When ultrasonic energy acts on the medium, it will cause the particle to vibrate at high speed and produce the changes of mechanical quantities such as velocity, acceleration, sound pressure and sound intensity, thus causing mechanical effects. Ultrasonic wave is the transmission form of mechanical energy, which is related to the wave process and can produce linear effect of vibration. When the ultrasonic wave propagates in the medium, the particle displacement amplitude is very small, but the particle acceleration caused by the ultrasonic wave is very large.
When the ultrasonic medium is not a uniform layered medium (such as biological tissue, human body, etc.), the acoustic impedance of each layer of medium is different, which will make the propagating sound wave reflect and form standing wave. The antinode and node of standing wave will cause the changes of pressure, tension and acceleration. Because the mass of different medium particles (such as biomolecules) is different, the vibration velocity caused by pressure change is different. The pressure change caused by the relative motion of medium particles is another reason for the ultrasonic mechanical effect. The mechanical effect of ultrasound should be used for processing (drilling, cutting)
Cutting, compaction, surface strengthening, welding, cleaning, polishing and removal of unwanted film and dirt, etc.) are also used to accelerate dispersion, homogenization, emulsification, crushing, sterilization and other processes.
The mechanical action of ultrasound has been widely used in production. The examples are as follows.
Mechanical stirring
The high frequency vibration and radiation pressure of ultrasound can form effective agitation and flow in gas and liquid. The strong jet and local micro jet produced by cavitation bubble vibration on the solid surface can significantly reduce the surface tension and friction of the liquid, and destroy the boundary layer of the solid-liquid interface. This function is the physical basis for the application of drugs, cosmetics, ultrasound, food and cosmetics.
Mutual diffusion
Ultrasonic vibration and cavitation pressure, high temperature effect, promote two kinds of liquid, two kinds of solid, or liquid-solid, liquid-gas interface, the occurrence of molecular mutual penetration, the formation of new material properties. Ultrasonic welding of metal or plastic, ultrasonic emulsification, cleaning, atomization can be classified as such.
Homogenization
The local shock wave generated by the closure of cavitation bubbles can crush the particles in the liquid and refine them; Make the crystal uniform; Dispersing large and uneven emulsion drops into small and uniform agents (such as medical contrast agents, cancer agents, etc.); It may even include thrombolysis.
Coacervation
Ultrasonic vibration can make the suspended particles move at different speeds in the air and liquid medium, increasing the chance of collision; Or the standing wave can make them tend to the antinode, which leads to the condensation process. Flue dust collection and artificial rainfall can belong to this category.
Mechanical cutting action
Due to the high acceleration of ultrasonic vibration and the acoustic corrosion of cavitation, it can be used for special precision machining of hard and brittle materials (GEM, ceramics, glass, magnetic steel, etc.).
Comminution
High intensity ultrasound pulse can crush kidney stones and gallstones in human body without damaging soft tissue.