Hydrodynamic Interactions Between Charged and Uncharged Brownian Colloids at a Fluid-Fluid Interface

Abstract: Hypothesis: The collective dynamics and self-assembly of colloids floating at a fluid/fluid interface is a balance between deterministic lateral interaction forces, viscous resistance to colloid motion along the surface and thermal (Brownian) fluctuations. As the colloid size decreases, thermal forces become important and can affect the self assembly into ordered patterns and crystal structures that are the starting point for various materials applications. Numerics: Langevin dynamic simulations involving two particles straddling a liquid/liquid interface with a high viscosity contrast are presented to describe the lateral interfacial assembly of particles in Brownian and non-Brownian dominated regimes. These simulations incorporate capillary attraction, electrostatic repulsion, thermal fluctuations and HI between particles (including the effect of the particle immersion depth). Simulation results are presented for neutrally wetted particles which form a contact angle of 90 degrees at the interface. Findings: Clustering, fractal growth and particle ordering are observed at critically large values of the Pe numbers, while smaller Pe numbers exhibit higher probabilities of yielding states in which particles remain uncorrelated in space and more widely separated.