We study the spreading phenomena when drops of hydrocarbon oil are placed on aqueous solution of surfactant (CTAB). The rate of spreading depends significantly upon the surfactant concentration (and is zero below a certain threshold). The time evolution of the spread layer and its structure are investigated by ellipsometry. The thickness is determined from analysis of the data for the ellipsometric angles Δ and Ψ. The signal behaviour vs. time, and the pattern of distribution of the experimental points, suggest that a very thin precursor layer (~1 nm) coexists with flat lenses (~100 nm). This features a pseudo-partial wetting of the surfactant solution by the hydrocarbon. In contrast, silicone oil is found to give complete spreading, with layers whose thickness is rather large and non-uniform. The surface pressure is measured with a Langmuir trough, at an A/W interface containing alkane – after a fast initial non-equilibrium spreading of drops of pure oil, saturation of the surface is rapidly reached. The complex spreading behaviour in the studied system can have implications to cases of practical relevance. We have recently shown [1] that the pseudo-partial wetting of the air/ water boundary by oil is of crucial importance for spontaneous formation of nano-droplets: in a fast dynamic process the spreading layer of oil can be dispersed into the bulk aqueous solution during rupture of air bubbles.
[1] Jie Feng et al., Nanoemulsions obtained via bubble bursting at a compound interface, Nature Physics 10 (2014) in press.
