Manipulating microbubbles in physiologically realistic flows using the ultrasound-induced Bjerknes force

Ultrasound contrast agents are micron-sized bubbles used for ultrasound imaging enhancement or drug delivery. They can be manipulated inside the cardiovascular system by utilizing the Bjerknes force, caused by the phase difference between the ultrasound pressure wave and the microbubble volume oscillations. Although the mechanism causing this force is well established, the balance between ultrasound-induced forces and hydrodynamic forces is poorly understood when the microbubbles are immersed in high Reynolds number and high Womersley flows. Experiments were conducted in a cylindrical tube under steady and pulsatile flows over a range of Reynolds and Womersley numbers relevant to drug delivery in the carotid artery. Microbubbles were imaged in the flow using a high-speed camera with a microscopic lens to determine the trajectories of individual bubbles and thus the relationship between acoustic and hydrodynamic forces. The relative scaling of these forces was computed for different acoustic pressure amplitudes and pulse repetition frequencies. The Bjerknes force scaled linearly with pulse repetition frequency and quadratically with acoustic amplitude. The ratio of Bjerknes to drag force decreased with increasing Reynolds number suggesting a threshold for clinical applications.