The Effect of Intermolecular Bond Strength on the Onset of Cavitation

Since bacterial destruction is a cavitation-dependent effect of ultrasound, it has been possible to draw conclusions regarding the factors affecting cavitation by studying bacteriological results. Micro gas bubbles are often described as nuclei for cavitation bubbles. In order to destroy bacteria, however, cavities must form and collapse within several microns of the bacterial cells so that the resulting collapse shock wave will have a high enough energy to rupture the cell wall. Cavitation near the cell occurs most readily when the interfacial linkages between the cell and the surrounding liquid are weakest. When surface-active agents, which strengthen the interfacial linkages, are added to the bacterial suspension, cavitation is inhibited and the ultrasonic death rate decreases markedly. If the bacterial suspension is heated, increasing the energy of the intramolecular bonds, the threshold sound pressure just necessary to kill bacteria is reduced, or, if the sound pressure is constant, the bacteria are destroyed at an increasing rate as the temperature increases. Likewise, if alternate cavitation nuclei which form weaker linkages with the surrounding liquid than the initial bacteria are added to the bacterial suspension, the killing rate of the original bacteria is decreased.