Anomalous Wetting of Underliquid Systems: Oil Drops in Water and Water Drops in Oil

We have investigated the wetting phenomena of two underliquid systems, i.e., oil (drop) in water medium and water (drop) in oil medium for two different substrates, poly(methyl methacrylate) (PMMA) and glass. We have conducted detailed static (equilibrium) and dynamic contact angle measurements of drops on substrates kept in air, water, and oils of varying densities, viscosities, and surface tensions. We compared the experimentally observed contact angles with those predicted by the conventional wetting theories, namely, Young's equation and the Owens and Wendt approach. The results reported herein showed that experimental values vary in the range of 8-20% with the conventional theoretical model for water (drop) in oil (viscous surrounding medium) on PMMA substrate. However, oil (drop) in water medium on PMMA does not show such an anomaly. By taking into consideration a thin oil film between a water drop and PMMA originating from the surrounding oil medium, the modified Young's equation is proposed here. We found that the percentage difference between experimentally observed contact angles with modified Young's equation is in the range of 0.88-5.88%, which is very less compared to percentage difference with classic Young's equation. For glass substrates, the standard Young's equation does not translate to the underliquid systems whereas the Owens and Wendt theory could not correctly predict the underliquid contact angles. 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. This present experimental study coupled with detailed theoretical analyses demonstrates the anomalous wetting signature of drops on substrates submerged in surrounding viscous medium, which is very different from the reported studies for drops on substrates kept in air (inviscid medium).