2D Stokesian Approach to Modeling Flow Induced Deformation of Particle Laden Interfaces

Hydrodynamic interactions between charged and uncharged Brownian colloids at a fluid-fluid interface

HYPOTHESIS

The cluster formation and self-assembly of floating colloids at a fluid/fluid interface is a delicate force balance involving deterministic lateral interaction forces, viscous resistance to relative colloid motion along the surface and thermal (Brownian) fluctuations. As the colloid dimensions get smaller, thermal forces and associated drag 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 for particle pairs 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 hydrodynamic interactions (HI) between particles (including the effect of the particle immersion depth). Simulation results are presented for neutrally wetted particles which form a contact angle θ=90⁰ at the interface.

FINDINGS

The simulation results suggest that clustering, fractal growth and particle ordering become favorable outcomes at critically large values of the Pe numbers, while smaller Pe numbers exhibit higher probabilities of final configurations where particle motion remains uncorrelated in space and particle pairs are found to be more widely separated especially upon the introduction of HI.

Effects of Interfacial Shear on Particle Aggregation at an Oil/Water Interface

Using a Stokesian dynamics simulation, the microstructure of particle aggregates at an oil/water interface with an applied Couette flow is studied. The results of the aggregation are consistent with previously published experimental work demonstrating multiple regimes of behavior based on the relative strength of shear and capillary forces. In previous work, densification of aggregates at low shear rates was theorized to occur due to short time scale fragmentation/reaggregation of aggregates with rigid particle bonds. In simulations, densification is observed at low shear rates but occurs due to local reorganization of particles due to capillary torques over long time scales. Moderate shear rates create mobile bonds between particles at shorter time scales, allowing aggregates to fragment without reaggregation into smaller isolated clusters, consistent with prior experimental work. At the highest shear rates, aggregation is inhibited completely.