CHEMICAL APPLICATIONS FOR ULTRASONIC WAVES* By E•.N•:ST YEAGEP. Dept. cf Chemistry, ld/estern Reserve University, Clevdand 6, Ohio 'I'HE APPLICATIONS for sound waves in chemistry may be divided into three classifications, (a) processing applications, (b) analysis and control and (c) fundamental studies of structure and the kinetics of chemi- cal processes. This paper is concerned primarily with the processing applications, particularly those of significance to the cosmetic chemist. The other two types of applications have been reviewed by several authors (3, 7, 16, 32, 44-46, 53). The acoustical spectrum is divided into three regions on the basis of the hearing limits of the human ear. Sound waves with frequencies between 20 and 20,000 cycles/sec. are referred to as sonic while sound waves with frequencies above 20,000 cycles/sec. are ultrasonic and below 20 cycles/sec. are infrasonic. Confusion has existed in the past concerning the terms supersonic and ultrasonic. Supersonic should be reserved to describe those phenomena which take place with a speed greater than that of sound under some reference condition such as in air at sea level. Thus, one should speak of supersonic flight and supersonic velocities, but in contrast, ultrasonic waves and ultrasonic frequencies. This division of the acoustical spectrum into three regions is somewhat artificial since the majority of the physical and chemical phenomena associated with sound waves bear little relationship to the hearing limits of the human ear. Many of the processing applications are not confined to ultrasonic frequencies but can be carried out successfully at sonic and in some cases even infrasonic frequencies. In such instances, ultrasonic waves may be preferred for secondary reasons. For example, personnel operating ultrasonic equipment are not annoyed by sound waves which they can not hear. Frequent reference will be made in this paper to acoustical intensity. This term represents the amount of sound energy per second transmitted through a 1-cm. = cross section perpendicular to the direction of propagation of the sound waves. Acoustical intensity is often expressed in watts/cm. = which corresponds to joules/sec.-cm 2. * Presented at the December 13, 1956, Meeting, New York City. 139

140 JOURNAL OF THE SOCIETY OF COSMETIC CHEMISTS Many of the processing applications are dependent on the formation of cavitation bubbles within liquids. For this reason it is worthwhile to consider the basic processes associated with cavitation before discussing the applications. The transmission of sound waves through a liquid is attended by periodic variations of pressure. With sound waves of high intensity, these varia- tions in pressure may be sufficient to develop tensions within the liquid provided the liquid does not rupture. In most cases, cavitation occurs within the liquid before tensions are developed. The nucleation of the cavitation bubbles, however, is a problem since the cohesive forces within the liquid are usually equivalent to tensions far greater than can be pro- duced acoustically. Under ordinary circumstances, a liquid contains micro dust particles which have at least partially hydrophobic surfaces. If the liquid also contains a dissolved gas, micro bubbles filled with both vapor and the gas are expected to exist within the cracks or surface irregu- larities of these particles even though the liquid is not completely saturated with the gas. With the introduction of sound waves into the liquid, these bubbles periodically vary in size. More dissolved gas diffuses into a bubble during the rarefaction than redissolved during the compressional part of the acoustical cycle. This results in the growth of the bubbles with each cycle through rectified diffusion to a size which may be considerably larger than that of the partially hydrophobic particles. When a bubble attains a size corresponding to a resonance condition, the expansion and partial collapse of the bubble with each cycle of the sound waves become extreme. Instantaneous pressures within the cavitation bubbles and in the liquid immediately adjacent to the bubbles may reach in excess of 10 a arm. primarily because of the finite momentum of the liquid as the bubbles partially collapse. The situation is somewhat analogous to the well- known water hammer which occurs when liquid flow is stopped abruptly within pipes. The destructive effects of the shock waves originating from these resonating cavitation bubbles are far greater than any associated with the sound waves responsible for the cavitation. Since the compression of the gases within the bubbles is at least partially adiabatic, relatively high instantaneous temperatures are believed to be realized within the bubbles (e.g., 1000øC.). Furthermore, there is evidence that larger cavitation bubbles in higher order resonance modes generate many addi- tional small bubbles (51) which in turn grow until a resonance condition is reached. For a more detailed discussion of cavitation, the reader is referred to references 3, 7, 16 and 51. With commercially available equipment cavitation can be produced easily at frequencies from a few hundred through two megacycles per second. In water only a fraction of a watt/cm. 2 is ordinarily required. At frequencies below 1 mc./sec., cavitation does not appear to be a partic-