During ultrasound processing, a series of compressions and rarefactions are created in the medium in which the waves are propagated in. At sufficiently high power, the energy in the ultrasonic waves exceeds the attraction forces between the molecules of the media which results in the formation of cavitation bubbles. The non-uniform acoustic field results in unstable bubbles which implode and cause localised ‘hotspots’ of high temperature (4000 K) and pressure (1000 atm). These 'hotspots' lead to chemical and mechanical effects manifested in the medium. The effects of ultrasound processing are dependent on the material involved such as, liquid – liquid systems, liquid – solid systems and gaseous systems.

 

Cavitation of bubbles affects the liquid systems in 2 different zones. The first is the bulk fluid immediately surrounding the bubble whereby the rapid collapse of the bubble results in shear force in the liquid responsible for the mechanical effect. This mechanical effect at liquid – liquid interfaces is responsible for the emulsification of two immiscible liquids forming emulsions.  The second zone is in the bubble in which any material introduced will experience extreme temperature and pressure due to the bubble collapse leading to chemical effects.

 

In solid – liquid systems, the effects of ultrasound are slightly different. Unlike liquid – liquid systems, collapse of cavitation bubbles on or near the surface is symmetrical due to the resistance to liquid flow. This results in an in rush of liquid from the side remote from the surface which creates liquid jets targeting the surface (imagine a high pressure jet) and making ultrasound effective in cleaning surfaces.