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Register a new userCritical motility-induced phase separation belongs to the Ising universality class
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A collection of self-propelled particles with volume exclusion interactions can exhibit the phenomenology of gas-liquid phase separation, known as motility-induced phase separation (MIPS). The non-equilibrium nature of the system is fundamental to the phase transition, however, it is unclear whether MIPS at criticality contributes a novel universality class to non-equilibrium physics. We demonstrate here that this is not the case by showing that a generic critical MIPS belongs to the Ising universality class with conservative dynamics.
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Using computer simulations and dynamic mean-field theory, we demonstrate that fast enough rotation of circle active Brownian particles in two dimensions generates a dynamical clustering state interrupting the conventional motility induced phase separation (MIPS). Multiple clusters arise from the combination of the conventional MIPS cohesion, and the circulating current caused disintegration. The non-vanishing current in non-equilibrium steady states microscopically originates from the motility ``relieved by automatic rotation, which breaks the detailed balance at the continuum level. This mechanism sheds light on the understanding of dynamic clusters formation observed in a variety of active matter systems, and may help examine the generalization of effective thermodynamic concepts developed in the context of MIPS.
We use large-scale Monte Carlo simulations to test the Weinrib-Halperin criterion that predicts new universality classes in the presence of sufficiently slowly decaying power-law-correlated quenched disorder. While new universality classes are reasonably well established, the predicted exponents are controversial. We propose a method of growing such correlated disorder using the three-dimensional Ising model as benchmark systems both for generating disorder and studying the resulting phase transition. Critical equilibrium configurations of a disorder-free system are used to define the two-value distributed random bonds with a small power-law exponent given by the pure Ising exponent. Finite-size scaling analysis shows a new universality class with a single phase transition, but the critical exponents $ u_d=1.13(5), eta_d=0.48(3)$ differ significantly from theoretical predictions. We find that depending on details of the disorder generation, disorder-averaged quantities can develop peaks at two temperatures for finite sizes. Finally, a layer model with the two values of bonds spatially separated to halves of the system genuinely has multiple phase transitions and thermodynamic properties can be flexibly tuned by adjusting the model parameters.
We investigate critical phenomena in colloids by means of the renormalization-group based hierarchical reference theory of fluids (HRT). We focus on three experimentally relevant model systems: namely, the Asakura-Oosawa model of a colloidal dispersion under the influence of polymer-induced attractive depletion forces; fluids with competing short-range attractive and longer-range repulsive interactions; solutions of star-polymers whose pair potential presents both an attractive well and an ultrasoft repulsion at shorter distance. Our results show that the ability to tune the effective interactions between colloidal particles allows one to generate a variety of crossovers to the asymptotic critical behavior, which are not observed in atomic fluids.
In nature, objects which are in thermal contact with each other, usually approach the same temperature, unless a heat source (or sink) cherishes a persistent flow of heat. Accordingly, in a well-isolated apartment flat, most items are at a similar temperature. This is a general consequence of equilibrium thermodynamics, requiring coexisting phases to have identical temperatures. Opposing this generic situation, here we identify a system showing different temperatures in coexisting phases, which are separated from each other by a sharp and persistent temperature gradient. Thermodynamically, such a hot and a cold phase are allowed to coexist, as the system we consider comprises active particles which self-propel relative to their environment and are thus intrinsically out-of-equilibrium. Although these microparticles are well known to spontaneously phase-separate into a liquid- and a gas-like state, different kinetic temperatures in coexisting phases occur if and only if inertia is introduced, which is neglected in standard models describing active particles. Our results, therefore, exemplify a novel route to use active particles to create a self-sustained temperature gradient across coexisting phases, a phenomenon, which is fundamentally beyond equilibrium physics.
Motility-induced phase separation is a purely non-equilibrium phenomenon in which self-propelled particles aggregate without any attractive interactions. One surprising feature of MIPS is the emergence of polar-nematic order at the interfacial region, whose underlying physics remains poorly understood. Here, I will show analytically and numerically that the many-body physics leading to the interfacial ordering behavior can be captured by an effective speed model. In this model, each particles speed depends on the systems density a short distance ahead of its direction of motion.
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