The role of viscosity and surface tension in bubble entrapment during drop impact onto a deep liquid pool

Abstract

The phenomenon of liquid drop impact onto the surface of a deep pool of the same liquid is studied in the context of bubble entrapment, using high-resolution digital photography. Three liquids, pure water, glycerin/water mixtures, and silicon oil, have been used to investigate the effect of viscosity (μ) and surface tension (σ) on regular bubble entrapment, and the associated impact crater signatures. The global viscous effect is seen as a shift in the classical inviscid bubble entrapment limits, whereas, at the impact crater, the local effect is seen as a weakening of the capillary wave, which is responsible for bubble pinching, and a weakening of the intensity of crater rebound. Bubble entrapment, which results from a competition between concentric capillary pinching of the crater cusp and viscous damping, is captured well by the capillary number Ca (Ca = mu Viσ, where Vi is the drop impact velocity). The measured peak entrapped bubble size decreases exponentially as capillary number increases, with the cut-off capillary number for bubble entrapment estimated to be around 0.6. The critical crater cone angle for peak bubble pinch-off weakly increases with capillary number, with the measured value in agreement with theory in the inviscid limit (low Ca). Additionally, the growth of the main body of the high-speed thin jet, formed immediately following bubble pinch-off, is fitted to a power-law singularity model. This suggests that the thin jet is similar to the hydraulic jets produced by the collapse of free-surface standing waves.