Dynamically altering the form of a three-dimensional colloidal crystal in real time is possible, research from the University of Bristol’s Department of Mechanical Engineering shows. This was done using an acoustic metadevice that is able to influence the acoustic space and control any of the ways in which sound waves travel.
The colloidal crystals in the study, called metamaterials, are artificially designed and structured materials which expand the properties of existing natural materials and compounds.
“Such materials will allow researchers to gain unprecedented control over a range of optical and acoustic wave phenomena. To date, whilst numerous examples of metamaterials now exist, none are reconfigurable in three-dimensions.”
Researchers used acoustic assembly techniques to trap a suspension of polystyrene microspheres, in a mixture of deuterium oxide and water, in patterns similar to crystal lattices. The study demonstrated a three-dimensional colloidal crystal that is reconfigurable in real time and that has the ability to quickly change its acoustic filtering attributes.
Acoustic Barriers and Reconfigurable Lenses
Real time reconfigurable metamaterials-based mechanisms with optical or acoustic wavelengths from ten microns to ten cm could have a wide range of applications.
In the optics field, they 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. Other applications in reconfigurable cloaks and lenses are also now plausible.
Phononic crystals are synthetic materials that formed by periodic variations of the material’s acoustic properties, i.e., elasticity and mass. One of the main properties of phononic crystals is the possibility of having a phononic bandgap. A phononic crystal with phononic bandgap prevents phonons of selected ranges of frequencies from being transmitted through the material.
The theoretical basis of phononic crystals actually dates back to Isaac Newton. He imagined that sound waves propagated through air the same way that an elastic wave would propagate along a lattice of point masses connected by springs with an elastic force constant E. This force constant is identical to the modulus of the material. In phononic crystals of materials with differing modulus, obviously the calculations are more complicated than this simple model.
Mihai Caleap and Bruce Drinkwater. Acoustically trapped colloidal crystals that are reconﬁgurable in real-time. PNAS, March 31, 2014
Image courtesy of Mihai Caleap, University of Bristol
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