Spontaneous bouncing of a droplet that impacts a surface is a mechanism that occurs for the moderate range of droplet impact velocities and is caused by the formation of a stable air layer (cushion) between the droplet and the surface. This bouncing behavior is more pronounced on Lubricant Impregnated Surfaces (LISs) inspired by the natural non-wetting surface of the pitcher plant, which relies on the stable formation of a thin lubricant film across its surface.
In this study, we performed modeling of the water-oil–air interfacial surfaces using the volume of fluid (VOF) methodology to simulate the impact of a water droplet onto a lubricant smooth surface with an oil as the lubricant. To resolve the effects of the air surrounding the droplet, computational cells were extensively small to capture the presence of the sub-micron layer of air trapped underneath the droplet during the impact.
The model was able to capture the initiation and subsequent effect of the air cushion on the droplet hydrodynamics. We found that the stability of the air cushion and the impact dynamics are independent of the oil viscosity for specific thicknesses of lubricant layers, whereas the impact conditions such as velocity and droplet properties played a significant role on the outcome of droplet impact. Hence, the dynamics of a droplet falling on a specific thickness of oil film was influenced by the squeezed air trapped between the two immiscible fluid (water and oil). In addition, the formation of high pressure dimple region was evident, which in some cases lead to entrapment of the air bubble. Finally, we validated the results with the existing experimental data in the literature.