Particle pinning suppresses spinodal criticality in the shear-banding instability

Absorbing-state transitions in particulate systems under spatially varying driving

Non-equilibrium transitions into absorbing states are widespread, and amorphous materials under cyclic shear have emerged as useful model systems in which to study their properties. Recent work has focused on homogeneous driving in which the shear amplitude is uniform in space, despite most real world flows involving spatially inhomogeneous conditions that are known to produce qualitatively distinct phenomenology. Here we study the absorbing state transition under inhomogeneous driving using a modified random organization model. For smoothly varying driving the steady state results map onto the homogeneous absorbing state phase diagram, with the position of the boundary between absorbing and diffusive states being insensitive to the driving wavelength. The phenomenology is well-described by a one-dimensional generalized continuum model that we pose. For discontinuously varying driving the position of the absorbing phase boundary and the exponent characterising the fraction of active particles are altered relative to the homogeneous case.

Effects of quenched disorder on the kinetics and pathways of phase transition in a soft colloidal system

Although impurities are unavoidable in real-world and experimental systems, most numerical studies on nucleation focus on pure (impurity-free) systems. As a result, the role of impurities in phase transitions remains poorly understood, especially for systems with complex free energy landscapes featuring one or more intermediate metastable phases. In this study, we employed Monte Carlo simulations to investigate the effects of static impurities (quenched disorder) of varying length scales and surface morphologies on the crystal nucleation mechanism and kinetics in the Gaussian core model system-a representative model for soft colloidal systems. We first explored how the nucleation free energy barrier and critical cluster size are influenced by the fraction of randomly pinned (or, static) particles (fp) and the size (np) of the pinned region or cluster. Both the nucleation free energy barrier and critical cluster size increase sharply with increasing fp but decrease as np grows for a given fraction of pinned particles, eventually approaching the homogeneous nucleation limit. On examining the impact of impurity's surface morphology on nucleation kinetics, we observed that the nucleation barrier significantly decreases with increasing the impurity (or, seed) size with crystalline surface morphologies with body-centered cubic showing the greatest facilitation. Interestingly, seeds with random surface roughness had little effect on nucleation kinetics. In addition, the polymorphic identity of particles in the final crystalline phase is influenced by both the seed's surface morphology and system size. This study further provides crucial insights into the intricate relationship between surface-induced local structural fluctuations and the selection of the polymorphic identity in the final crystalline phase, which is essential for understanding and controlling crystallization processes in experiments.

Athermal quasistatic cavitation in amorphous solids: Effect of random pinning

Amorphous solids are known to fail catastrophically via fracture, and cavitation at nano-metric scales is known to play a significant role in such a failure process. Micro-alloying via inclusions is often used as a means to increase the fracture toughness of amorphous solids. Modeling such inclusions as randomly pinned particles that only move affinely and do not participate in plastic relaxations, we study how the pinning influences the process of cavitation-driven fracture in an amorphous solid. Using extensive numerical simulations and probing in the athermal quasistatic limit, we show that just by pinning a very small fraction of particles, the tensile strength is increased, and also the cavitation is delayed. Furthermore, the cavitation that is expected to be spatially heterogeneous becomes spatially homogeneous by forming a large number of small cavities instead of a dominant cavity. The observed behavior is rationalized in terms of screening of plastic activity via the pinning centers, characterized by a screening length extracted from the plastic-eigenmodes.