Drop oscillation dynamics on viscous thin immiscible liquid films: slip to pin transitions

Drop oscillation dynamics as well as drop mobility studies on thin liquid films such as on lubricant infused surfaces (LIS) have been developed extensively in recent years - relevant for many applications such as in water harvesting and enhancing condensation. However, the influence of how drop oscillation behaves in tandem with the viscous resistance on LISs remains unclear. In this study, the droplet height, contact angle and frequency were observed by a high-speed camera from a lightly deposited water drop onto liquid oil films of various viscosities and types. The droplet contact line mobility was used to categorize the natural oscillation frequencies to either the slip contact line (SCL) or pinned contact line (PCL) cases. By comparing the experimental frequency results with SCL and PCL numerical simulations, we find good agreement between the two methods and demonstrate a transition of droplet oscillation from the SCL to the PCL cases while varying the viscosity and wettability of the liquid film at fixed film thicknesses.