The critical conditions for coalescence in phase field simulations of colliding droplets in shear.

Simulations have been performed using the free-energy binary-liquid lattice Boltzmann method with sufficient resolution that the critical capillary number for coalescence was determined for collisions between droplets in simple shear with a small initial offset in the shear gradient direction. The simulations were used to study the behavior of the interacting interfaces and the film between them during collisions over a wide range of capillary numbers with emphasis on near-critical conditions. From these three-dimensional simulations with deforming interfaces, several features of the evolution of the film between the drops were observed. The critical film thickness was determined to be similar to the interface thickness, a power law described the dependence of the minimum film thickness on the capillary number in collisions without coalescece, and an inflection point was found in the dynamics of the minimum distance between drops that eventually coalesce. The rotation of the film and the flow in it were also studied, and a reversal in the flow was found to occur before coalescence. The mobility of the phase field was therefore important in the continued thinning of the film at the points of minimum thickness after the flow reversal. A comparison of the critical capillary number and critical film thickness in the simulations with the values for experiments in confined simple shear indicated that the effective physical radius of the simulated droplets was on the order of several micrometers. The results are significant for simulations of droplet interactions and emulsion flows in complex geometries and turbulence because they demonstrate the necessary scale of the computations and how parameters, such as the interface thickness and phase field mobility, should be selected for accurate results.