Predicting the Splash of a Drop Impacting a Thin Liquid Film.

An experimental study is carried out to investigate droplet-film interactions when a drop impinges on a thin stagnant film of the same liquid. The impacting drop causes either liquid deposition or splash, consisting of prompt generation of secondary drops or a delayed process. By varying the drop diameter and impact velocity, measurements are made to characterize the phenomena using five different liquids that are chosen to cover a wide range of liquid properties (viscosity and surface tension). The drop impact dynamics are captured with a high-speed digital camera with real-time, high-resolution image processing. The drop-splash threshold is found to scale with inertial and viscous forces, or Reynolds number (Re), as well as capillary forces, as described by the balance of gravitational and interfacial tension forces, or Bond number (Bo); fluid properties are described by their Morton number (Mo). A correlation, functionally expressed as Re = ϕ(Bo,Mo), is devised to determine the splash/no-splash (or deposition) boundary, and the predictions for the splash/no-splash outcomes agree well with the experimental outcomes as well as those readily available in the literature.