Viscoelastic model of phase separation in colloidal suspensions and emulsions

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.

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Phase separation in charge-stabilized colloidal suspensions: Influence of nonlinear screening

Physical Review E ◽

10.1103/physreve.73.041407 ◽

2006 ◽

Vol 73 (4) ◽

Cited By ~ 16

Author(s):

A. R. Denton

Keyword(s):

Phase Separation ◽

Colloidal Suspensions

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A unique route of colloidal phase separation yields stress-free gels

Science Advances ◽

10.1126/sciadv.abb8107 ◽

2020 ◽

Vol 6 (41) ◽

pp. eabb8107

Author(s):

Hideyo Tsurusawa ◽

Shunto Arai ◽

Hajime Tanaka

Keyword(s):

Phase Separation ◽

Mechanical Stress ◽

Volume Fraction ◽

Colloidal Suspensions ◽

Biological Science ◽

Substantial Impact ◽

Gel Network ◽

Selection Of ◽

Particulate Systems

Phase separation often leads to gelation in soft and biomatter. For colloidal suspensions, we have a consensus that gels form by the dynamical arrest of phase separation. In this gelation, percolation of the phase-separated structure occurs before the dynamical arrest, leading to the generation of mechanical stress in the gel network. Here, we find a previously unrecognized type of gelation in dilute colloidal suspensions, in which percolation occurs after the local dynamical arrest, i.e., the formation of mechanically stable, rigid clusters. Thus, topological percolation generates little mechanical stress, and the resulting gel is almost stress-free when formed. We also show that the selection of these two types of gelation (stressed and stress-free) is determined solely by the volume fraction as long as the interaction is short-ranged. This universal classification of gelation of particulate systems may have a substantial impact on material and biological science.

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