(518g) Stokesian Dynamic Simulations and Capillary Forces Between Colloids Floating on a Fluid Interface

The collective dynamics and self-assembly of colloids straddling a gas/liquid or a liquid/liquid interface underlies the physics of many technologies and is relevant to ore flotation, stabilization of Pickering Emulsions, colloid  “armoring” of bubbles and drops, and the self-assembly of particles at interfaces into ordered structures for colloidal crystals and materials fabrication.   

In this presentation we study the self-assembly of colloid particles due to capillary attractive forces between particles, which are heavier than the bounding phases. Surface colloid dynamics is governed by a balance between the lateral interaction forces between the particles on the surface, and the hydrodynamic viscous resistance to motion along the surface. In this presentation, we focus on dynamic assembly driven by capillary attractive forces. The gravitational force depresses the meniscus around the particle, and the overlap of the menisci between particles creates an asymmetric meniscus around the colloids which generated the capillary attraction.  We use Stokesian dynamics to follow the colloid assembly. Our simulations are based on the Langevin equation for the motion of each particle, and include the stochastic Brownian thermal fluctuation force for the case of low Peclet numbers, the inter-particle interactions and the hydrodynamic resistance, each formulated as a pairwise contribution. Significant attention is focused on calculating the pair-wise fluid resistance, incorporating the effects of the immersion depth of the particles and the separation distance.  Simulations from states in which the particles are widely separated demonstrate the organization into clusters. These clusters fractally grow to a critical size for which the collective mass can no longer be supported by surface tension, and instead the mass becomes submerged into the underlying liquid.