| Summary: | When a liquid drop falls and hits a dry surface, it may spread smoothly on the surface or dramatically splash, emitting many small droplets. Although it is a very common process, there are few experimental studies, which explore all control properties of the breakup of drops, notably the ambient gas pressure. As pressure is lowered, a splash is completely suppressed. The ambient gas pressure is determined for the onset of splashing of low-viscosity liquid drops on smooth dry surfaces as I change the control parameters: drop impact velocity, drop radius, viscosity, surface tension, density, and gas molecular weight. This threshold pressure indicates that there are two distinct regimes when drop impact velocity is varied. By rescaling data using functions of only three dimensionless numbers, the commonly used Reynolds and Weber numbers, as well as the ratio of drop radius to gas mean free path, all data is collapsed to a single curve that encompasses both regimes.
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