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

Abstract

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.

Collisions and rebounds of chemically active droplets

“…To retain the relative simplicity of an axisymmetric problem, the configuration considered here is highly symmetric as only the normal approach of a single droplet to a flat wall is considered. Yet, this provides an important physical insight into the interaction and rebound dynamics, which could contribute significantly to a better understanding of experimental studies involving confined active droplets: several recent contributions have indeed suggested that the collective behaviour of many self-propelled droplets is greatly influenced by the role of confinement on their interactions [21,59]. The present analysis also provides a critically-valuable benchmark analysis for the validation of simpler models (e.g.…”

Modeling chemo-hydrodynamic interactions of phoretic particles: A unified framework

“…Thanks to these generic common features, multiple interaction routes can be envisioned for this class of systems, either directly through the flow generated by the motion of one particle in the vicinity of its neighbors or through its physico-chemical signature and resulting gradients near other particles. How such long-range interaction routes compete and condition the collective dynamics of these systems as observed in experiments, and how this interplay is modified by the varying particle density or their environment are key questions, currently at the center of attention of the physical community [42][43][44].…”

Bouncing, chasing, or pausing: Asymmetric collisions of active droplets

“…Instead, the emergence of spontaneous dynamics stems from the nonlinear coupling of physico-chemical processes and convective transport. Beyond steady and directed selfpropulsion, active droplets may exhibit complex curling trajectories [14][15][16], random diffusive behaviour [15,17] and unsteady self-propulsion [18] at the individual level, and collective dynamics such as avoidance [19], aggregation [20,21], polar alignment [5], crystallization [22] and train formation [23].…”