Soft matters whose constituents are deformable are ubiquitous in nature especially in biological systems-including cells and their organelles-as well as in foams and emulsions. The capacity for deformation in these soft materials gives rise to a range of intriguing phenomena, such as glassy behavior without any size dispersity, cluster crystal formation, and re-entrant melting. Deformability also plays a crucial role in facilitating essential biological processes, such as the flow of blood through veins and arteries. In this work, we investigate assemblies of two-dimensional (2D) polymeric, non-overlapping rings, which mimic deformable particulates in 2D using extensive molecular dynamics simulations. The rings are confined in a rectangular channel with hard walls perpendicular to the flow direction, mimicking natural flow conditions. We analyze the flow properties of these deformable particle assemblies at two different stiffness values. To further asses the impact of deformability, we examine the same monodisperse system at higher densities for the stiffer rings, where deformation is necessary and a fluid layer emerges at the channel edges. Finally, we explore a mixture of rings with two distinct stiffnesses and observe effective segregation of soft and hard particles at small channel widths.

We introduce a Hamiltonian Active Brownian Particle (HABP) model that connects equilibrium dynamics with the non-equilibrium, two-dimensional overdamped behavior of standard Active Brownian Particles (ABPs). In equilibrium, the system follows overdamped Langevin equations that strictly satisfy the fluctuation-dissipation theorem. Coupling translational and rotational degrees of freedom to separate heat baths (T_θ > T_tr), drives the system out of equilibrium. In free space, matching the diffusion coefficients yields quantitative agreement with the ABP model in the limit T_θ/T_tr→∞, whereas in a harmonic potential, this matching is recovered even at finite temperature ratios. Entropy production analysis demonstrates that in the ABP limit, all energy injected by the swim force dissipates into the translational bath, leaving the rotational bath as a zero-cost entropy source. These insights provide the first steps toward equilibrium-inspired descriptions of active matter, facilitating the development of new theoretical frameworks to study activity-induced fluctuations, transport, and phase behavior in living and non-living soft matter systems far from equilibrium.

Directional memory in amorphous solids is commonly quantified through the Bauschinger effect, yet the observation of the inverse Bauschinger effect suggests that the sign of memory can invert, pointing to distinct underlying plastic organization. Here, we connect directional memory to the nature of yielding in steadily sheared amorphous solids. Using simulations of two-dimensional polydisperse glasses, we show that the type of directional memory (Bauschinger versus inverse Bauschinger) is jointly controlled by deformation history, strain rate, and parent temperature. We identify a critical history amplitude gamma N,crit(Tp, gamma(center dot) ) and construct a phase diagram that delineates regimes with memory inversion from those showing only conventional Bauschinger response. Microscopically, memory inversion correlates with networklike shear-band morphology and plastic healing, whereas conventional memory is associated with persistent localization and cumulative damage. These results establish directional memory as an order parameter for a shear-rate and annealing-controlled brittle-ductile crossover and suggest that plastic healing provides a generic route to memory inversion in disordered solids.

Viscosity is a critical determinant of a liquid's ability to form a glass upon cooling from a high-temperature state. As glass-forming liquids are cooled, their viscosity, or equivalently the structural relaxation time, increases rapidly near the glass transition temperature, a universal hallmark of glasses. The temperature dependence of viscosity is characterized by fragility, which varies widely among glassy liquids: some show the Arrhenius temperature dependence of viscosity or relaxation time ("strong" liquids), while others exhibit super-Arrhenius behavior ("fragile" liquids). We performed extensive molecular dynamics simulations on a realistic glass-forming system, sodium-lead-borate (Na2O-PbO-B2O3), to investigate how increasing lead oxide (PbO) content at the expense of boron oxide (B2O3) influences viscosity and fragility. Our results show a transition from strong to fragile behavior with increasing PbO concentration, elucidating the role of chemical composition in driving this transition. We further examine the Stokes-Einstein (SE) relation, the Kohlrausch-Williams-Watts (KWW) stretch exponent (beta(kww)), and dynamical heterogeneity across the strong-to-fragile spectrum. We find that SE violation becomes more pronounced with increasing fragility, while beta(kww) decreases, indicating stronger deviation from exponential relaxation in fragile glasses. Interestingly, dynamical heterogeneity, characterized by the four-point susceptibility [chi(4)(t)] and the non-Gaussian parameter [alpha(2)(t)], is slightly enhanced in strong glasses despite weaker SE breakdown and higher beta(kww) values. Furthermore, our results suggest that different local structural units play distinct roles in shaping dynamical heterogeneity in strong and fragile glasses. These findings underscore the intricate interplay between fragility, the Stokes-Einstein relation, and dynamical heterogeneity, while emphasizing the crucial role of glass composition in tuning viscosity in real glass-forming systems.

Understanding how amorphous solids yield under shear is central to predicting material failure, yet prescribing reliable local yielding criteria remains a fundamental challenge. Here we introduce the soft matrix method, which creates a minimally constrained and elastically coupled environment that allows localized regions of an amorphous solid to yield naturally. This method overcomes key limitations of earlier approaches and provides a robust platform for probing failure mechanisms in soft disordered materials. Using this framework, we analyze localized yielding by systematically varying the size of the local probe region in our microscopic simulations, and we uncover an intrinsic length scale (ζ) that governs local failure, showing that ζ grows with the age of the system. The age dependence appears not only in the distribution of local yield stresses but also in the pseudogap exponent θ, which quantifies the marginal stability of amorphous solids. These insights offer a pathway toward improved elastoplastic modeling of disordered materials.

The physics of active matter, wherein constituent particles consume energy to generate autonomous motion, has revolutionized non-equilibrium statistical mechanics. While a large body of work has successfully elucidated the behavior of dilute active systems, the dense regime – characterized by “active glasses and active solids” – presents profound challenges that defy conventional theoretical frameworks. Recent observations reveal two striking features in these dense systems: an apparent enhancement of Mermin-Wagner-Hohenberg (MWH) fluctuations leading to anomalous long-wavelength density fluctuations, and a remarkable correspondence between activity-induced annealing and annealing via oscillatory shear. In this perspective article, we propose a novel approach toward a deeper understanding of dense active matter: by developing active Hamiltonian models as equilibrium reference frameworks, we map out pathways toward non-equilibrium active systems. This strategy allows us to elucidate both the correspondence between driven and active systems and the enhanced MWH fluctuations, which likely arise from a strong coupling between spatially random active forces and long-wavelength density (phonon) modes. We outline a comprehensive roadmap employing complementary approaches, including the active Hamiltonian formalism, comparative studies of oscillatory shear in active and passive solids, and investigations of chiral active matter. Establishing this activity-oscillatory shear correspondence across diverse systems is essential to demonstrate its universality, reveal the underlying large-scale emergent physics, and place our hypothesis on a firmer theoretical ground.

The yielding transition marks the onset of irreversible plastic deformation in amorphous solids and plays a central role in determining the mechanical stability and failure of metallic glasses, colloidal suspensions, and biological assemblies. Despite extensive research, the microscopic factors governing the nature of yielding, particularly the transition between brittle and ductile mechanical responses, remain poorly understood. Recent studies have identified kinetic fragility as a key parameter governing yielding in passive glasses; whether this connection persists in active glasses remains open. Here, using molecular dynamics simulations of a Kob-Andersen glass former doped with Run-and-Tumble (RTP) active particles under oscillatory shear, we show that activity systematically reduces kinetic fragility and consequently alters the mechanical response. The common yield point γ_c decreases monotonically with activity and exhibits a power-law dependence on the Arrhenius activation barrier. Increasing activity suppresses the dependence of the yield strain on thermal history and transforms the response from brittle-like to increasingly ductile, with smoother stress relaxation and reduced stress discontinuities. The timescale to reach steady state near yielding retains a critical power-law divergence, indicating that activity does not alter the underlying critical character of the transition. Active glasses also develop broader, more diffuse shear bands. Our results establish kinetic fragility as a unifying parameter governing yielding in both passive and active glasses and demonstrate that activity offers a powerful route to tune the mechanical response of amorphous materials.

Enhancing the mechanical strength and stability of amorphous solids is crucial for material design, with microalloying being a common yet poorly understood method. Using molecular dynamics simulations, we investigate the effect of aspherical impurities on the yielding transition of amorphous solids in the context of the ductile-to-brittle transition associated with microalloying. While spherical impurities larger than the constitutive particles lead to a higher yield strain and increased brittleness, we observe that rod-shaped impurities with an aspect ratio slightly larger than one, which introduce rotational degrees of freedom, have a more pronounced effect. As the aspect ratio increases, their rotational freedom is reduced, leading to more brittle yielding. Completely freezing the rotational degrees of freedom can produce both mechanically and kinetically stable amorphous solids that exhibit brittle yielding. Thus, enhancing brittleness through higher fractions of aspherical impurities presents an opportunity to explore the ductile-to-brittle transition, which is easily accessible in experiments, particularly in colloidal systems. Our finite-size scaling analysis provides compelling evidence for the existence of a finite-disorder critical point as the boundary between ductile and brittle yielding.

Mechanically stable glasses, notably well-annealed and ultrastable ones, typically exhibit brittle failure through shear-band formation. Whether this localization is an inevitable consequence of annealing or can be dynamically controlled remains unknown. We address this by doping amorphous solids with self-propelled particles that perform run-and-tumble or active Brownian dynamics. Through extensive simulations of a polydisperse model, we show that shear-band formation and propagation are governed by a competition among three timescales: the imposed shear time 1 / γ ° , the activity persistence time τp, and the intrinsic time for a shear band to span the system. At low persistence time, active doping progressively converts a single system-spanning shear band into a network of shear bands, yielding continuously with increased yield stress and yield strain, and inter-band spacing that decreases as a power law with active force. Interestingly, we uncover a universal compensatory relationship between active forces and global shear rates: a rise in one offsets a decline in the other, arising from isomorphic-like behavior across force-strain-rate combinations. We further identify a non-monotonic relationship between yielding and persistence time: while the yield stress increases at small τp with increasing active forces, it tends to decrease at large τp. Under creep, increasing active force reduces the steady-state strain rate and delays fluidization, extending the compensation to the stress-controlled regime. These results establish that shear localization in brittle glasses is not dictated solely by preparation history but can be dynamically altered through internal activity, with broad implications for yielding in living and synthetic active matter.

Two-dimensional (2D) systems have attracted renewed interest within the scientific community due to their anomalous dynamical behaviors, which arise from long-wavelength density fluctuations as predicted by the Mermin-Wagner-Hohenberg (MWH) theorem. In equilibrium, it is well established that continuous spontaneous symmetry breaking (SSB) in 2D is prohibited at any finite temperature (T > 0), resulting in the absence of true long-range positional order and establishing d_l = 2 as the lower critical dimension. Recent studies have demonstrated that, in active systems, the lower critical dimension can shift from d_l = 2 to 3. This study examines the impact of MWH theorem violation in active systems on dynamical heterogeneity (DH). As a minimal model, glassy systems of active particles undergoing run-and-tumble (RT) motion are considered. Glass-like dynamical behavior, including anomalously enhanced DH, is observed in various biological systems such as collective cell migration, bacterial cytoplasm, and ant colonies. Furthermore, the study investigates the influence of local positional order, or medium-range crystalline order (MRCO), on DH in the presence of activity. The results indicate that the growth of DH with increasing activity differs significantly between systems with and without MRCO. These findings may have important implications, as many biological systems exhibit local structural ordering, and DH could serve as a useful indicator for quantifying the degree of ordering.

Amorphous materials, especially metallic glasses, are known for their exceptional mechanical properties, such as high yield strength and large yield strain. Understanding the microscopic mechanisms behind their failure, particularly the yielding transition, remains an active area of research. Previous studies have shown that yielding behaviour depends on the initial age of the sample. Through extensive computer simulations, we demonstrate that this age dependence varies across different materials and is influenced by the specific characteristics of the initial glass former, particularly its fragility. Both strong and fragile glass formers exhibit similar yielding behaviour in poorly annealed conditions with a critical yield strain, γc that does not depend on the initial conditions. However, below a critical degree of annealing, the yield point increases significantly with further annealing for fragile glasses, while it remains relatively constant for strong glasses. The results are found to be universal across a wide variety of model glassy systems with varying fragility, including metallic glasses, molecular glasses, model granular glasses, and network-forming glasses like Silica. We rationalise these findings by introducing a modified mean-field elastoplastic model that explicitly incorporates the crucial role of changing energy barrier with increasing annealing in the yielding process. This simple model reproduces all the simulation results and provides critical insights into how energy barriers influence the physics of the yielding transition including the critical yield strain under oscillatory shear deformation.

Universal behavior in far-from-equilibrium systems is driven by interactions between transport processes and noise structure. The Kardar-Parisi-Zhang (KPZ) framework predicts that extensions incorporating conserved currents or temporally correlated noise give rise to distinct growth morphologies and universality classes, yet direct experimental realization has remained elusive. Here, we report atomically resolved Sn thin-film growth on Sb-doped MnBi2Te4, revealing a sharp dynamical crossover between two fundamentally different regimes. Early-stage growth follows Villain-Lai-Das Sarma (VLDS) scaling, forming two-dimensional islands and stanene layers. Beyond a critical deposition time, temporally correlated noise becomes the dominant factor, driving the nucleation of α-Sn clusters, their development into faceted grains, and the coexistence of faceted β-Sn. Molecular dynamics simulations and Auger electron spectroscopy reveal that adatom escape serves as the microscopic origin of this temporally correlated noise, offering a mechanism for the observed universality crossover. These findings establish that temporal noise correlations can fundamentally alter the universality class of a growing interface, linking atomistic kinetics to emergent universal behavior.

The dynamics of dense particle packings near the jamming transition is characterized by correlated particle motion. The growth of dynamical heterogeneities, or strong spatial variations in the motion of the particles constituting the system, is a hallmark feature of slow glassy dynamics. We report here a systematic confocal microscopy study that characterizes the cooperative dynamics of fluorescently-labelled colloidal particles in dense aqueous suspensions. We demonstrate that jammed particulate suspensions can be fluidized by increasing the width of the particle size distribution. Our molecular dynamics simulations, performed to numerically investigate the effects of continuous-size polydispersity on dense particle packing dynamics, show an excellent match with our experimental results. Besides shedding light on the fundamental aspects of particle-scale dynamics at the jamming-unjamming transition, our findings are significant in the processing of commonly-encountered dense suspensions such as paints, cosmetics, and food.

Using particle based simulation of a model glass former, we demonstrate a transition from brittle yielding to ductile yielding in amorphous solids by introducing quenched disorder in the form of randomly pinned particles. The well-annealed samples, which exhibit brittle yielding, undergo a transition to increasingly ductile yielding with increasing pinning concentrations while exhibiting an enhanced stress overshoot. Extensive finite size analysis is performed to demonstrate the critical nature of the transition at a finite pinning concentration and the various scaling exponents obtained are found to be in good agreement with the reported values for the random field Ising model universality class. Finally, we establish a connection between inherent disorder strength of amorphous solids that controls the nature of the yielding and quenched disorder strength due to particle pinning.

Memory effects in amorphous materials have been widely studied because of their possible widespread future applications. We show here that ultrastable glasses can exhibit a transient reversible memory effect when subjected to both a local driving force via run-and-tumble active particles and global shear. We investigate the system's response across different yielding regimes by selectively switching the shear direction at different strains. We analyze how changes in shear direction influence yielding, postyield behavior, and structural evolution in active amorphous solids. Our model active system exhibits an enhanced anisotropic response, displaying both conventional and inverse Bauschinger effects, depending on the deformation history. The results indicate that activity-induced shear band networks create structural memory, enabling the system to heal upon shear reversal due to the transient nature of this phenomenon. Additionally, shear softening under cyclic loading produces irreversible and less branched networks with increasing cycles, a structural evolution underlying the transition from inverse Bauschinger effect to conventional Bauschinger effect. These findings provide novel insights into how activity and shear collectively contribute to mechanical response, including memory formation in ultrastable disordered systems.

Using extensive molecular dynamics simulations, we have performed finite-size scaling (FSS) in the aging regime of a model glass-forming liquid to investigate how the length scales associated with amorphous order (static length) and dynamic heterogeneity (dynamic length) evolve with waiting time. The α-relaxation time in the aging regime reveals non-monotonic finite-size effects with a peak at an intermediate system size, which, as far as we know, are not found in the equilibrium systems, and the peak position shifts to larger system sizes with decreasing temperature and increasing waiting time, indicating a growth of a characteristic length scale with waiting time. The extracted correlation volume associated with amorphous order increases logarithmically with the waiting time. Detailed analysis of the dependence of the length scale on waiting time allowed us to estimate the static length scale in the deep supercooled liquid regime. The dynamic length scale, obtained from FSS and block analysis of the four-point dynamic susceptibility, follows a power-law growth with waiting time. The values of the length scales obtained agree well with those obtained from different spatial correlation functions.

We consider a class of nonstandard, two-dimensional Hamiltonian models that may show features of active particle dynamics, and therefore, we refer to these models as active Hamiltonian (AH) systems. The idea is to consider a spin fluid where-on top of spin-spin and particle-particle interactions-spins are coupled to the particle's velocities via a vector potential. Continuous spin variables interact with each other as in a standard XY model. Typically, the AH models exhibit nonstandard thermodynamic properties (e.g., for temperature and pressure) and equations of motion with nonstandard forces. This implies that the derivation of symplectic algorithms to solve Hamilton's equations of motion numerically, as well as the thermostatting for these systems, is not straightforward. Here, we derive a symplectic integration scheme and propose a Nosé-Poincaré thermostat, providing a correct sampling in the canonical ensemble. The expressions for AH systems that we find for temperature and pressure might have parallels with the ongoing debate about the definition of pressure and the equation of state in active matter systems. For a specific AH model, recently proposed by Casiulis et al. [Phys. Rev. Lett. 124, 198001 (2020)0031-900710.1103/PhysRevLett.124.198001], we rationalize the symplectic algorithm and the proposed thermostatting, and investigate the transition from a fluid at high temperature to a cluster phase at low temperature where, due to the coupling of velocities and spins, the cluster phase shows a collective motion that is reminiscent to that observed in a variety of active systems.

The Stokes-Einstein (SE) relation, which relates diffusion constants with the viscosity of a liquid at high temperatures in equilibrium, is violated in the supercooled temperature regime. Whether this relation is obeyed in nonequilibrium active liquids is a question of significant current interest to the statistical physics community trying to develop the theoretical framework of nonequilibrium statistical mechanics. Via extensive computer simulations of model active glass-forming liquids in three dimensions, we show that SE is obeyed at a high temperature similar to the equilibrium behavior, and it gets violated in the supercooled temperature regimes. The degree of violation increases systematically with the increasing activity which quantifies the amount the system is driven out of equilibrium. First passage-time (FPT) distributions helped us to gain insights into this enhanced breakdown from the increased short-time peak, depicting hoppers. Subsequently, we study the wave vector dependence of SE relation and show that it gets restored at a wave vector that decreases with increasing activity, and the crossover wave vector is found to be proportional to the inverse of the dynamical heterogeneity length scale in the system. Our work showed how SE violation in active supercooled liquids could be rationalized using the growth of dynamic length scale, which is found to grow enormously with increasing activity in these systems.

Active glasses are dense and disordered systems consisting of motile particles that display phenomenology observed in many biological systems. Here we investigate motility-driven annealing and fluidization in these systems and establish a correspondence between the yielding behaviour of glassy systems under active dynamics and their yielding under oscillatory shear. The yielded region of the phase diagram correlates with tissue fluidization, whereas the annealing region explains age-related maturation and stiffening. This suggests that some mechanical changes observed in ageing tissues can partially stem from processes analogous to enhanced ageing observed in active glasses. In addition to showing similar yielding diagrams, we strengthen the correspondence to oscillatory shear by demonstrating diverging time scales to steady states, the possibility of memory encoding and reading, and the importance of stress reversals in the annealing process in both cases. Finally, we study yielding in active solids and demonstrate that given the correct geometry, one can either suppress or promote brittle failure via shear band formation by tuning activity. Active glasses are dense, disordered structures made up of motile constituents. Simulations now show that motility-driven annealing in such systems leads to mechanical changes, including increased brittleness.