On the bubble trapped underneath a droplet impacting a moving hydrophilic surface: From perfect slip to no slip

Much experimental evidence supports the commonly held assertion that fluid does not slip along a solid surface. Here, we demonstrate circumstances in which slip occurs. This happens when the fluid velocity relative to the surface is sufficiently large for the acceleration required to prevent the slip cannot be achieved. To demonstrate the slip, we examine the evolution of a bubble trapped underneath a droplet impacting a moving surface and estimate the parameters determining when the slip occurs. We demonstrate that the terminal slip distance, the maximum distance between where the bubble velocity reaches the surface velocity and the impact point, increases with the surface velocity and weakly depends on the Weber number. Its observed length was up to 1.39 mm. An acceleration extracted from the experiments quantifies the slip and provides a simple tool for predicting the terminal slip distance.