CHEMICAL APPLICATIONS FOR ULTRASONIC WAVES 143 PHASE A Large amplitude ¾' '•J' •• /-' "-, / • ../ ", /'\ \ .- ,, / •., / x ..- ........ . ........... Low amplitude • • o. ,, ,:\ PHASE B Figure 2.--Surface waves at the interface between two phases. requires more intense sound waves. If the solid is already subdivided, dispersion through further reduction of particle size proceeds more readily. Ultrasonic waves are effective in breaking up agglomerated particles. When intense ultrasonic waves within a liquid impinge on a gas-liquid interface, the liquid is dispersed as a fog. Sollner (40) has obtained evi- dence that cavitation within the liquid phase near the interface is involved in the formation of the aerosol. On the basis of unpublished research at Western Reserve University, the author believes that surface waves of the type represented in Fig. 2 at the liquid-gas interface also contribute to the formation of fog and may be more important than cavitation. While the togs generated with ultrasonic waves are often dense optically, the droplet size is relatively large, and hence, the fogs are usually unstable. McCubbin (25) with ultrasonic waves at 2.4 mc./sec. in water found the mean droplet size to be between 1 and 10 microns. The fogs are more difficult to produce with viscous liquids. In Fig. 3 is a photograph of the fog produced when ultrasonic waves at a frequency of 600 kc./sec. are focused from within the liquid phase so as to converge at the water-air interface. No immediate industrial applications are anticipated for the ultrasonic formation of aerosols on the basis of present information. Coagulation and Precipi/ation Ed•ects Ultrasonic waves are capable of producing appreciable increments in the rate of coagulation or precipitation of roetastable suspensions in liquids (42). Such effects are observed only in the absence of cavitation with suspensions which lack adequate protection. In the presence of cavitation the dispersing effects usually are predominant. As a result, most of the work reported in the literature has been carried out at low intensities so as to avoid cavitation. The results at these low intensities have not proved sufficiently great to warrant industrial application. It should be noted, however, that cavitation can also be prevented even at moderately high

144 JOURNAL OF THE SOCIETY OF COSMETIC CHEMISTS • 2:•"•i :5. :...• •. .5 .!: ß ??" ,.- "%5: .=. "- ß '-'" :q •. .... •. 5:.: .4• -,.'•.:. %.':5 '•. ':'. . •,• '•. ... Figure &--Fog produced bv ultrasonic waves at 600 kc./sec. focused to converge at the surface of water. intensities by degassing the liquid and by using increased static pressure on the liquid. Experiments with much higher acoustical intensities without cavitation might yield faster rates of agglomeration and precipitation than have been reported to date. The coagulation effects are the result of an increased probability of col- lision between particles in the presence of the sound waves. On the basis of various second order properties of sound waves, attempts have been made to predict the increase in the rate of collision. At least three factors are involved. In the sound field the displacement imparted to the large suspended particles is smaller and lags behind that of the small particles. This results in an increased number of collisions per unit time. When the sound waves encounter two particles located side by side, the periodic variations in the velocity of the solvent molecules are greater between the two particles than in the bulk of the solvent because of the partial con- striction of flow caused by the suspended particles. On the basis of Bernoulli's principle, an apparent attractive force is expected between the two particles. A third factor is radiation pressure. In the presence of standing waves, radiation pressure caused the suspended particles to move to either the nodes or antinodes of the standing waves, • depending on the acoustical properties of the suspended particles. With aerosols the situation is more promising because the acoustical effects are considerably larger. The optimum frequency for most particle size distributions is sonic rather than ultrasonic (i.e., 10 a to 10 4 cycles/sec.).