KINETICS OF WETTING IN WASHOUT OF DUST
285
short times, the delay time measured is a little too long because the
indication of contact may be too early and the indication of wetting is
inherently too late.
A voltage was applied across the holders for the water drop and
the solid by means of a battery and the circuit shown in Fig. 1. Most
measurements were made at voltages below 50 volts. Higher voltages
were tried occasionally, but the high field tended to deform the water
drop by pulling out a cone. In allthe cases studied, the optimum voltage
with respect to the duration of the delay time was below 30 volts.
Much experimentation was devoted to the preparation of the sur-
faces of the solids. It was thought that surface oxide and other contaminants,
e.g., dirt or grease from handling, might have decisive
effects. It was found, however, that the surface state has a remarkably
small influence. In one extreme case, two drops of water were used,
each of which was covered with a thin but visible film of oil. The delay
time did not differ markedly from that determined with clean drops.
Glass beads were used in one series of experiments after various surface treatments (and lack of treatment), cleaning with HNO, or a grease
solvent, and rinsing in water. There was no marked effect of these
treatments upon the delay time. Nevertheless, the solids were handled
with forceps, and attempts were made to avoid contaminating the materials,
The experiments were initially conducted in the ambient laboratory
air, except for some experiments conducted at a controlled humidity in
a glove box. The results were fairly reproducible. However, experi-
ments with steel surfaces gave a very wide scatter of the data. This
was first attributed to dust in the air issuing from another experiment.
Thereafter the experiments with steel were conducted in filtered air in
a glove box. However, this made no difference in the scatter and an
extensive investigation was made. The results showed that the scatter
was caused by varyingcontents of hydrogen in the steel. This phenom-
enon is of particular importance with respect to the experimental technique used, and mentioning it here may be useful to those attempting
to reproduce our results.
RESULTS
Table 1 summarizes the results and gives at the same timea list
of the materials used. The results are given in further detail in Figs.
4to 17. For comparison, measurements were made with a pair of water
drops. The data obtained at high voltages are presented in Fig. 3. These
data give tv’ = 29 at high voltages. For establishmentof the value of
tV at low voltages, numerous measurements were made at 4 volts,
and the average value was taken. This gave tV = 7.48 at low voltages.