Some liquids may contain antibubbles. An antibubble consists of a liquid core droplet surrounded by a gas bubble. Surface-active agents on both gas-liquid interfaces of the antibubble are typically electrically charged, preventing the core droplet from touching the outer bubbles surface. Antibubbles have been observed in various milk products, in Belgian beer, and in ultrasound contrast agents.
Just like regular bubbles, antibubbles oscillate upon sonication. Commercial ultrasound contrast agents, consisting of microscopically small bubbles with stabilising shells, express higher harmonics when subjected to ultrasound with intermediate or high acoustic amplitudes. However, as the antibubble core droplet can be considered incompressible, an antibubble expands more readily than it contracts. Owing to this asymmetry in antibubble oscillation, even at low acoustic amplitudes, swinging antibubbles generate higher harmonics. For this reason, antibubbles are suitable candidates for ultrasonic harmonic imaging.
In previous studies by the Experimental Acoustics group of the University of Bergen, Norway, the suitability of antibubbles as ultrasound contrast agent was investigated. In addition, a mathematical model for an oscillating antibubble with a Newtonian viscous shell was derived.
Earlier, it had been speculated that the liquid cores of antibubbles might be replaced by therapeutic agents, and thus to create vehicles for ultrasound-guided local drug delivery.
For diagnostic imaging and drug delivery applications, the dynamic behaviour of antibubbles under sonication needs to be predicted in greater detail, with a focus on harmonics generation and antibubble disruption.
This study will result in a mathematical model of antibubble dynamics, incorporating solid and viscoelastic shells. Furthermore, the harmonic response of antibubbles will be studied experimentally using multi-frequency probes.