Acoustically trapped colloidal crystals that are reconfigurable in real time

Photonic and phononic crystals are metamaterials with repeating unit cells that result in internal resonances leading to a range of wave guiding and filtering properties and are opening up new applications such as hyperlenses and superabsorbers. Here we show the first, to our knowledge, 3D colloidal phononic crystal that is reconfigurable in real time and demonstrate its ability to rapidly alter its frequency filtering characteristics. Our reconfigurable material 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 milliliters in volume with spheres arranged in an orthorhombic lattice in which the acoustic wavelength is used to control the lattice spacing. Transmission acoustic spectroscopy shows that the new colloidal crystal behaves as a phononic metamaterial and exhibits clear band-pass and band-stop frequencies which are adjusted in real time.Artificially engineered metamaterials have attracted significant research interest due to their useful wave guiding and transmission properties. The interest in these materials stems from the possibility of gaining previously unheralded control over wave phenomena, for example, controlling the path of waves leads to incredibly efficient lenses (1) or invisibility cloaking (2, 3) and controlling their transmission and reflection leads to highly efficient filters (4), diodes (5–7), or superabsorbers (8).Photonic and phononic crystals are periodic metamaterials typically created from multiple elementary repeating cells. They exhibit a well-known series of band-pass and band-stop frequencies that have attracted significant attention for use as filters and absorbers. Here we demonstrate a phononic crystal reconfigurable in real time, although the same principles could be used to fabricate other metamaterials including photonic crystals, as well as the more elaborate configurations required for applications such as cloaking.The frequencies of the band gaps in phononic crystals can be tuned by changing the lattice geometry (9), and the width of the gaps depends on the contrast between the densities and sound velocities of the component materials. Most of the work to date has not involved any reconfigurability, for example, 2D phononic crystals with a periodicity millimeter order and above have been assembled manually and used to explore their basic properties (10, 11). A reconfigurable phononic crystal has been created in 2D using optical tweezers but this is limited to a relatively small scale ( area) and to transparent or dielectric particles (12). Three-dimensional experimental realizations of phononic crystals have either been manufactured with millimeter periodicity or as colloids (13, 14). Colloids represent a particularly attractive option as they are known to self-assemble into simple 3D crystalline structures. Recent work on 3D hypersonic (GHz) colloidal crystals has led to experimental observation of the hypersonic Bragg gaps (15). However, whereas the choice of particle shape, size, volume fraction, etc. allows the colloidal crystal to be tuned, they do not facilitate real-time reconfigurability and so cannot be used in an active mode.In this article we report for the first time, to our knowledge, the experimental realization of a 3D colloidal metamaterial that is reconfigurable in real time and demonstrate its ability to rapidly alter its acoustic filtering characteristics. The reconfigurable metamaterial is assembled from a low-density aqueous suspension of microspheres. Our metadevice generates a high-amplitude megahertz-frequency (MHz) acoustic standing wave which results in acoustic radiation forces that lead to particles moving to, and becoming trapped in, a simple orthorhombic lattice (although here we explore only simple tetragonal forms). By varying the frequency of the acoustic standing wave in the metadevice we are able to control the lattice geometry. We use transmission acoustic spectroscopy to explore the reconfigurability of the resulting metamaterial. These measurements reveal a complex and tunable distribution of band-gap and band-stop phenomena over a wide range of frequencies.