Ultrasonic waves are sound waves with frequencies greater than 20kHz. Their wavelength and wavespeed vary depending on the medium, or cleaning solution, they are formed in. Inaudible to humans, ultrasound waves are longitudinal waves that vibrate in the direction they are travelling, forming areas of high pressure (compressions) and low pressure (rarefactions) as shown in the image below.
Ultrasound waves create microscopic bubbles as they pass through certain fluids. This occurs as the areas of low pressure in the longitudinal waves drop below the vapour pressure of the cleaning solution forming a ‘hole’ or vacuum bubble. Vapour pressure is the pressure that a vaporised fluid exerts on the surface of the same fluid in liquid form, when they are both at thermal equilibrium. These bubbles oscillate due to an acoustic field then implode, forming shock waves as the surrounding fluid flows to fill the void left by the bubble. This process of bubble formation and implosion is called non-inertial cavitation. These cavitation bubbles are microscopic and exist for an extremely short period of time, thus they cannot be seen with the naked human eye.
Cavitation bubbles can produce local pressures of up to 20,000 psi and local temperatures of up to 5,000°C (approximately the temperature of the surface of the Sun). Shock waves formed by cavitation at the boundary between a fluid and a solid can dislodge contaminants and dirt from the surface of the solid. In this way, a variety of solid objects can be efficiently cleaned to a high standard.
The size and life cycle of the cavitation bubble will depend primarily on the frequency. Lower frequencies produce larger, longer lasting bubbles with more powerful implosion forces, whilst higher frequencies produce smaller, shorter duration bubbles with weaker implosion forces. Typically, the diameter of cavitation bubbles varies from 40 microns at higher frequencies (like 80kHz) to 150 microns at lower frequencies (like 20kHz).