Non-contact actuation of elastic lattices using Bjerknes forces

Noncontact actuation of small elastic structures is a challenging problem given the poor scaling of electronic actuators. Sound offers a convenient and steerable power source for actuation, but the direct conversion for actuation via primary acoustic forces is typically inefficient. We recently demonstrated that the secondary scattering forces between bubbles can be dramatically amplified by geometric patterning and used for precise actuation in a fluidic environment. Here, we build on this idea and introduce a dynamic metamaterial composed of trapped gas bubbles in a fluid. We show carefully designed structures that achieve wirelessly switchable mechanical motion via secondary acoustic radiation forces. We introduce a theoretical model of the system to describe the equilibrium configurations of the structures, and support this model with experimental results. Building on our model, we introduce design principles to achieve different actuation behaviors. Our results highlight the potential of such acousto-elastic actuators for applications in soft robotics, programmable matter, and smart materials capable of reconfigurable actuation.