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Ultrasonic mechanical effect

2021-04-30 10:42:25

Ultrasonic energy acts on the medium, which will cause high-speed and subtle vibrations of the particles, resulting in changes in mechanical quantities such as speed, acceleration, sound pressure, and sound intensity, which will cause mechanical effects. Ultrasound is the propagation form of mechanical energy, which is related to the wave process and will produce a linear effect of vibration. When the ultrasonic wave propagates in the medium, although the particle displacement amplitude is small, the particle acceleration caused by the ultrasonic wave is very large.

If 20kHz, 1W/cm2 ultrasonic waves propagate in water, the sound pressure amplitude generated is 173kPa, which means that the sound pressure amplitude changes between plus and minus 173kPa 20,000 times per second, the acceleration of the largest particle Up to 1440km/s2, which is about 1500 times the acceleration of gravity, such intense and fast-changing mechanical motion is the mechanical vibration effect of power ultrasound.

When the ultrasonic medium is not a uniform layered medium (such as biological tissue, human body, etc.), the acoustic impedance of each layer of the medium will reflect the propagating sound wave and form a standing wave. The antinodes and nodes of the standing wave cause pressure and tension. And acceleration changes. Since the masses of particles of different media (such as biomolecules) are different, the vibration speeds caused by pressure changes are different. The pressure changes caused by the relative movement of the media particles are another cause of ultrasonic mechanical effects. The mechanical effect of ultrasonic should be processed (punching, cutting
Cutting, compacting, surface strengthening, welding, cleaning, polishing and removing undesirable films and dirt, etc.), it is also used to accelerate dispersion, homogenization, emulsification, crushing, sterilization and other processes.

The mechanical action of ultrasound has been widely used in production. Examples are as follows.

Mechanical stirring
Ultrasonic high-frequency vibration and radiation pressure can form effective agitation and flow in gas and liquid. The strong jet and local micro-rush flow generated by the vibration of the cavitation bubble on the solid surface can significantly reduce the surface tension and friction of the liquid, and destroy the surface layer of the solid-liquid interface, thus reaching the level that ordinary low-frequency mechanical agitation cannot reach. effect. This effect is the physical basis for the application of drugs to penetrate people, beauty products to guide people's skin, ultrasonic degassing, and food and cosmetics blending and thinning.

Mutual diffusion
The pressure and high temperature effects of ultrasonic vibration and cavitation are used to promote molecular interpenetration between two liquids, two solids, or between liquid-solid and liquid-gas interfaces, forming new material properties. Ultrasonic welding of metals or plastics, phacoemulsification, cleaning, and atomization can be classified as such effects.

Homogenization
The local shock wave generated after the cavitation bubble is closed can crush the particles in the liquid to make it thin; make the crystal uniform; disperse large and uneven emulsion droplets into tiny uniform drugs (such as medical contrast agents, cancer treatment agents, etc.) ; It may even include the effects of dissolving thrombus.

Cohesion
Ultrasonic vibration can cause the suspended particles in the gas and liquid media to move at different speeds, increasing the chance of collision; or using standing waves to make them tend to the antinodes, so that the condensation process occurs. The flue dust collection and artificial rainfall fall into this category.

Mechanical cutting
Due to the high acceleration of ultrasonic vibration and the acoustic corrosion effect of cavitation, hard and brittle materials (jewels, ceramics, glass, magnetic steel, etc.) can be precisely processed with special shapes.

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