KINETICS OF WETTING IN WASHOUT OF DUST
299
2. Wetting is a rate process. Unless the rate is great enough,
colliding solid particles and water drops bounce apart.
3. A solid salt that is hydrated so as to be effectively in the form
of a saturated solution at the surface is wetted by the mechanism of
coalescence and at the rate of coalescence of two water drops. Rate
determining is then the reorientation of hydrogen bonds across the in-
terface. This requires an orientating electric field; i.e., the colliding
particles must be charged.
4. A solid that is covered by OH is more rapidly wetted than one
that is covered by H,O because the reorientation of bonds is required
on the part of the water drop alonein the former case and on the part of
both in the latter case. Again, the colliding particles must be charged.
5. In principle, a hydrogen-replacing metal may decompose liquid
H,O to form chemisorbed OH without breaking the bonds between the
liquid H,O and the chemisorbed OH. In this mechanism the orientation
of bonds is automatically satisfied. Atomic hydrogen in solution in the
metal is much more powerful in this mechanism than is the metal it-
self, namely, according to Eq. 4.
It appears that iron is not a hydrogen-replacing metal but decomposes water only by virtue of its dissolvedhydrogen. As a consequence,
iron free of hydrogen does not decompose water in the short period of
contact in collision and therefore is not wetted by this mechanism.
6. Iron free of hydrogen, e.g., after pickling it in concentrated
HNO;, does not decompose water and therefore does not coveritself
with chemisorbed OH even during prolonged exposure to the ambient
air. Iron free of hydrogen is therefore not wetted by any mechanism in
collision.
REFERENCES
1. T. G. Owe Berg, G. C. Fernish, and T. A. Gaukler, The Mechanism of Co-
alescence of Liquid Drops, J. Atmospheric Sci., 20: 153 (1963).
2. T. G. Owe Berg, Sur la Théorie de la Conductivité Electrique des Solutions
Aqueuses d’Acides, J. Chim. Phys., 50: 247 (1953).
3. M. R. Basila, Hydrogen Bonding Interaction Between Adsorbate Molecules
and Surface Hydroxyl Groups on Silica, J. Chem. Phys., 35: 1151 (1961).
4. T. G. Owe Berg, The Kinetics of Wetting of Solid Hydrocarbons, Special Report No. 0780-01(01) SP, Aerojet-General Corporation, September 1963.
5. Lord Rayleigh, The Influence of Electricity on Colliding Water Drops, Proc.
Roy. Soc., 28: 406 (1879).
6. C. V. Boys, Soap Bubbles
New York, 1959.
(3rd revised edition),
Dover
Publications,
7. C. R. McCulley et al., Scavenging Action of Rain on Airborne Particulate
Matter, Ind. Eng. Chem., 48: 1512 (1956).
8. Lord Rayleigh, Further Observations upon Liquid Jets, Proc. Roy. Soc., 34:
130 (1882).
9. U. S. Army Signal Corps, Contract No. DA-36-039-AMC-03378-(E).
10. C. Orr, F. K. Hurd, and W. J. Corbett, Aerosol Size and Relative Humidity,
J. Colloid Sci., 13: 472 (1958).