What is Ultrasonic Cavitation?
When the pressure of the sound wave propagated by ultrasonic vibration in liquid reaches one atmosphere, the peak pressure of the ultrasonic wave can reach vacuum or negative pressure. However, in reality, there is no negative pressure present, so a large force is generated in the liquid, which pulls the liquid molecules into voids. This cavity is very close to vacuum, and it ruptures when the ultrasonic pressure reaches its maximum reverse direction. The strong impact generated by the rupture hits the dirt on the surface of the object. The shock wave phenomenon generated by the rupture of countless small cavitation bubbles is called the "cavitation" phenomenon.
When the ultrasonic energy is high enough, tiny bubbles (cavitation nuclei) existing in the liquid vibrate, grow, and continuously gather acoustic field energy under the action of the ultrasonic field. When the energy reaches a certain threshold, cavitation bubbles collapse and close rapidly.
The lifespan of cavitation bubbles is about 0.1 μ s. When they collapse rapidly, they can release enormous energy and generate micro jets with a velocity of about 110m/s and strong impact force, resulting in a collision density of up to 1.5kg/cm2. At the moment of rapid collapse, cavitation bubbles generate local high temperature and pressure (5000K, 1800atm), with a cooling rate of up to 109K/s. The cavitation effect of ultrasound greatly increases the rate of heterogeneous reactions, achieves uniform mixing between heterogeneous reactants, accelerates the diffusion of reactants and products, promotes the formation of solid new phases, and controls the size and distribution of particles.
Additional Information:
When ultrasound propagates in a medium, the interaction between ultrasound and the medium causes physical and chemical changes in the medium, resulting in a series of mechanical, thermal, electromagnetic, and chemical ultrasound effects, including the following four effects:
1. Mechanical effects. The mechanical action of ultrasound can promote the emulsification of liquid, the liquefaction of gel and the dispersion of solid. When standing waves are formed in ultrasonic fluid media, tiny particles suspended in the fluid condense at the nodes due to mechanical forces, forming periodic accumulations in space.
The induced polarization and magnetization caused by the mechanical action of ultrasonic waves during propagation in piezoelectric and magnetostrictive materials (see dielectric physics and magnetostriction).
2. Cavitation effect. Ultrasonic waves can generate a large number of small bubbles when applied to liquids. One reason is that local tensile stress occurs inside the liquid, forming negative pressure. The decrease in pressure causes the gas originally dissolved in the liquid to become supersaturated and escape from the liquid, becoming small bubbles.
Another reason is that strong tensile stress tears apart the liquid into a cavity, known as cavitation. The cavity contains liquid vapor or another gas dissolved in the liquid, and may even be a vacuum. Small bubbles formed by cavitation will continuously move, grow, or suddenly burst with the vibration of the surrounding medium.
3. Thermal effect. Due to the high frequency and energy of ultrasound, it can produce significant thermal effects when absorbed by the medium.
4. Chemical effects. The action of ultrasound can promote or accelerate certain chemical reactions. For example, pure distilled water produces hydrogen peroxide after ultrasonic treatment; Nitrous acid is produced after ultrasonic treatment of water containing nitrogen gas; The aqueous solution of dye will change color or fade after ultrasonic treatment.
