1•,2 JOURNAL OF THE SOCIETY OF COSMETIC CHEMISTS Dispersion of Solids and Liquids Intense sound waves are capable of dispersing materials in both the liquid phase (41, 42) and the gas phase (25, 40). Dispersion in the liquid phase will be considered first. Two mechanisms have been proposed to ex- plain the dispersion of solids and liquids by intense sound waves. In most instances the dispersing effects are contingent on acoustically pro- duced cavitation within the liquid. The extreme mechanical effects associated with cavitation appear to be the primary agency by which solids as well as liquids are dispersed in a liquid phase. In the case of a few liquid pairs (e.g., mercury and water), it has been observed that emulsions can be formed with ultrasonic waves even in the absence of cavitation. Surface waves are excited at the interfaces between the two liquids. The situation is represented in Fig. 2. As the amplitude of the transverse waves becomes greater, droplets of one liquid are thrown into the other liquid and vise versa. Cavitation, however, is the pre- dominant agency for most emulsions. While almost any solid or liquid can be suspended in a second liquid by means of cavitation, emulsions are by far the easiest to form. A survey of the literature as well as current experimental work in the author's labora- tory indicate that the mean particle size for emulsions is often of the order of 1 micron, a value which is somewhat disappointing since it is relatively large for stable colloidal suspensions. For emulsions of such large particle size to be stable for any appreciable time, stabilizing agents usually must be added to provide protection. It is interesting to note that mineral oil suspensions in water produced ultrasonically even in the absence of stabiliz- ing agents at concentrations of the order of 1 per cent oil have proved stable for periods of the order of one month. The ultimate or limiting concentration of one liquid phase in a second liquid phase is contingent on the nature of the sound source as well as the properties of the liquids. While only a small percentage of oil can be suspended in water in the ab- sence of a stabilizing agent, concentrations in the excess of 75 per cent can be obtained with surface active agents present. The literature provides insufficient data upon which to base any statement as to the rate of emulsification, particularly in terms of commercial proc- essing. Crawford (7) reports that whistle-type transducers (to be de- scribed later) have been used in England to produce emulsions in the pharmaceutical, cosmetic and food'processing industries. While ultra- sonic waves offer promise as a means for preparing emulsions, overoptimism is to be discouraged since current indications are that ultrasonic emulsifi- cation is not yet competitive with more conventional' techniques even for relatively small scale industrial applications. The suspension of solids in liquids is more difficult to accomplish and

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