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CeNS Colloquium

Kleiner Physikhörsaal, LMU
Date: 26.01.2018, Time: 15:30h

Phase separation in active matter: MIPS and FIPS

Dr. Shashi Thutupalli, National Centre for Biological Sciences Bangalore, India

The complex interplay between the geometry of the macroscopic system and the microscopic interactions among individuals is often hard to resolve. We combine experiments, theory and numerical simulations, to study the influence of boundaries on the fluid-mediated, dissipative, many-body forces and torques that determine the collective dynamics of self-propelled particles. Using experiments with active emulsion droplets whose motion is fully three dimensional, we demonstrate that geometric confinement alters the far-field flow of the particles and, thereby, their hydrodynamic interactions. These changes give rise to distinct states of collective organization: two- dimensional crystals arrested at free interfaces, three-dimensional crystals stabilized by vorticity, and one-dimensional quasi-stable lines that travel in both two and three dimensions. We rationalize these experimental results by computing the slow viscous flow produced by the droplets in the presence of boundaries. Numerical simulations based on the theory are in excellent agreement with experiment. Our work elucidates how macroscopic boundaries, by altering the microscopic interactions between constituents, influence the emergence of long-ranged order and long-lived structures in non equilibrium systems. We establish the paradigm of flow-induced phase separation (FIPS) in self-propelled particle populations with relevance to the formation of biological aggregates at multiple scales and to the emerging field of geometric and topological synthetic active matter. I will end by contrasting FIPS with Motility-Induced Phase Separation (MIPS) a complementary mechanism for the aggregation of active particles and will discuss MIPS in the context of our recent work on the fruiting body formation of Myxococcus Xanthus, a soil bacterium.