Wetting of Water Drops on a PMMA Substrate in Viscous Medium

There is a significant surge in the development of water repellant (superhydrophobic), oil repellent (superoleophobic) surfaces. Though these surfaces are well studied for air medium (inviscid), still there is a lack of fundamental understanding of wetting behavior in presence of surrounding viscous medium. In the present work, we investigate the wetting behaviour of water drops on a PMMA substrate surrounded by viscous oil medium for a wide range of viscosity ratio. The sessile drop is generated at the needle tip (J-needle for denser oil) close to the PMMA substrate to initiate the spreading of a water drop on the substrate. Experimentally measured contact angle at static equilibrium can well interpret the wetting behaviour of water drop on PMMA substrate placed in oil (surrounding medium). It is also observed that the theoretical values of water (drop)-oil and oil (drop)-water system satisfy the Young’s equation of two liquid system, but certain percentage errors are observed when compared to experimental results. These differences are interpreted in terms of interfacial energies of the two-liquid systems. In addition, ‘complementary hysteresis’ model recently put forward by Ozkan et al. [Surf. Topogr.: Metrol. Prop.2017,5,024002] is modified to study the wetting characteristics. Finally, based on the comparison of experimental and theoretical values, a short perspective is provided on the potential of a stable thin lubricant film under the droplet that changes the wetting characteristics of the substrate.