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
R
a
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 R
a
is the determination of the condition for the ejection of secondary droplets by the excitation of an instability in the developing watersheet; provided
R
a
≪
R
, it is possible to evaluate the condition to a high degree of accuracy, and for
R
a
= 0.84 μm it is found to be α4/3
RV
1.69
> 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
R
a
; 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.
The physical products of splashing water drops were investigated with respect to several parameters: impact velocity, drop size, surface tension, radius of curvature and roughness of the target surface, and the depth of liquid film covering the surface of the target. It is shown that the number of droplets produced by a splash increases with surface roughness, impact velocity and drop size, but decreases with an increase in liquid film depth and with a reduction in surface tension of the drop. For a given drop size, the number of splash products is proportional to the kinetic energy of the drop at impact. The size distribution of splash products is approximately log-normal; the mean size of the ejected droplets (approx. 120 *m diameter) increases with drop size, surface roughness and depth of liquid film, but decreases with increasing impact velocity and with a reduction in surface tension. Certain empirical relationships are established which permit the number of splash products, N, to be estimated in terms of the conditions of impact. One such relationship gives N=3.4 R3V2-63, where R(mm) is the radius of a water drop impacting a flat solid surface with velocity V(ms-1). It is also shown that water drops of less than 0.75mm in radius require an impact velocity greater than their terminal velocity if they are to eject droplets.
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