University of Cambridge > Talks.cam > DAMTP Statistical Physics and Soft Matter Seminar > From active liquids to active solids: a tale of criticality, phase transitions and phase separations

From active liquids to active solids: a tale of criticality, phase transitions and phase separations

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Active matter provides a fertile ground for discovering novel physics. Active systems display a wealth of emergent phenomena which are not found in equilibrium systems. For active liquids, previous works observed: motility-induced phase separation (MIPS), long-ranged ordered (collective motion) phase in two dimensions, and order-disorder phase co-existences (banding and reverse-banding regimes). In the first part of this talk, I will show how we unify these diverse phase transitions and phase co-existences into a single formulation based on generic hydrodynamic equations for polar active fluids. In doing so, we reveal the existence of a novel co-moving phase co-existence and of a novel multicritical point. In the second part of the talk, I will consider very dense active systems. In recent years, much progress has been made in the understanding of the so-called athermal jamming transition, a rigidity transition particulate matter generically undergo as their volume fraction is increased. However, our current understanding of active jamming is crucially lacking. While it is natural to expect that applying high enough self-propulsion forces to initially passive jammed systems will result into unjamming of the systems, what kind of universal behavior (if any) is observed around the athermal critical jamming point when active or thermal fluctuations are added remains unknown. By means of numerical simulations and mean-field analytical arguments, we probe the effect of 1) persistent active and 2) thermal fluctuations on a dense, athermal jammed system of spheres in 2D and ask the question of whether a strictly jammed system (understood as remaining above isostaticity) exists at non-zero fluctuations strengths. We uncover critical behaviour in the vicinity of the athermal jamming critical point which is strikingly independent of the type of perturbation (active or thermal).

This talk is part of the DAMTP Statistical Physics and Soft Matter Seminar series.

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