Acoustic Cavitation


Acoustic cavitation is the growth and collapse of pre-existing microbubbles when subjected to an ultrasonic field in liquids.  The cavitation bubbles are the result of oscillations resulting from the effect of the maximum temperature and pressures reached when they collapse. These bubbles grow when subjected to low frequency and implode at higher frequencies due to compression. During the implosion phase, the temperature and pressure rise and the heat exchange occurs in microseconds.  In the surrounding fat tissue, the resulting cavitation can cause thermal, mechanical and chemical actions leading to the modification of the membranes.  Acoustic cavitation can be investigated based on the theoretical single bubble model.  However, in a multibubble system, the formation of bubble clusters makes it challenging to characterise the cumulative properties of these bubbles.  In 1917 Lord Rayleigh provided a partial explanation by showing that significant pressures could be generated from the implosion of the spherical vapour bubbles.  During the 1930s, Robert Knapp managed to capture the behaviour of individual cavitation bubbles by using motion pictures. These films showed that the cavitation bubbles possessed a high degree of non-linearity during the low-pressure phase.  This was compared to the gentle growth of the bubbles when subjected to low pressure in the surrounding liquid.  However, when they collapse the liquid pressure increases and produces a violent action. This non-linear behaviour is captured in the Rayleigh-Plesset equation for spherical bubble dynamics.  In solutions, the bubble collapses to a size much smaller than its original dimension and therefore compressing the non-condensable gas in the bubble to very high pressure and temperatures resulting in the emission of light.