Agglomeration and coalescence behavior of microbubbles under ultrasonic irradiation

Microbubbles have large specific interfacial areas and fast dissolution rates into liquid in comparison with conventional bubbles. Because of the difficulty of controlling of microbubbles in a liquid, however, microbubble-related technologies have yet to be applied in chemical reaction processes with gas-liquid contact operations. In this study, the effects of solution conditions on the dynamic behavior of microbubbles in an ultrasonic field were investigated. When microbubbles in ion-exchanged water were irradiated with ultrasound at a frequency of 2.4 MHz, the microbubbles in the neighborhood of antinodes moved toward to nodes by the Bjerknes force. In the stagnant space on the nodes, microbubbles formed agglomerates and were rapidly removed upward. When a surfactant was added, microbubble aggregates became difficult to form, because of steric inhibition by the adsorption of surfactant to the microbubble surface. In addition, when the z potential of microbubbles was close to 0mV, some of the agglomerated microbubbles coalesced. This suggests the possibility of controlling not only agglomeration and coalescence behavior of microbubbles, but also their ascent velocity in a liquid by suitable setting of solution conditions.