to other degrees of freedom. Here we consider magnetic amorphous solids where mechanical and magnetic degrees of freedom interact, leading to rather complex plastic events whose nature must be disentangled. In this paper we uncover the anatomy of the various contributions to some typical plastic events. These plastic events are seen as Barkhausen Noise or other "serrated noises". Using theoretical considerations we explain the observed statistics of the various contributions to the considered plastic events. The richness of contributions and their different characteristics imply that in general the statistics of these "serrated noises" cannot be universal, but rather highly dependent on the state of the system and on its microscopic interactions., Comment: arXiv admin note: text overlap with arXiv:1401.4326

The concept of a Shear Transformation Zone (STZ) refers to a region in an amorphous solid that undergoes a plastic event when the material is put under an external mechanical load. An important question that had accompanied the development of the theory of plasticity in amorphous solids for many years now is whether an STZ is a {\em region} existing in the material (which can be predicted by analyzing the unloaded material), or is it an {\em event} that depends on the loading protocol (i.e., the event cannot be predicted without following the protocol itself). In this Letter we present strong evidence that the latter is the case. Infinitesimal changes of protocol result in macroscopically big jumps in the positions of plastic events, meaning that these can never be predicted from considering the unloaded material., Comment: 4 pages, 5 figures

We study the glass transition in fluids where particles are endowed with spins, such that magnetic and positional degrees of freedom are coupled. Novel results for slowing down in the spin time-correlation functions are described, and the effects of magnetic fields on the glass transition are studied. Aging effects in such systems and the corresponding data collapse are presented and discussed.

In this paper we study the four point correlation function $\chi_4$ of collective density fluctuations in a nonequilibrium liquid. The equilibration is controlled by a modified stretched exponential behavior ( $\exp [-{(t_\mathrm{w}/\tau)}^\beta]$ ) having the relaxation time $\tau$ dependent on the aging time $t_\mathrm{w}$. Similar aging behavior has been seen experimentally in supercooled liquids. The basic equations of fluctuating nonlinear hydrodynamics (FNH) are solved here numerically to obtain $\chi_4$ for equilibrium and non equilibrium states. We also identify a dynamic length scale $\xi$ from the equilibrated function. $\xi(T)$ grows with fall of temperature $T$. From a broader perspective, we demonstrate here that the characteristic signatures of dynamical heterogeneities in a supercooled liquid, observed previously in computer simulations of the dynamics of a small number of particles, are also present in the coarse grained equations of generalized hydrodynamics., Comment: 6 figures

We discuss a model metallic glass in which Barkhausen Noise can be studied in exquisite detail, free of thermal effects and of the rate of ramping of the magnetic field. The mechanism of the jumps in magnetic moment that cause the Barkhausen Noise can be fully understood as consecutive instabilities where an eigenvalue of the Hessian matrix hits zero, leading to a magnetization jump $\Delta m$ which is simultaneous with a stress and energy changes $\Delta \sigma$ and $\Delta U$ respectively. Contrary to common belief we find no "movements of magnetic domain boundaries" across pinning sites, no fractal domains, no self-organized criticality and no exact scaling behaviour. We present a careful numerical analysis of the statistical properties of the phenomenon, and show that with every care taken this analysis is tricky, and easily misleading. Without a guiding theory it is almost impossible to get the right answer for the statistics of Barkhausen Noise. We therefore present an analytic theory, showing that the probability distribution function (pdf) of Barkhausen Noise is not a power law times an exponential cutoff.

We present results of atomistic simulations of a new model of a magnetic amorphous solid subjected to external mechanical strains and magnetic fields. The model employed offers new perspectives on important effects like Barkhausen noise and magnetostriction. It is shown that the plastic response in such systems exhibit singularities characterized by unexpected exponents requiring careful theoretical reasoning. The spatial structure of the plastic events requires a new coarse grained elasto-magnetic theory which is provided here.

Amorphous magnetic solids, like metallic glasses, exhibit a novel effect: the growth of magnetic order as a function of mechanical strain under athermal conditions in the presence of a magnetic field. The magnetic moment increases in steps whenever there is a plastic event. Thus plasticity induces the magnetic ordering, acting as the effective noise driving the system towards equilibrium. We present results of atomistic simulations of this effect in a model of a magnetic amorphous solid subjected to pure shear and a magnetic field. To elucidate the dependence on external strain and magnetic field we offer a mean-field theory that provides an adequate qualitative understanding of the observed phenomenon.

We solve numerically the equations of nonlinear fluctuating hydrodynamics (NFH) for the supercooled liquid. The time correlation of the density fluctuations in equilibrium obtained here shows quantitative agreement with molecular dynamics(MD) simulation data. We demonstrate numerically that the 1/rho nonlinearity in the NFH equations of motion is essential in restoring the ergodic behavior in the liquid. Under nonequilibrium conditions the time correlation functions relax in a manner similar to that observed in the molecular dynamics simulations in binary mixtures. The waiting time t_w dependence of the non-equilibrium response function follows a Modified Kohlrausch-Williams-Watts(MKWW) form similar to the behavior seen in dielectric relaxation data., Comment: 10 pages, 5 figures

In this report we address the question whether aging in the non equilibrium glassy state is controlled by the equilibrium alpha-relaxation process which occur at temperatures above Tg. Recently Lunkenheimer et. al. [Phys. Rev. Lett. 95, 055702 (2005)] proposed a model for the glassy aging data of dielectric relaxation using a modified Kohlrausch-Williams-Watts (KWW) form. The aging time dependence of the relaxation time is defined by these authors through a functional relation involving the corresponding frequency but the stretching exponent is same as the alpha-relaxation stretching exponent. We present here an alternative functional form directly involving the relaxation time itself. The proposed model fits the data of Lunkenheimer et. al. perfectly with a stretching exponent different from the alpha-relaxation stretching exponent., Comment: 1 TeX file, 10 eps figures