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Proc Natl Acad Sci U S A. 2018 May 22;115(21):5403-5408. doi: 10.1073/pnas.1718807115. Epub 2018 May 7.

Flow-induced phase separation of active particles is controlled by boundary conditions.

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Simons Centre for the Study of Living Machines, National Centre for Biological Sciences, Bangalore 560065, India;
International Centre for Theoretical Sciences, Tata Institute of Fundamental Research, Bangalore 560012, India.
Department of Mechanical and Aerospace Engineering, Princeton University, Princeton, NJ 08544.
Joseph Henry Laboratories of Physics, Princeton University, Princeton, NJ 08544.
Department of Theoretical Physics, The Institute of Mathematical Sciences-Homi Bhabha National Institute, Chennai 600113, India.
Department of Applied Mathematics and Theoretical Physics, Centre for Mathematical Sciences, University of Cambridge, Cambridge CB3 0WA, United Kingdom.


Active particles, including swimming microorganisms, autophoretic colloids, and droplets, are known to self-organize into ordered structures at fluid-solid boundaries. The entrainment of particles in the attractive parts of their spontaneous flows has been postulated as a possible mechanism underlying this phenomenon. Here, combining experiments, theory, and numerical simulations, we demonstrate the validity of this flow-induced ordering mechanism in a suspension of active emulsion droplets. We show that the mechanism can be controlled, with a variety of resultant ordered structures, by simply altering hydrodynamic boundary conditions. Thus, for flow in Hele-Shaw cells, metastable lines or stable traveling bands can be obtained by varying the cell height. Similarly, for flow bounded by a plane, dynamic crystallites are formed. At a no-slip wall, the crystallites are characterized by a continuous out-of-plane flux of particles that circulate and re-enter at the crystallite edges, thereby stabilizing them. At an interface where the tangential stress vanishes, the crystallites are strictly 2D, with no out-of-plane flux. We rationalize these experimental results by calculating, in each case, the slow viscous flow produced by the droplets and the long-ranged, many-body active forces and torques between them. The results of numerical simulations of motion under the action of the active forces and torques are in excellent agreement with experiments. Our work elucidates the mechanism of flow-induced phase separation in active fluids, particularly active colloidal suspensions, and demonstrates its control by boundaries, suggesting routes to geometric and topological phenomena in an active matter.


active matter; boundary effects; hydrodynamics; phase separation

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