(522d) The Influence of Surface Properties and Interfacially-Active Species on the Coalescence-Mediated Wetting of a Surface By an Emulsion Drop

When an emulsion is applied on a surface, the wetting of the surface by the drops of the emulsion is traditionally thought to be preceded by the formation of a bridge between the drop and surface across the intervening suspending medium film. Recently, we have identified a new mechanism of wetting, whereby even a small solubility of the drop phase in the suspending medium leads to diffusion of the dissolved drop fluid to the surface and nucleation into islands. The parent drop then forms a bridge with one of the islands, and spreads on the surface. In this work, we will present our more recent investigations that further characterize the coalescence-based wetting phenomenon. We studied the following important variables: the contact angle of the drop with the surface, the type of oil, the presence of surfactants and the presence of impurities such as dissolved water in the oil. Experiments were conducted with glycerol as the drop fluid, silicone oil or castor oil as the suspending fluid against a mica, pristine SU-8 or plasma treated SU-8 (p-SU-8) substrate. Film drainage dynamics and wetting behavior were captured using the technique of Reflection Interference Contrast Microscopy (RICM).

When drops of glycerol settled towards a p-SU-8 substrate in castor oil, which is a small molecule oil compared to our previous studies that employed a high molecular weight silicone oil (SO), glycerol islands were found to nucleate and grow, although they grew only just beyond sizes resolvable using RICM. Moreover, coalescence and wetting were ‘instantaneous’ upon contact between the parent glycerol drop and one of the islands, unlike observations in the glycerol-SO system, where coalescence was significantly delayed.

The addition of a non-ionic surfactant, Span 80, to castor oil at concentrations above the critical micellar concentration (CMC) caused dramatic nucleation of glycerol islands with a foam-like cellular structure. So, while the surfactant molecules compete with glycerol for adsorption on the substrate, the reduction in the interfacial tension and hence the barrier to nucleation is likely the predominant effect for this surfactant, oil, water and substrate system. An industrially relevant system with castor oil containing span-80 and presaturated with glycerol was also investigated. The island growth rate was weak in the absence of the parent drop near the surface, but was strongly enhanced once a drop approached the surface. Ultimately, coalescence was observed between the parent drop and one of the rapidly growing islands.

The coalescence-based mode of wetting throws open an entirely new suite of variables for controlling wetting: drop solubility and diffusivity in the medium, substrate roughness and wettability, and interfacially active species. Therefore, it has tremendous implications in the design of emulsion based coatings, and in the petroleum, cosmetics and other industries that handle emulsions.