Flash photography was used to examine the impact of water drops with flat, solid surfaces in order to determine the influence of drop size R and surface roughness parameter Ra upon the critical impact velocity VT above which splashing occurs. The existence of a critical impact velocity may be inferred from the literature but has not been proven prior to these experiments. It is shown that for a given surface roughness the product ST=RVT1.69 remains constant for the range R=0.70-2.25mm, suggesting that the condition for splashing is determined uniquely by the Reynolds Number and the Weber Number. The critical impact velocity VT is a decreasing function of Ra (the mean absolute deviation of surface contour from the mean surface level) and the variation of VT with Ra is of the order 3ms-1μm-1 for Ra1μm; for surfaces of Ra1μm, the corresponding variation is of the order 0.05ms-1μm-1. The data obtained here suggest that the number of drops produced per collision N may be predicted by the relationship N=k[R3V2-R182ST1.18] where k is an experimentally determined constant and V is the actual impact velocity of the drop. The relationship is superior to that proposed by Stow and Stainer (1977) in that it provides the correct functional relationship between R and VT, and correctly predicts values of N observed by Stow and Stainer. For drops impacting at their terminal velocity, the effect of the roughness of the target is believed to be crucial in predicting the occurrence of a splash only if drops possess radii in the range 0.5-0.8mm; for smaller drops no splash is likely, even on a very rough surface, and larger drops will splash on a perfectly smooth surface. Although of, perhaps, secondary importance, it was also noted that VT is higher for the more prolate drops of any given mass; the effect of drop eccentricity on the splash process has not been previously documented. These data may facilitate computations of those microphysical processes within clouds which involve collisions between ice particles and water drops and which may lead to the production of secondary drops.
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