Emergent vortices in populations of colloidal rollers

Bricard, Antoine and Caussin, Jean Baptiste and Das, Debasish and Savoie, Charles and Chikkadi, Vijayakumar and Shitara, Kyohei and Chepizhko, Oleksandr and Peruani, Fernando and Saintillan, David and Bartolo, Denis (2015) Emergent vortices in populations of colloidal rollers. Nature Communications, 6. 7470. ISSN 2041-1723

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    Abstract

    Coherent vortical motion has been reported in a wide variety of populations including living organisms (bacteria, fishes, human crowds) and synthetic active matter (shaken grains, mixtures of biopolymers), yet a unified description of the formation and structure of this pattern remains lacking. Here we report the self-organization of motile colloids into a macroscopic steadily rotating vortex. Combining physical experiments and numerical simulations, we elucidate this collective behaviour. We demonstrate that the emergent-vortex structure lives on the verge of a phase separation, and single out the very constituents responsible for this state of polar active matter. Building on this observation, we establish a continuum theory and lay out a strong foundation for the description of vortical collective motion in a broad class of motile populations constrained by geometrical boundaries.