|Title||Flow-induced phase separation of active particles is controlled by boundary conditions|
|Publication Type||Journal Article|
|Year of Publication||2018|
|Authors||Thutupalli S, Geyer D, Singh R, Adhikari R, Stone HA|
|Journal||Proceedings of the National Academy of Sciences|
|Keywords||Active matter, boundary effects, hydrodynamics, phase separation|
Active particle suspensions comprise energy-consuming and hydrodynamically interacting units, such as swimming microorganisms, autophoretic colloids, and active droplets. Though it is now recognized that emergent self-organization in such systems is driven by their spontaneous hydrodynamic flow, it is still not well-understood how the modification of this flow by confining boundaries impacts self-organization. Here we combine experiments, theory, and simulations to elucidate the effect of boundaries on the spontaneous flow in a suspension of active emulsion droplets. Our results establish a widely applicable paradigm for flow-induced phase separation in active fluids and offer routes to manipulating their microstructure.
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.
|Short Title||Proc Natl Acad Sci USA|