Acoustic manipulation and actuation of bubbles in complex environments: Beyond the Bjerknes force

Micron-sized gas bubbles are notoriously difficult to isolate, handle and remotely control. Their large buoyancy in common liquids will usually force them to rise and burst at any gas/liquid interface or remain trapped against a solid boundary until dissolution. While bubble stability issues against dissolution have found numerous practical workarounds, the challenge remains at isolating and maneuvering a single bubble in free space to, for instance, perform precise single bubble dynamics experiments with applied ultrasound or to use them as active carriers for a specific payload deliverable on demand. Here we demonstrate that single-beam acoustical tweezers [D. Baresch et al, Phys. Rev. Lett., 116, (2016)] can trap and manipulate in 3D a single bubble with the radiation pressure of helicoidal ultrasonic beams. Contrary to the situation where bubbles are trapped in the antinodes of a standing wave, the trapping vortex beam does not require oscillating volume changes of the bubble to generate a force, i.e., the trapping mechanisms cannot be explained in terms of Bjerknes forces. Viscous boundary layer effects and large bubble stability will be discussed. Opportunities for our manipulation technique to probe the high speed dynamics and rheology of particle-laden armored bubbles will also be presented [V. Poulichet et al., PNAS, 112, (2015)].