Recently, acoustic metamaterials have been used to assist in remote manipulation of large objects (i.e., objects greater than at least twice the wavelength) using acoustic waves transmitted through the air. Without the assistance of metamaterials, acoustic waves can only manipulate objects smaller or comparable to the wavelength, for example, as can be trapped by acoustic standing waves or at the potential wells generated by either a transducer array or a hologram. By using an acoustic metamaterial, however, acoustic waves reflected by the object’s surface produce an exchange of momentum between the waves and the object, resulting in an acoustic radiation force on the object allowing larger objects to be both suspended and manipulated. However, metamaterials developed for in-air levitation and manipulation cannot be applied in an aqueous environment. They cannot generate strong enough radiation forces in water because of the small acoustic property mismatch with water.
UW Madison researchers have developed a new acoustic metamaterial for aqueous environments. Raised patterns on the reflecting surface provide an acoustic reflection tailored to desired motion including translation, centering, and rotation. Available buoyancy allows acoustic energy to be channeled to non-levitating purposes. They’ve integrated different manipulation forces within a single metamaterial patch through surface pattern superimposition and frequency multiplexing. They’ve also demonstrated more complex examples of multi-object, multi-path, non-invasive, and 3D underwater manipulation using multiple metamaterial patches and transducers.