Superspreading of water—silicone surfactant on hydrophobic surfaces

We have studied the spreading of aqueous mixtures of six different silicone surfactants on Parafilm, a hydrophobic surface. Two of the surfactants were soluble in water and did not superspread. The other surfactants were insoluble and their aqueous dispersions superspread. Thus we found that a dispersed surfactant-rich phase is needed for superspreading and that the initial rate of spreading increases with decreasing size of the particles of the dispersed phase. We also found that the radius of the spreading drop varies as the square root of time during the initial spreading. This time dependence is consistent with but not unique to Marangoni flow. The initial spreading rate passes through a maximum with increasing concentration, but the equilibrium area, i.e. the area wetted by the drop after spreading has ceased, is proportional to the surfactant concentration. When the spreading experiment was conducted in a dry atmosphere the dispersions did not superspread. The dispersions were superspreaders at 100% humidity and spreading rates were even greater in supersaturated air. Thus we found that the presence of water vapor is necessary for superspreading. We speculate that the water vapor provides a thin high tension film at the leading edge of the spreading drop, and so spreading is driven by a Marangoni effect, but we do not know whether a pre-existing film is formed by the vapor or whether it is recruited during spreading. A water film mechanically deposited near a spreading drop accelerates the spreading in the direction of the mechanically deposited film. It was initially thought that a hammer-like geometry was responsible for superspreading. However, Hill et al. (R.M. Hill, M. He, H.T. Davis and L.E. Scriven, Langmuir, (10 1994) 1724) found that an analogous linear trisiloxane surfactant is a superspreader. The natural next question was whether surfactant-rich dispersions of the hydrocarbon polyoxyethylenes (CmEn) are superspreaders. In separate work we studied a series of dispersions of both hammer-like and linear hydrocarbon polyoxyethylene surfactants and found that none were superspreaders. Thus, among the surfactants compared, we found that a silicone hydrophobe is required for superspreading, but the molecular geometry of the trisiloxane surfactant does not appear to be a critical parameter.