Cavitation. When ultrasonic wave acts on liquid, it will produce a large number of small bubbles. One reason is that the local tensile stress in the liquid forms negative pressure, and the decrease of pressure makes the gas originally dissolved in the liquid supersaturate and escape from the liquid into small bubbles. Another reason is that the strong tensile stress "tears" the liquid into a cavity, which is called cavitation. The cavity is filled with liquid vapor or another gas dissolved in the liquid, even vacuum. Small bubbles formed by cavitation will suddenly move, grow up or burst with the vibration of the surrounding medium. When it bursts, the surrounding liquid suddenly rushes into bubbles, producing high temperature, high pressure and shock wave. Internal friction related to cavitation can form charge, and discharge in bubbles can produce luminescence. Ultrasonic treatment technology in liquid is mostly related to cavitation.