Microscale dynamics in emulsions: The effect of concentration and non-axisymmetry

The morphology of an emulsion depends on the dynamics of the dispersed phase under the prescribed flow conditions. This work is a two part study of the microscale dynamics of drops in emulsions.;In the first part, we developed an experimental technique to study the influence of the many-body interactions on the microscale dynamics (at the level of individual drops) in concentrated emulsions. The deformation, break-up and coalescence of micron-sized polydimethylsiloxane (PDMS) drops in a refractive index-matched suspension of polymethylmethacrylate (PMMA) particles under-going a planar hyperbolic flow was investigated. The presence of particles produces various short-ranged stochastic effects and macroscopic mean-field effects, both of which influence the dynamics of drops at the microscale. In our experiments, the relative influence of the local and macroscopic effects was varied by changing the particle to drop size ratio (dp/d). At small dp/d, the macroscopic effects dominated and the particles provided only an increased viscosity contribution to the break-up and deformation of the drop. For the parameters of this study, coalescence was enhanced and the thin film formed during coalescence was particle-free. At large dp/d, the local flow fluctuations and direct interactions with particles had a dominant influence on the drop dynamics.;In the second part, we examined the possibility of a non-axisymmetric rupture of the thin film formed between two approaching drops by carrying out a three-dimensional linear stability analysis. The stability calculation was interfaced with the full axisymmetric simulation of two drops approaching in an external flow in order to obtain the correct base state film shape at each time interval. We showed that the thin film first becomes unstable to a non-axisymmetric disturbance under all conditions studied here. A comparison of the critical thickness from the stability calculations with the full drop axisymmetric calculations was made. The drainage time for coalescence obtained from the stability calculations showed a good agreement with the experimental observations. The calculations showed that the curvature of the base-state film shape had a pronounced effect on its stability to perturbations.