Optically profiling a draining aqueous film confined between an oil droplet and a solid surface: Effect of nonionic surfactant

Abstract

We have profiled the deformation of an oil droplet during its approach to a silica surface, monitoring the drainage of the intervening aqueous film in real time, using imaging ellipsometry/reflectometry. Specifically, we investigated how film drainage and droplet deformation are affected by the addition of the nonionic surfactant C12E8. The droplet dimples at the beginning of film drainage at its center. At low surfactant concentrations (<10% of the cmc) we observe film recovery, due to the Marangoni effect reversing the direction of fluid flow. High surfactant concentrations minimize interfacial tension gradients, thereby hindering Marangoni flow. Lowering of the oil-water interfacial tensi..