292
OWE BERG, GAUKLER, AND SQUIER
plied voltage.
The data should then be representative of an unknown
and uncontrolled floating potential. Even so, the data show that the
wetting of glass is a slow process.
The two processes involved in wetting, namely, the supply of a
bond by the water drop and the formation of a bond with the solid surface, are particularly conveniently studied with metals. With a metal,
there is no doubt about a sufficient conductivity for the application of
the voltage to the surface.
Figure 11 shows the results obtained with a magnesium ribbon. At
low voltages the rate is apparently independent of the polarity, but at
high voltages the rate is almost twice as high for positive magnesium
as for negative magnesium.
Figures 12 and 13 show the results obtained with sheet copper and
copper-plated steel. The data for the two materials are not significantly different although, on the whole, the copper-plated steel gave
higher rates than did the copper sheet. Thereis no appreciable polarity
effect. This is the only case found so far that gives a constant product
tv% It is noteworthy that the data scatter more in the case of copper
than in the case of magnesium.
Experiments were conducted with various types of steel. The material used in the first experiments was copper-plated air-rifle pellets
pickled in concentrated HNO; to remove copper, rinsed in distilled
water, and dried. This material was not wetted within 7 msec at any
voltage up to +25 volts. The material was then treated in the same way
as before but, in addition, pickled in 2NH,SO, overnight. This treat-
ment led to rapid wetting of the pellets; the rate was independentof the
voltage.
There appeared to be two possible explanations for these observa-
tions. Either the surface becomes passivated by the HNO, or hydrogen
dissolved in the steel is instrumental in the wetting process. Pickling
in H,SOQ, gives a high concentration of hydrogen in the steel; concentrated HNO, removes dissolved hydrogen from the steel.
The air-rifle pellets pickled in HNO, presented the first case of
nonwetting encountered in the investigation. Since a change in treat-
ment (pickling in 2N H,SO,) caused rapid wetting, it is apparent that the
state of the solid is essential to the wetting and that the rate of wetting
is not determined by the phenomenain or on the water drop alone. The
treatment with H,SO, gave a rate of wetting that was independentof the
voltage applied.
It was therefore decided to investigate the wetting of the steel with
a view to the effect of the solid upon the rate of wetting. Of course, it
is clear from a comparison of the data for the other solids used with
those for a pair of water drops that the solid takes part in the ratedetermining process, but it appeared that steel would be particularly
well suited for the studyof the effect.