A Microfluidic Acoustic Metamaterial using Electrowetting: Enabling Active Broadband Tunability

While acoustic metamaterials provide extraordinary control to manipulate sound waves, their physical realization and applicability are severely impeded by the limited tunability, narrow operational frequency range, and non-compact designs. Integrating liquids with active actuation mechanism in the metamaterials provides broader material and design scope. Active microfluidic techniques for liquid actuation, never used in metamaterials before, will enable active tunability for liquid-embedded metamaterial designs, leading toward a novel class of "microfluidic acoustic metamaterials (MAM)." This work demonstrates deep-subwavelength ultra-compact tunable MAM, consisting of a slit aperture which is tuned by electrically moving a liquid droplet over it using electrowetting-on-dielectric. The proposed design makes MAM inherently multi-stable, and the ability to tune the acoustic field by moving the emission source point provides widescale efficient precision and multiple degrees of freedom. MAM realizes active acoustic switching and amplitude modulation of more than 20 dB and phase modulation from 0 to 2 pi with greater than 80% transmission efficiency analyzed analytically, experimentally, and numerically. MAM also delivers broadband operations ranging from 35 to 45 kHz. This design strategy opens state-of-the-art pathways for automating, tuning, and miniaturizing metamaterials by using microelectromechanical (MEMS) and microfluidic concepts.