Engineering three dimensional ultrasonic colloidal systems with tuneable anisotropy Seminar

Abstract

Acoustic metamaterials capture the imagination with breath-taking promises of super-resolution in imaging and invisibility cloaking. Here we describe a new class of acoustic metamaterial that is reconfigurable in real-time and demonstrate its ability to rapidly alter its frequency filtering characteristics. Building on the experience physicists have gained over the last few decades in making “optical lattices” by shining criss-crossing laser beams through a dilute gas of ultra-cold atoms, we have developed a state-of-the-art technique [M. Caleap and B.W. Drinkwater, Proc. Natl. Acad. Sci. 111, 6226 (2014)] for creating three-dimensional colloidal systems that are reconfigurable in real-time. The method is based on acoustic assembly to trap a suspension of micro-particles in patterns resembling crystal lattices. We further show that quasi-crystalline patterns can also be created by using arrays of sources with 'forbidden' symmetries. The resulting systems represent a fundamentally new type of colloidal order and permit the first real-space imaging of quasi-crystals. Promising technological applications, ranging from tuneable filters to acousto-optical devices, are anticipated. Indeed, the structural diversity of natural crystalline materials has inspired intense efforts to build artificial crystalline materials to achieve similar properties in engineered devices. The development of photonic and phononic crystals and their use in electromagnetic and acoustic cloaking, as well as their ability to control optical, thermal, and acoustic waves and fields has demonstrated the vast range in functionality of artificial crystals. Both crystalline and quasi-crystalline structures are likely to be particularly well suited for metamaterial applications (i.e. for engineering the space and controlling the propagation of waves).