Dynamic behaviour of drops in oil/water/oil dispersions

Abstract

The dynamic behaviour of drops of oil/water/oil (O/W/O) and water/oil (W/O) in abnormal polymer/water/surfactant systems was investigated. The size of internal oil droplets continuously decreased with time until it reached a steady-state value. Whereas the size of multiple water drops showed a minimum. After the minimum, the size of multiple water drops either reached a steady-state value or continued enlarging until phase inversion occurred. The phase inversion occurred because inclusion of oil droplets into water drops resulted in a continuous increase in effective volume fraction of dispersed phase. The time evolution of the size of multiple drops was described in terms of a balance between (a) drop break-up and escape and (b) drop coalescence and inclusion. The inclusion events retarded the initial decrease in the size of multiple water drops with time and increased the drop size after the minimum. By reducing the surfactant concentration, the ability of the dispersed phase to entrain the continuous phase decreased so that no minimum was achieved for the size of multiple drops with time, similar to conventional systems with simple drops. The size distribution of the multiple water drops initially narrowed and then widened again, whereas the size distribution of internal oil droplets continuously narrowed with time until it reached a constant value. Generally, the size distribution of drops narrowed as the average size of drops decreased. The possible mechanisms for complex drop formation were discussed and drop deformation was suggested as the main cause for inclusion at a low dispersed phase ratio.