Experimental study of bubble dynamics in a droplet driven by an ultrasound standing wave

A good understanding of bubble dynamics in the ultrasonic vibration (UV) field can advance the development of a range of bubble-based emerging technologies such as atomisation of liquids and nucleation/solidification of droplets. However, previous studies have mainly focused on bubble dynamics in bulk liquids; the bubble dynamics in a confined liquid is poorly understood. To address this problem, in this study, UV with a frequency of 40 kHz was applied to form a standing wave in a small water tank. An oil droplet (diameter: 6.7 mm, 6.4 mm, and 6.0 mm) was chosen as the confined space for bubbles (diameter: 0.9 mm, 1.4 mm, and 2.2 mm). The bubble-droplet mixture was generated using co-axial needles driven by syringe pumps, which was then released at a certain height above the tank bottom. The UV was controlled to start at 100 mus after the release of the bubble-droplet mixture. Meanwhile, a high-speed video camera was activated simultaneously with a particular focus on bubble dynamics. Significant motion of bubbles inside the confined liquid was found. Small bubbles tended to have a higher moving speed than large bubbles due to the relatively long acceleration distance and small drag force. As a result, small bubbles penetrated the oil-water interface more often. For large bubbles, it was found that the oil-water interface confined their movement very well. Particularly, for the bubble of 2.2 mm in diameter, it could not escape from the oil-water interface; instead, the trapped bubble significantly accelerated the motion of the droplet. This study provides information that helps the understanding of bubble dynamics in a confined liquid.