CHEMICAL APPLICATIONS FOR ULTRASONIC WAVES 1•! to be the limiting value for polystyrene solutions (36). Similar results have been found for other polymers. If the molecular weight distribution of the polymer before ultrasonic degradation is relatively wide, distribution for the degraded polymer is considerably narrower. No extensive industrial use of ultrasonic waves for the degradation of polymers is anticipated since most chemists are interested in increasing rather than decreasing the molecular weight of polymers. In addition to the mechanical degradation associated with cavitation, intense ultrasonic waves can cause thermal degradation. The absorption of the sound energy within a liquid results in the progressive heating of the system. As a result, it is important to provide a means for cooling the liquid in order to prevent pyrolysis. The absorption coefficient for sound waves increases as the square of the frequency in most cases hence, proc- essing applications involving viscous liquids should be carried out pref- erably at relatively low frequencies to minimize thermal problems. Of the various processing applications mentioned above, the most promising with respect to cosmetic chemistry appear to be the production of colloidal suspensions (particularly emulsions), the degassing of liquids and the expediting of mass and heat transfer in fluids. While all of these applications can be accomplished relatively easily, engineering data are generally not available. ULTRASONIC EQUIPMENT FOR PROCESSING APPLICATIONS For the most part, the choice of frequencies for processing applications is not critical since cavitation occurs over a wide range of frequencies. When cavitation is to be produced throughout a relatively large tank, low ultrasonic frequencies (e.g., 20 kc./sec.) are used because the sound energy can be distributed more uniformly throughout the tank. Often very high intensities are required in a restricted region as is the case with continuous flow processing systems. In such instances, higher ultrasonic frequencies in the range 105 and 106 cycles/sec. are favored since ultrasonic waves of shorter wavelengths are more readily focused. Several excellent reviews (3, 6, 7, 16, 17) on ultrasonic equipment for industrial applications have been published in recent years. Only a brief r•sum• of ultrasonic generators will be presented in this article. Samsel (34) has also considered a number of the factors involved in the choice of generating devices fo'r various applications. The majority of the processing applications involve the propagation of ultrasonic waves through liquids. Three types of ultrasonic generators have been used extensively for this purpose: (a) hydrodynamic devices, (b) piezoelectric transducers and (c) magnetostrictive transducers. The term transducer refers to any device which changes one form of energy to another, e.g., alternating electrical energy 'to sound waves. • The hydro-

152 JOURNAL OF THE SOCIETY OF COSMETIC CHEMISTS Direction of flow Resonator Figure 7.--Hydrodynamic generator. dynamic and magnetostrictive generators are best suited to sonic and low ultrasonic frequencies (5000-50,000 cycles/sec.). Piezoelectric transducers are practical for frequencies as low as 10,000 cycles/sec. but have found most extensive use in the range 100 to 1000 kc./sec. for processing appli- cations. Hydrodynamic Devices Hydrodynamic generators convert the mechanical energy associated with liquid flow to acoustical energy. One of the simplest types for produc- ing moderately intense ultrasonic waves in liquid is the resonant wedge whistle of Janovski and Pohlman (18) as shown in Fig. 7. A jet of liquid impinges in a wedge-shaped resonator. Intense sound waves are then propagated into the liquid from this resonator and cavitation results. Units of this type are commercially available. This type of sound source is inexpensive both in terms of the initial cost and operation. Crawford (7) reports that such liquid phase whistles operating at 30 kc./sec. have been used for the dispersion of water and oil in the manufacture of hair cream. Several other types of hydrodynamic oscillators (4) have been developed to convert hydrodynamic flow to oscillatory motion and may become feasible for the generator of acoustical waves at sonic and low ultrasonic frequencies for processing applications with a minimum of cost. Piezoelectric Transducers If a single crystal of a material with an anisotropic lattice is placed in an electrical field, the crystal will change its dimensions. This effect, known as the inverse piezoelectric effect, is utilized as a means for converting electrical energy to acoustical energy. Periodic variations in the di- mensions are produced through the use of an alternating electrical field. Sound waves are then propagated into any medium in contact with the vibrating surfaces of the transducer. The electrical power to drive these transducers is obtained from an electronic radio-frequency generator. Prior to World War II, a-quartz was used almost exclusively for the generation of intense ultrasonic waves by the piezoelectric effect. A diagram of a transducer of this type is shown in Fig. 8. The alternating electrical field is applied by means of a metal conducting coating on the two plane surfaces of the circular quartz plate. The frequency of the