Amplifying secondary Bjerknes forces for precise acoustofluidic actuation*

Acoustofluidic forces, such as streaming and radiation forces, are often enhanced by resonant bubbles built into devices. However, since bubbles are deeply-subwavelength resonators, there is limited ability to use these effects for precise manipulations. While the secondary radiation forces between bubbles could lead to more precise actuation, such forces are extremely weak – typically nanonewtons – and thus have been largely ignored. Here, we demonstrate that patterning microbubbles into arrays can geometrically amplify the secondary Bjerknes forces, producing surprisingly high forces that overcome the effects of streaming and primary radiation forces. We experimentally demonstrate the ability of a Bjerknes actuator to precisely position a 1 cm object with 15 μm accuracy, using sound with a 50 cm wavelength. By modifying the bubble patterns, the Bjerknes actuator can also act as a rotational motor, driving unidirectional motion of a freely-floating structure. We describe the amplification and manipulation behavior using a theoretical model, and show that the amplified Bjerknes forces can outperform comparable magnetic actuators. By embedding arrays of resonant bubbles, new capabilities can be integrated into acoustofluidic devices, providing precise, switchable actuation.