Mechanisms of ultrasound foam interactions

An experimental investigation into the effects of two frequencies of low-power ultrasound on detergent (Teepol) stabilised air-water foams is presented. Foam was subjected to ultrasound at 28 and 40 kHz with a powerto-foam volume ratio of approximately 3 W l-1 with particular consideration being given to the acoustic impedance discontinuity between air and water. The foam height, liquid drainage and collapse behaviour were compared to experiments conducted without ultrasound. In the case of 40 kHz ultrasound, an increased liquid drainage rate was observed and a pronounced increase and subsequent peak in mean liquid hold-up, which occurred at 4 min, was observed. These results appear to be independent of the initial liquid hold-up of the foam. Liquid drainage and mean liquid hold-up results were related to foam collapse mechanisms of homogeneous rupture (HR) and rupture front breakage (FR), concluding that rupture front breakage dominates the collapse of foam under the influence of 40 kHz ultrasound. © 2009 Curtin University of Technology and John Wiley & Sons, Ltd..