Binary droplet interactions in shear water-in-oil emulsion: A molecular dynamics study

The shear field and binary droplet interaction regimes of water-in-oil emulsions between parallel gold plates were investigated by molecular dynamics (MD) methods. The developed MD model was subjected to dimensional analysis and the dimensionless numbers that guarantee the similarity between the model and the actual system were identified as the capillary number (Ca), Reynolds number (Re), Weber number (We), and dimensionless time (τ). The effects of different pressures were compared by applying different forces perpendicular to the shear direction to the parallel plates. The heptane formed adsorbed layers on the gold plates, exhibited a shear thinning characteristic, and formed a no-slip uniform shear field driven by the parallel plates. The interactions between gold atoms and a heptane molecule have been investigated in depth by quantum chemical methods and it has been found that the nature of the adsorption is a van der Waals interaction. Three main binary droplet interaction regimes are produced in the presence of shear fields: coalescence, a temporary bridge formation, and sliding. The effects of capillary forces and shear effects on liquid bridges were discussed. Droplet trajectories and deformations under different pressure were analyzed. Finally, the effect of initial offset (ΔYin/2R) and Ca on the droplet interaction regime was investigated through regime maps. It was found that the coalescence regime has upper critical points for ΔYin/2R and Ca; droplet slipping occurs mainly at higher ΔYin/2R; the temporary bridge formation regime appears mainly at higher Ca. This study provides a new idea for understanding binary droplet interactions in shear emulsions.