Trapping and sorting of micro-sized objects is one important application of lab on a chip devices, with the use of acoustic bubbles emerging as an effective, non-contact method. Acoustically actuated bubbles are known to exert a secondary radiation force (FSR) on micro-particles and stabilize them on the bubble surface, when this radiation force exceeds the external hydrodynamic forces that act to keep the particles in motion. While the theoretical expression of FSR has been derived by Nyborg decades ago, no direct experimental validation of this force has been performed, and the relationship between FSR and the bubble's ability to trap particles in a given lab on a chip device remains largely empirical. In order to quantify the connection between the bubble oscillation and the resultant FSR, we experimentally measure the amplitude of bubble oscillations that give rise to FSR and observe the trapping and release of a single microsphere in the presence of the mean flow at the corresponding acoustic parameters using an acoustofluidic device. By combining well-developed theories that connect bubble oscillations to the acoustic actuation, we derive the expression for the critical input voltage that leads to particle release into the flow, in good agreement with the experiments.
Bibliographical noteFunding Information:
This research has been supported by a DARPA Young Faculty Award through grant N66001-11-1-4127. YC and SL thank Dr. Sascha Hilgenfeldt (UIUC) for fruitful discussions and Dr. Tony Yu for help with image processing. The authors would also like to acknowledge Dr. Feng Zhao (WSUV) for resonant frequency measurements and Darius Saadat-Moghaddam for his help with experiments. BM thanks the support from the WSUV ENCS Undergraduate Summer Research Program
© The Royal Society of Chemistry 2016.