Bubble coalescence and dissolution effects on sonochemical activity in pulsed wave ultrasound systems

Pulsed wave (PW) ultrasound offers a promising alternative to continuous wave (CW) ultrasound for contaminant degradation, yet the mechanisms governing PW sonochemical effectiveness remains underexplored. This study investigates the relative influence of bubble coalescence and dissolution on sonochemical activity in PW systems using calorimetry, terephthalate (TA) dosimetry, and the capillary method. Experiments were conducted at 337 kHz with 10 and 100 ms on-times with varying off-times and surfactant concentrations using fluorotelomer sulfonates (FtS) of differing chain lengths. Calorimetry and hydroxyterephthalate (HTA) fluorescence consistently showed less sonochemical activity with increasing off-times. However, bubble coalescence, measured by the capillary method, decreased with increasing off-times. These findings, taken together, indicate that bubble dissolution, rather than coalescence, is the dominant factor limiting sonochemical efficiency in PW systems. In the presence of FtS and compared to CW systems, PW did not affect bubble coalescence, suggesting surfactants affect bubble coalescence similarly in CW and PW systems. Finally, calorimetry proved to be a reliable, chemical-free dosimeter for assessing cavitation activity, offering a practical alternative to traditional dosimeters.