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

Published online by Cambridge University Press:  17 June 2010

HSIEN-HUNG WEI  and
JENG-SHIUNG JAN
HSIEN-HUNG WEI*
Affiliation:
Department of Chemical Engineering, National Cheng Kung University, Tainan 701, Taiwan
JENG-SHIUNG JAN
Affiliation:
Department of Chemical Engineering, National Cheng Kung University, Tainan 701, Taiwan
*
†Email address for correspondence: [email protected]
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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 as R−2, resembling Coulomb attraction (repulsion), where R is 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.

Type
Papers
Information
Journal of Fluid Mechanics , Volume 657 , 25 August 2010 , pp. 64 - 88
Copyright
Copyright © Cambridge University Press 2010

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