Reconfigurable Acoustic Metamaterials: From Random To Periodic In A Split Second
Authors: Mihai Caleap and Bruce Drinkwater
Faculty of Engineering, University of Bristol, United Kingdom.
Using an acoustic metadevice that can influence the acoustic space and can control any of the ways in which waves travel, we have demonstrated, for the first time, that it is possible to dynamically alter the geometry of a three-dimensional colloidal crystal in real time .
The colloidal crystals designed in our study, called metamaterials, are artificially structured materials that extend the properties of existing naturally occurring materials and compounds.
The reconfigurable metamaterial is assembled from microspheres in aqueous solution, trapped with acoustic radiation forces. The acoustic radiation force is governed by an energy landscape, determined by an applied high amplitude acoustic standing wave field, in which particles move swiftly to energy minima. This creates a colloidal crystal of several millilitres in volume with spheres arranged in an orthorhombic lattice in which the acoustic wavelength is used to control the lattice spacing.
Dynamically reconfigurable metamaterials based devices with optical or acoustic wavelengths from tens of microns to tens of centimetres could have a wide range of applications. In optics it could lead to new beam deflectors or filters for terahertz imaging and in acoustics it might be possible to create acoustic barriers that can be optimised depending on the changing nature of the incident sound. Further applications in reconfigurable cloaks and lenses are also now conceivable. The reconfigurable acoustic assembly method developed in this study is an important step as it has clear advantages over other possible approaches, for example optical trapping and self-assembly. In particular, acoustic assembly is scalable with wavelength from microns to metres. The method will work on a vast range of materials, such as nearly all solid-fluid combinations, it will also enable almost any geometry to be assembled and it is cheap and easy to integrate with other systems.
Figure 1: Acoustic metadevice capable of manipulating the acoustic space and controlling the propagation of waves (Image credit: Mihai Caleap, University of Bristol, copyright © 2014)
This first realization of an acoustic metamaterial that is reconfigurable in real-time represents a new and versatile means by which wave propagation phenomena can be controlled. The approach will allow the design and synthesis of a wide range of new colloidal crystals based on micrometre-sized particles arranged in crystal lattices. Using our acoustic assembly technique, low-density acoustic colloids could be conceived to form quasi-crystalline structures , which would otherwise be difficult by the self-assembly technique. The range of structures that can be constructed is limited only by the ability to produce the appropriate acoustic potential energy landscape. The technique can also provide a platform for performing bio-assays and cell-assays in a non-contact way using acoustic or magnetic standing waves.
 Mihai Caleap and Bruce W. Drinkwater, "Acoustically trapped colloidal crystals that are reconfigurable in real time". Proceedings of the National Academy of Sciences of USA, 111 (17), 6226–6230 (2014). Full Article.
 Jules Mikhael, Johannes Roth, Laurent Helden, Clemens Bechinger Mikhael, "Archimedean-like tiling on decagonal quasicrystalline surfaces". Nature, 454, 501–504 (2008). Abstract.