(690f) Effect of Surface Wettability Variation on Emulsion Morphology and Stability

Emulsions, liquid droplets dispersed in another immiscible liquid, are widely used in a myriad of applications in chemical engineering, analytical chemistry, pharmaceuticals, material engineering, oil and gas industry, and food technology. One of the most important characteristics of emulsion is its stability against separation. In this study, we carry out numerical simulations to investigate (oil-in-water) emulsion stability and morphology in a microchannel of heterogeneous surface wettability. The objective here is to elucidate the effect of surface wettability on drop motion via flow dynamics simulations.

We apply a conservative diffuse-interface lattice Boltzmann method (LBM) to perform three-dimensional numerical simulations of liquid-liquid flow. The numerical problem consists of a single (oil) drop immersed into an immiscible continuous phase (water) in a square microchannel with alternating surface wettability: hydrophilic, hydrophobic, and again hydrophilic sections. The two liquids have equal densities, and we vary the viscosity ratio between drop (oil) and continuous phase (water). Depending on the drop length and capillary number, calculated based on the velocity of the drop and dynamic viscosity of the continuous phase, four different flow patterns are observed when the oil drop passes through the hydrophobic section. These distinct flow patterns include (a) passing without any changes in drop shape or dynamics, (b) adhesion of the dispersed liquid to the walls, (c) phase inversion (i.e., water becomes the dispersed phase), and (d) drop breakage. These outcomes are in close agreement with the existing experimental data. The morphology of the oil emulsion and the corresponding velocity field for each case are shown in Fig 1.

Our simulation results reveal multiple recirculations present inside the oil droplet and within the surrounding thin-wetting film. Our preliminary data show that the thin-film recirculation tends to tear the film apart, leading to a change of emulsion morphology. These results suggest that the thin-film flow field plays a vital role in keeping emulsion stability.

In this presentation, we will elucidate how the emulsion stability and morphology under heterogeneous surface wettability are influenced by the following crucial factors: (i) the fluid viscosity ratio, (ii) surface wettability values, and (iii) the interfacial tension between the dispersed and continuous phases.