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Glassy Dynamics and Jamming in Dense Persistent Active Matter

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SPL - New statistical physics in living matter: non equilibrium states under adaptive control

In several biological systems, such as bacterial cytoplasm, cytoskeleton-motor complexes and epithelial sheets of cells, self-propulsion or activity is found to fluidize a glassy state that exhibits characteristic glassy features in the absence of activity. Recent experiments on dense systems of Janus colloids and vibrated granular systems have provided a lot of information about how activity affects glassy dynamics and jamming. To develop a theoretical understanding of these non-equilibrium phenomena, we have studied, using molecular dynamics and Brownian dynamics simulations, the effects of activity in several model glass-forming liquids. The activity in these systems is characterized by two parameters: the magnitude of the active force and its persistence time. If the persistence time is short, then the observed behaviour is similar to that near the usual glass transition. The introduction of activity reduces the glass transition temperature and decreases the kinetic fragility. Some of these effects can be understood from a generalization of the Random First Order Transition (RFOT) theory of the glass transition to active systems. For large but finite persistence times, the approach to dynamical arrest at low propulsion force goes through a phase characterized by intermittency. This intermittency is a consequence of long periods of jamming followed by bursts of plastic yielding, akin to the response of dense amorphous solids to an externally imposed shear. In the limit of infinite persistence time, the homogeneous liquid state obtained at large values of the active force exhibits several unusual properties: the average kinetic energy increases with increasing system size and a length scale extracted from spatial velocity correlations increases with system size as a power law with exponent close to one. This active liquid evolves to a force-balanced jammed state when the self-propulsion force is decreased below a threshold value. The jamming proceeds via a three-stage relaxation process whose timescale grows with the magnitude of the active force and the system size. We relate the dependence on the system size to the large correlation length observed in the liquid state. Some of the properties of the jammed state obtained for small active force are found to be substantially different from those of passive jammed systems.

This talk is part of the Isaac Newton Institute Seminar Series series.

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