The Influence of Dissolved Carbon Dioxide on Cavitation Intensity in Ultrasound Cleaning Systems

A study is made of the effect of dissolved CO2 on cavitation inception and activity in an ultrasound system with a sound frequency of 928 kHz. It is shown that measurable cavitation activity is completely absent at moderate acoustic power densities (≤1.7 W cm-2) when only CO2 is dissolved in ultra pure water. It is suggested that the enhanced stability of CO2 bubbles against coalescence might be the origin of the delayed cavitation inception when compared to other dissolved gases. A combination of dissolved O2 and CO2 can lead to a measurable cavitation activity at an acoustic power of 420 mW cm-2, but an increase of the dissolved CO2 level reduces, in general, the observed cavitation activity. In order to compare results with different dissolved gas concentrations, measurements are performed with acoustic pulses and the pulse off time is varied. An optimal pulse-off time exists, where a maximum of cavitation activity is observed. However, the pulse-off time interval with enhanced cavitation activity is narrowed with increasing dissolved CO2 concentrations. Again, a decrease in bubble coalescence might explain the narrowing of the “enhancement peak”.