Coalescence-induced coalescence of inviscid droplets in a viscous fluid

A comprehensive simulation of the coarsening mechanism coalescence-induced coalescence (CIC) is developed to predict the growth rate of inviscid dropl ets in a viscous matrix fluid. In CIC, the shape relaxations of coalescing droplets establish flow fields that drive other droplets into contact, thus creating a cascade of coalescence events. It is believed that CIC is respo nsible for droplet growth in some demixed polymer solutions, such as isotac tic polypropylene (iPP) and diphenyl ether (DPE). A cascade of coalescence events is simulated using a three-dimensional molecular dynamics-like simul ation of a dispersed two-phase isopycnic fluid system. The coalescence-indu ced flow is driven mostly by the strong gradients in curvature at the neck of a coalescing pair of droplets, and the flow is modeled analytically by a pproximating it as due to a ring of point forces. The resultant velocity of each droplet in the suspension is calculated by superimposing all of the c oalescence-induced flow fields and applying Faxen's Law. The mean droplet s ize [a] grows like t(xi), where t is the coarsening time and xi a growth ex ponent that increases with increasing minority phase volume fraction to. Go od agreement with experimental values of xi (0.22 < <xi> < 0.47) is obtaine d for a phase-separated iPP-DPE solution for <phi> greater than or equal to 0.23. It is also shown that the droplet size distribution broadens for sem idilute suspensions (phi less than or equal to 0.42) but remains relatively narrow for highly concentrated suspensions (phi greater than or equal to 0 .54). A phenomenological kinetic theory of coalescence is proposed. It is b elieved that in nondilute emulsions, CIC can account for coarsening that ha s been attributed previously to more traditional coalescence mechanisms. (C ) 2000 Academic Press.