Self-propulsion and dispersion of reactive colloids due to entropic anisotropy

Abstract

In this paper, self-motion of reactive colloids and their dispersion behaviour are theoretically examined. The motion is driven by an osmotic force imbalance arising from non-uniform atmospheres of reactive solutes around the colloids. The propulsion here is not limited to Janus-like particles. It can also occur to particles having ‘uniform’ reactivity due to the more universal mechanism – entropic anisotropy created by breaking in rotational symmetry. The idea is demonstrated by examining the motion of a reactive particle due to asymmetry in its shape or to the presence of an additional particle. In the two-particle problem, in particular, we find that sink (source) particles can self-migrate towards (apart from) each other at velocities varying asR−2, resembling Coulomb attraction (repulsion), whereRis the inter-particle distance. Because of this Coulomb-like nature, a suspension of sink particles could undergo collective flocculation due to unscreened osmotic attraction. The criterion for an occurrence of the flocculation is also established. It reveals that the flocculation can occur if the particle volume fraction is within a certain window in terms of the solute concentration and the particle reactivity. The stability of reactive suspensions is also discussed using the modified Derjaguin–Landau–Verwey–Overbeek (DLVO) theory that takes account of the competition between long-range reaction-induced osmotic forces and short-range colloidal forces. A more generalized view for the present self-driven particle motion is elucidated by a simple scaling theory, providing lucid accounts for the self-motion of two particles, composite bodies, and Janus particles – all are driven by dipolar distortions in potential energy. Comparison with phoretic self-swimmers is also discussed.