Ultrasonic wave is a mechanical elastic wave with a frequency of more than 20kHz, the wave velocity is generally 1500m/s and the wavelength is 0.01~10cm. Ultrasonic wave is a mechanical vibration in the media propagation process, with the beam, orientation, reflection and transmission characteristics, it mainly produces 2 kinds of vibration in the medium, namely, shear wave and longitudinal wave, the former can only be produced in solids, and the latter can be in solid, liquid, gas generation.
The interaction mechanism between ultrasonic wave and medium is divided into three kinds: thermal mechanism, mechanical mechanism and cavitation mechanism.
Thermal effect (Zhou, 2009) refers to the process of ultrasonic wave propagation in the medium, its vibration energy is continuously absorbed by the medium, and converted to heat energy, so that their own temperature rise. The absorption of sound energy causes the whole heating in the medium, the local heating outside the boundary and the cavitation forming the shock wave, which leads to the local heating of the wavefront. The greater the ultrasonic strength, the stronger the thermal effect.
Mechanical action (Zhou, 2009) refers to the ultrasonic is a mechanical vibration energy transmission form, will form an effective flow and agitation in the liquid, resulting in the destruction of the media structure, the liquid particles are crushed to achieve ordinary low-frequency mechanical agitation can not achieve the effect. The mechanical action of ultrasound is often used in crushing, agglutination, cutting and so on.
Cavitation is when the ultrasonic action in the medium, its strength exceeds an air valve value, it will produce cavitation phenomenon.