Wetting of underliquid systems

Abstract

Wetting studies can be tracked back over the last few decades due to its applications in the development of water repellent (superhydrophobic), oil repellent (superoleophobic) surfaces. Despite the fact that these surfaces are well explored for air medium (inviscid), still the subject remains an emerging and challenging field due to the dearth of fundamental studies of wetting in surrounding viscous medium. Analyzing the interaction of a liquid droplet with a surface when kept in another liquid medium is vital for evolving applications in aquatic environment, oil-spillage, designing functional interfaces etc. The present study identifies and addresses a systematic study of two different underliquid systems : oil (drop) in water (surrounding medium) and water (drop) in oil (surrounding medium) with two different substrates viz., Poly (methyl methacrylate) PMMA and glass. Conventional theories namely, Young's equation and Owens-Wendt approach were corroborated with experimentally observed results. It was found that that experimental values vary largely with the conventional theoretical model for water (drop) in oil (viscous surrounding medium) on PMMA substrate. However, oil (drop) in water medium on PMMA substrate do not show such an anomaly. Therefore we hypothesized that a thin oil-film is sandwiched between water drop and substrate. Accordingly, we presented a modified theoretical model of Young's equation considering a thin oil film beneath the water drop originating from surrounding viscous medium. On the other hand, the standard Young's equation do not translate to the underliquid systems on a glass substrate. Also, Owens-Wendt theory could not correctly predict the underliquid contact angles on glass. Therefore similar to PMMA, we hypothesize that a thin oil film is present beneath the water drop on glass substrate. However, the modified Young's equation with thin-film consideration agrees very well with the experimental values and thereby demonstrated the presence of a thin film between a drop and glass substrate originating from the surrounding viscous medium.