History force effects on contrast agent microbubbles in an ultrasound field

We study experimentally the radial and translational dynamics of an ultrasound contrast agent microbubble pair pulsating in an ultrasound field. The two bubbles attract each other through the so-called secondary Bjerknes force; quantifying these bubble-bubble interactions is therefore crucial for optimized medical imaging protocols. Using optical tweezers, we trap and control the distance between two microbubbles (BR-14, Bracco Research S.A., Geneva). We position the bubble pair away from the sample chamber wall, to prevent wall effects and quantify purely the acoustic bubble-bubble interaction and the dissipation due to viscosity in the fluid. The ultra-high speed Brandaris camera recorded the bubble dynamics at 15 million frames per second; from the optical measurements we track the instantaneous bubble radii and positions. We write a force balance for each bubble, assuming a no-slip boundary condition since the bubble interface is coated with a lipid monolayer to prevent dissolution. By comparison with the experimental results, we find that history effects are crucial to correctly account for the viscous forces.