ENGINEERING, DESIGN, AND PROCESS DEVELOPMENT significant. From the inlet, which is fitted with a sterile connection, the liquid is fed into the top portion of the apparatus, where it forms a pool. From the surface of this pool it runs down the inside of a number of cooled vertical stainless steel tubes and leaves the equipment by an outlet which is also fitted with a sterile connection. When no more liquid is passed through, the pool is automatically drained through a few small holes drilled through the vertical tubes just above their welded entry into the top portion of the heat exchanger. Cleaning of the straight tubes is an easy matter after the top and bottom lids have been removed. They are attached to the cylindrical part of the apparatus by means of a special trap which has been described (2).
Acknowledgment
The authors wish to acknowledge the assistance of Sven Warenius in making a number of test models of flexible tube couplings, and of Sven Goranson for valuable collaboration in designing the heat exchanger. literature Cited (1) H e d h , C.-G., Smed. Patent 145,828 (1954). (2) HedBn, C.-G., Malmgren, B., Sundstrom, K. E., and Tornqvist, B., Acta Path. Microbial. Scand., 30, 284 (1952). (3) Phillips, C. R., and Kaye, S., Am. J . Hag., 50, 270 (1949). ACCEPTED &fay 25, 1954. RECEIVED for review April 13, 1954.
Bactericidal Effects of Ultrasound Instrumentation and Techniques for Quantitative Studies LILLIAN A. RUSSELL
AND
ARTHUR M. BUSWELL
Illinois State W a t e r Survey, Urbana, Ill.
FRANCIS J. FRY
AND
ROBERT McL. WHITNEY
University of Illinois, Urbana, 111.
B
4 RICIDAL effects of ultrasound have been conclusively demonstrated by other workers in the field; however, in the equipment employed, quantitative control of the ultrasonic variables could not be obtained. A body of quantitative data based on the measurement and control of the physical variables of the ultrasonic field and sufficient to define the conditions most favorable to bacterial destruction would be of considerable advantage in evaluating this tool for commercial use and in outlining specifications for equipment to be used for municipal sterilization of xater and commercial sterilization of milk. The research described in this paper was initiated to supply this quantitative data. In the earlier studies a commercially available generator with quartz transducer was used on a pure culture of a single strain of Escherichza coli. More recent studies are being made on the same organism but uith the use of equipment designed for research purposes. This first section of the study is primarily concerned with describing instrumentation and techniques that are suitable for initial quantitative studies of the bactericidal effects of ultrasound.
Knowledge of Physical Phenomena of Ultrasonic Field Is Basic to Study
An experimental study that proposes to achieve the measure ment and control of the physical variables of the ultrasonic field, with the intention of correlating bacteriological effects with them, must be concerned first with an analysis of the characteristic phenomena of the system and the changes in the variables that produce them. Fry (6) reviewed these phenomena for biological systems in general and included in this category changes in temperature and in pressure, forces resulting from radiation pressure, and cavitation and its concomitants. Temperature changes in the absence of cavitation are of two kinds. A periodic temperature change arises from the adiabatic compression and expansion associated with a sound wave in a liquid medium. According t o Fry, this is of minor significance; i t is of the order of 0.01" C. for a sound-pressure amplitude of 10 atmospheres (an intensity of 35 watts per square em.) in water. September 1954
A gradual temperature increase is due to the progressive absorption of sound energy and its conversion into heat. For a plane traveling wave, the intensity decreases logarithmically with distance, and the amount of sound energy absorbed per unit volume of liquid per second a t any given distance from the crystal is expressed by the product of the coefficient of absorption and the intensity of sound a t that point. The magnitude of this coefficient increases with the viscosity of the medium and with the square of the sound frequency. Periodic pressure changes a t high frequencies are accompanied by a unidirectional radiation pressure. In relative magnitude much smaller than the alternating-pressure amplitude, radiation pressure manifests itself a t an acoustically reflecting interface with a force proportional to the intensity of the sound field. The alternating sound-pressure amplitude is proportional to the voltage applied a t the crystal and thus is a function of the square root of the intensity. A sound-pressure amplitude of 10 atmospheres is associated with a radiation pressure of 0.0024 atmosphere. At ultrasonic frequencies the rapid periodic recurrence of pressure changes becomes as important a factor as their absolute magnitude; for example, a sound-pressure amplitude of 10 atmospheres gives rise to a velocity amplitude of 70 om. per second, a particle amplitude of 0.11 micron, and an acceleration of 4 X IO8 cm. per second (40,000 g's). Unless measures are taken to prevent it, a body immersed in a liquid under such accelerations and velocity amplitudes becomes subject to cavitation, which occurs when the pressure on the surface of a body is reduced so low by the incapability of the flow to maintain contact with the body that a vacuum appears, or a t least a region saturated with the vapor of the liquid forms. The critical low pressure limit that can be reached before cavitation occurs is dependent upon the dissolved gas content of the liquid. Yumachi (14) found that with water saturated with air the limiting pressure was approximately equal to the vapor pressure of the liquid, but with even partially deaerated distilled water it was minus 1 atmosphere (absolute). Briggs, Johnson, and Mason ( 1 ) found that when liquids are degassed their natural cohesive power brcomes effective and they will withstand a
INDUSTRIAL AND ENGINEERING CHEMISTRY
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ENGINEERING, DESIGN, AND PROCESS DEVELOPMENT negative acoustic pressure. Harvey and coworkers (8, 9) demonstrated that Nithout gas nuclei cavity or bubble formation do not occur even a hen the liquid is under considerable tension. Two types of cavitation, vaporous and gaseous, are usually distinguished. By adding a iew drops of a volatile liquid to a noncavitating system, Grabar ( 6 ) readily produced vaporous cavitation and used this means to distinguish the mechanical effects that were produced by vaporous cavitation and the chemical effects that he found were produced only by gaseous cavitation. Air bubbles serve as nuclei for the formation of vapor cavities when the pressure is reduced to the vapor pressure of the liquid. When these cavities collapse the air content plays an important role, since the air cannot be as readily redissolved as the vapor can be condensed. The cavity must then collapse on an air nucleus, and the abruptness and violence of the shock of collapse are dependent upon the size of the residual nucleus. This is perhaps the most important reason for the difference in the effects of gaseous and vaporous cavitation.
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