An experimental investigation of fluid flow resulting from the impact of a water drop with an unyielding dry surface

Abstract

The flow of fluid associated with the impact of water drops of radius R at a speed V onto unyielding dry metal surfaces of known roughness Ra is described. Spatial dimensions of the deforming drop are normalized by transformations of the kind x' — x/R, and time scales are normalized according to t' = tV/R, to permit comparison of events where or differ. It is shown that the primary influence of the surface roughness parameter Ra is the determination of the condition for the ejection of secondary droplets by the excitation of an instability in the developing watersheet; provided Ra≪ R, it is possible to evaluate the condition to a high degree of accuracy, and for Ra = 0.84 μm it is found to be α4/3RV1.69 > 7.4, where α is the eccentricity of the drop at the moment of impact. Deceleration of the drop apex does not commence until > 0.6, contrary to the prediction of Engel (1955) but in good agreement with that of Savic & Boult (1957). Close examination of the very early stages of impact suggests strongly that the so-called watersheet originates at a moment t' — 0.01 after first contact, regardless of the absolute values of R, V or Ra; the initial normalized watersheet velocity is of order 5. Where there is ejected material, its normalized velocity at the moment of ejection is of the order of 20 % greater than that of the watersheet substrate. Simple calculations also suggest that initial fluid velocities greater than 10 are required immediately before the initiation of the watersheet (t'< 0.01). Impacts at speeds considerably greater than the appropriate terminal fall speed in air show no deviations in character from those investigated at much lower speeds. A simple subsidiary experiment also suggests that greater impact velocities are required to produce splashing on inclined targets.