Influence of liquid viscosity on growth of the surface of an air bubble subjected to ultrasonic cavitation

An air bubble is injected into a liquid near the surface of an ultrasonic transducer and exposed to an ultrasonic field to increase the surface of the bubble–liquid interface. The purpose of this theoretical and experimental work is to investigate the effect of liquid viscosity on the relative increase in the bubble–liquid interface with ultrasonic cavitation. Numerically, the estimation of the relative increase in the air bubble surface is based on an iterative process. The distance between the air bubbles and the transducer, as well as the liquid viscosity, greatly impacts the surface relative increase. The optimal distance from the transducer is approximately 10 mm. At low viscosity, the surface area can increase by up to 1.7 times, but as the liquid viscosity increases above 7 mPa s, there is no significant increase. Experimentally, air bubbles subjected to ultrasonic cavitation are observed and analyzed. The image analysis algorithm is based on analyzing the external contour of the bubble combined with the brightness gradients in the image. The experimental observations are similar to those observed numerically. The relative increase in the surface area decreases with the viscosity. Above a viscosity of approximately 7.4 mPa s, there is almost no further relative increase.