Fundamentals of Ultrasound. Factors Affecting Cavitation

Characteristics of ultrasound and solution conditions impact acoustic cavitation. Ultrasonic factors include frequency and power. Solution factors include bulk temperature and type of dissolved gas. Unlike solution parameters, ultrasonic conditions can be controlled by user discretion, depending on the available equipment. Common ultrasonic transducers include horn systems, flat plate transducers, and baths. These ultrasonic transducers are often used to generate cavitation in liquids for remediation purposes. Horns deliver high-intensity cavitation by delivering power over a small surface area.

Different ultrasonic frequencies used affect the critical size of cavitation bubbles. Higher frequency ultrasound has a shorter cycle, and thus, less time for bubble growth during rarefaction and more cycles before bubbles reach a critical size for cavitational collapse than low-frequency ultrasound. Moreover, shorter growth times limit time for sound energy absorption, causing high-frequency ultrasound to have smaller bubble radii. However, a moderate frequency has higher •OH production rates than lower or higher frequency ultrasound.

Shorter bubble collapse times and smaller cavitation bubbles at higher frequencies result in more effective radical migration to the bulk solution. Low-frequency ultrasound has longer growth periods and increased sound wave absorption. Low-frequency ultrasound also has higher acoustic amplitude pressures than high-frequency ultrasound, and consequently produces more physical effects (Section 4).

Acoustic power has strong impacts on acoustic cavitation. Acoustic power yields a larger pressure amplitude. This larger amplitude pulls the walls of the bubble apart more strongly, resulting in larger oscillations and faster, nonlinear bubble growth rates. Increased power increases sound wave absorption in cavitation bubbles, resulting in larger bubble radii and a shorter lifetime. Bubbles with larger radii have more potential energy to be released as kinetic energy upon collapse. Therefore, increased power increases collapse temperature and pressure.

Bulk temperature also affects cavitation. Increases in bulk temperature increase vapor pressure in an oscillating bubble. The bond dissociation of water in the cavitation bubble to •OH and •H is an endothermic reaction, requiring a lot of heat. With more water vapor in the gas phase at higher temperatures, more energy is needed to heat water to the high temperatures needed to dissociate •OH and •H from water, reducing the temperature in the bubble. This energy used to heat the extra water reduces energy available as kinetic energy of the collapsing bubble, thereby reducing the compression of the bubble, and consequently the collapse temperature. Therefore, under the same ultrasonic power settings, solutions irradiated at higher bulk temperatures produce less •OH than lower bulk temperatures.

Dissolved gases are crucial for cavitation nuclei formation. Inert sparging gases are used to mimic anoxic sediments. The number of atoms in the gas indicates the specific heat ratio, or polytropic index (y). Diatomic gases have lower specific heat ratios than monatomic gases because diatomic gases have more vibrational and rotational modes. Diatomic gases, therefore, spend more kinetic energy on molecular motion and less energy toward collapse temperature relative to monatomic gases. Gases with higher thermal conductivities lose more heat to the bulk liquid. Dissolved gases also affect radical production. •OH production is greater in monatomic gases with high polytropic indices and low thermal conductivities.