Numerical Investigation of non-head-on collision and coalescence in oil-in-water emulsions

The stability of oil-in-water emulsions depends on the ease with which the dispersed oil phase droplets come in contact (or collide) with each other, and subsequently coalesce. Whether a phase separation is desired or not, controlling this coalescence process requires a thorough understanding of the approach dyanmics of the dispersed phase droplets, which in turn, governs the drinage (of the thin continuous film between the droplets) dynamics. Theoretical and Experimental studies mainly focus on head-on collisions between such droplets and the subsequent coalescence process. In reality, however, concentrated emulsions consist of a large number of droplets in a small amount of space, resulting in non-head-on contact/collision among droplets where the dynamics is surely different to that for head-on collisions. In this work, the effect of the angle of attack, θ, on coalescence in oil-in-water emulsions has been investigated using the Lattice Boltzmann Method. We notice that an increase in the angle of attack leads to a decrease in the collision time and the coalescence time. The decrease in the collision time is due to the geometric layout of the collision course and a local flow caused by the shear stress exerted by the droplets on the thin film in between. Once the film has drained enough, film rupture occurs, and a thin bridge forms between them, which defines the moment of collision. Finally, we also incorporate local changes in viscosity and interfacial tension during the non-head-on collisions.