The Driving Force Behind Ultrasonic Cavitation Treatment
Liquid exposure to high-intensity ultrasound can cause acoustic cavitation. This phenomenon can usually be seen as a bubble cloud formed near an ultrasonic source (such as an ultrasonic horn) and is regarded as a strong hissing sound. Cavitation is a low-pressure cavity (i.e. vacuum bubble or cavity) formed in a liquid, which continuously increases, oscillates briefly, and then asymmetrically implodes with enormous intensity.
Chemical and physical effects of cavitation :
The collapse of violent foam causes local extreme temperature (~5000 ℃), heating/cooling rate (~10 billion ℃/s) and pressure (~1000 atm) in cavitation, generates free radicals, and causes many chemical (chemical) reactions (such as oxidizing pollutants, sterilization, polymerization, desulfurization, long-chain molecule degradation, etc.). At the same time, the cavitation field generates flow, extremely fast micro jets (about 500 m/sec), and enormous shear forces, promoting a wide range of physical (mechanical) effects such as emulsification, particle fragmentation, cell fragmentation, homogenization, dispersion, de aggregation, and degassing. Under appropriate conditions, cavitation can even generate light - an effect known as sonoluminescence.
The pure chemical effect of ultrasonic cavitation is enhanced to a certain extent with increasing frequency, while the mechanical effect has a strong opposite dependence with increasing frequency. The design of industrial ultrasonic processors introduces strong shear forces to the processed liquid, therefore, they tend to operate at the lower edge of the ultrasonic spectrum (~20 kHz).
Explanation of potential mechanisms :
The schematic diagram of the dynamics of cavitation bubbles is shown in the figure, which illustrates the growth and asymmetric collapse of low-pressure cavities, resulting in the generation of microjets. The simultaneous occurrence of bubble oscillation and bubble growth is not displayed. As the bubble oscillates and expands, it draws the surrounding liquid vapor and dissolved gas into the interior of the bubble. This process is called "rectification diffusion". The pressure in the foam is kept at a relatively low level, which is conducive to its final implosion. In the final stage of implosion, the velocity of the bubble wall can exceed the speed of sound inside its gas. This generates shock waves within the bubble (similar to those produced when an airplane crosses a sound barrier), which break down the bubble into tiny fragments that subsequently become the starting point for further cavitation events.
Protect your hearing :
The collapse of cavitation bubbles generated during the ultrasound process produces strong broad-spectrum "hissing" noise, with decibels exceeding 100 decibels. Therefore, it is very important to take health and safety precautions when using ultrasonic liquid processors.
