Diffuse-Interface Simulations of Drop Coalescence and Retraction in Complex Fluids

Drop dynamics plays a central role in defining the interfacial morphology in two-phase complex fluids such as emulsions and polymer blends. In such materials, the components are often microstructured complex fluids themselves. To model and simulate drop behavior in such systems, one has to deal with the dual complexity of non-Newtonian rheology and evolving interfaces. Recently, we developed a diffuse-interface formulation which incorporates complex rheology and interfacial dynamics in a unified framework. This paper describes applications of our method to simulate drop coalescence after head-on collision and drop retraction in a quiescent matrix. One of the two phases is Newtonian and the other is rheologically complex; we have considered viscoelastic fluids modeled by an Oldroyd-B equation and nematic liquid crystals described by a modified Leslie-Ericksen model. After two drops collide, film drainage is enhanced when either phase is viscoelastic and drop coalescence happens more readily than in a comparable Newtonian system. The retraction of drops from a stationary state of zero-velocity and zero-stress is initially hastened but eventually hindered by viscoelasticity in either component. Newtonian theories may be used to back out the interfacial tension only if the polymer relaxation time is much shorter than the drop retraction time. When retracting from an initial state with pre-existing stress, as produced by steady shearing, viscoelasticity in the matrix hinders retraction from the beginning while that in the drop initially enhances retraction but later resists it. The retraction of nematic drops reveals interactions among surface tension, surface anchoring and bulk orientation. The dynamic interfacial tension drives a Marangoni flow near the isotropic-nematic interface. In general, the retraction process cannot be used as a means of measuring the interfacial tension between a liquid crystalline polymer and a flexible polymer.