(543a) Understanding the Collision and Coalescence Mechanism of Micro-Droplets on Thin Liquid Coating

Interactions between deformable liquid droplets and liquid coatings play important roles in numerous industrial processes including downstream processing, emulsification, and large-scale oil-water separation. Understanding the interaction mechanisms of an oil droplet and an opposing oil layer is therefore of fundamental and practical importance to investigate many complex and challenging topics, especially for structural optimization of several industrial facilities in field-processing. In this presentation, we will provide systematic quantification of interaction forces and the spatiotemporal evolution of oil droplets colliding on oil film dynamically in an aqueous medium with various film thicknesses and surface tension. The full Approaching-Collision-Coalescence behaviors are fully described together with the specific role played by the finite film thickness of the oil layer. The constraint due to solid substrate on oil and water drainage is studied through extending the current SRYL theory which only focuses on specific cases of droplet-droplet or droplet-substrate interactions to include an additional draining layer so that a more general model for all film thicknesses can be developed. A colloidal probe AFM experiment is performed to validate our theoretical model with a novel surface processing technique applied to form a uniform and stable oil film in water. Our results provide key control parameters like oil/water viscosity ratio, together with their scaling laws in the dynamic droplet-film collision process with valuable implications on the stabilization/adhesion/wetting mechanisms of oil/water/solid systems in presence of surfactant and multiple liquid/liquid and liquid/solid interfaces.