282
OWE BERG, GAUKLER, AND SQUIER
particles and raindrops. This process occurs in two steps: (1) collision
and (2) coalescence or wetting during contact in collision. The last step
has been Studied. This paper summarizes the results of the study.
A similar phenomenon, namely, the coalescence of two liquid drops
in contact with each other, was studied previously.' This investigation
Showed that there is a delay, t, between contact and coalescence and
that the product of t and the voltage, V, across the drops is a constant.
Accordingly, colliding drops do not coalesce but bounce apart unless
they are electrically charged. Experiments with various liquids showed
that
(e—1)* tV = constant
(1)
(where € is the dielectric constant) and that
M = (€~1)”
(2)
is the induced dipole moment,
It follows from this result that all the intermolecular hydrogen
bonds are engaged within each drop and that coalescence requires the
reorientation of such bonds across the contact surface. This reorientation is a slow process and may be effected by an applied electric field.”
The same conclusion should apply to the wetting of a solid particle in
its collision with a water drop. Wetting is the formation of bonds be-
tween the liquid and the solid, notably, hydrogen bonds between H,O
molecules in the drop and OH groups in the solid.** Thus there are
two conditions for the rapid wetting in collision: (1) the presence of OH
groups in the solid and (2) the reorientation of hydrogen bonds in the
H,O molecules.
It was shown by Rayleigh® that colliding water drops bounce apart
unless they are charged. This experiment was repeated by Boys.° More
recently, McCulley e¢ al.’ showed that powder particles blown onto a
stationary water drop bounceoff if they are not wettable but are swallowed by the drop if they are. The particles must have been charged in
this experiment,
EXPERIMENTAL TECHNIQUE
The experimental technique used was similar to that used in the
preceding investigation with a pair of liquid drops.' Figure 1 shows the
arrangement schematically as used previously. In this investigation,
the movable drop was replaced by a solid. The drop was approximately
2 mm in diameter. The solid had the shape of a sphere or a flat sheet.
Experiments were also conducted with pointed wires. However, the end
of the wire formed a depression in the water drop, and it was not pos-