When the ultrasonic wave propagates in the medium, due to the interaction between the ultrasonic wave and the medium, the medium undergoes physical and chemical changes, resulting in a series of mechanical, thermal, electromagnetic and chemical ultrasonic effects, including the following four effects:
1. Mechanical effects
The mechanical action of ultrasound can promote liquid emulsification, gel liquefaction and solid dispersion. When a standing wave is formed in the ultrasonic fluid medium, the tiny particles suspended in the fluid are condensed at the nodes due to mechanical force, forming periodic accumulation in the space. When ultrasonic waves propagate in piezoelectric materials and magnetostrictive materials, induced electrical polarization and induced magnetization due to the mechanical action of ultrasonic waves (see dielectric physics and magnetostriction).
When ultrasonic waves act on liquids, a large number of small bubbles can be produced. One reason is that partial tensile stress appears in the liquid to form negative pressure. The decrease in pressure makes the gas originally dissolved in the liquid supersaturate and escape from the liquid and become small bubbles. The other reason is that the strong tensile stress "tears" the liquid into a hollow, which is called cavitation. Inside the cavity is liquid vapor or another gas dissolved in the liquid, and it may even be a vacuum.
The small bubbles formed due to cavitation will continue to move, grow up or suddenly burst with the vibration of the surrounding medium. When it bursts, the surrounding liquid suddenly rushes into the bubble, generating high temperature, high pressure, and shock waves. The internal friction accompanied by cavitation can form electric charges, and cause light emission in the bubbles due to discharge. The technology of ultrasonic treatment in liquid is mostly related to cavitation.
3. Thermal effect
Due to the high frequency and high energy of ultrasonic, it can produce obvious thermal effect when absorbed by the medium.
4. Chemical effects
The effect of ultrasound can promote or accelerate certain chemical reactions. For example, pure distilled water undergoes ultrasonic treatment to produce hydrogen peroxide; nitrogen-dissolved water produces nitrous acid after ultrasonic treatment; dye aqueous solution will change color or fade after ultrasonic treatment. The occurrence of these phenomena is always accompanied by cavitation. Ultrasound can also accelerate the hydrolysis, decomposition and polymerization of many chemical substances.
Ultrasound also has a significant effect on photochemical and electrochemical processes. After the aqueous solutions of various amino acids and other organic substances are ultrasonically processed, the characteristic absorption bands disappear and show a uniform general absorption, which indicates that the cavitation has changed the molecular structure.
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