Is Young’s equation valid for under-liquid wetting?

The aim of the present work is to understand the wetting of two under-liquid systems i.e., oil (drop) in water medium and water (drop) in oil medium on two substrates- glass and Poly (methyl methacrylate) (PMMA). Theoretical contact angles calculated by Young’s equation and Owens-Wendt approach were compared with the experimental contact angle. Four different oils were used – laser oil, two type of silicone oils and DBP. It was found that both the theories could not correctly predict under-liquid contact angles on glass. However, with a modified formulation of Owens-Wendt approach by considering a thin water film beneath the oil drop on a glass substrate, close agreement with the experimental data was found. Furthermore, for water (drop) in oil medium, we also applied the modified Young’s equation with thin oil film beneath the drop and the experimental results show close agreement with the model. In contrast, such a variance was not observed for oil (drop) in water medium on PMMA substrate. But we found that measured contact angles vary with the theoretical contact angles for water (drop) in oil on PMMA substrate. Therefore, similar to the glass substrate, we conjecture here also that a thin oil film is sandwiched between the water drop and the substrate.