Radio-frequency actuated polymer-based phononic meta-materials for control of ultrasonic waves

Radio-frequency (RF) control of an ultrasonic phononic crystal was achieved by encapsulating it in a composite of high k-10% KF-doped BaTiO3 dielectric nanoparticles with poly(N-isopropylacrylamide) (PNIPAm)-based hydrogel. The combination of the nanoparticles and hydrogel produced a composite with elastic properties susceptible to RF actuation. The novel acoustic meta-material enables the regulation of sound waves by electromagnetic waves, which is not possible in a conventional medium as elasto-mechanical waves, and electromagnetic waves do not directly couple. Compared with light waves, radio waves can penetrate deeper into bulk structures and enable the control of propagation of ultrasonic waves through a macroscale phononic crystal. An RF antenna emitting at 318.6 and 422.5 kHz was used to modulate the device in water and ambient air, respectively. An increased transparency of the ultrasonic wave in the material was observed due to an increase in the bandwidth of the modulated device exceeding 8 kHz with a 30-fold increase in the signal modulation at select frequencies. The radio waves induced changes in the transmission and demonstrated the control of ultrasound with applied RF. The synthetic acoustic properties in the resultant meta-material device were actively manipulated through the interaction of electromagnetic waves with the material. Mechanical waves can be controlled by radio waves using a material designed by researchers in the USA and China. Phononic crystals are materials engineered to manipulate sound waves in a desired way. Now, Arup Neogi and co-workers from the University of North Texas and the University of Electronic Science and Technology of China have used a nanoparticle-doped hydrogel to make a phononic crystal sensitive to radio waves — an important development since mechanical waves and electromagnetic waves do not directly interact in nature within conventional materials. The team created an ‘active’ phononic crystal by encapsulating a square lattice of stainless-steel cylinders in a polymer. The polymer contained nanoparticles that are sensitive to radio-frequency electromagnetic waves. The transmission of ultrasonic waves through the material varied depending on the intensity and frequency of an incident radio-frequency signal. Radio-frequency (RF) was used to control ultrasound wave propagating through a phononic crystal based metamaterial device. The tunable metamaterial was realized by interstitially filling the spacing in the phononic crystal with high-k, 10% KF doped BaTiO3 nanoparticles dispersed in poly(N-isopropylacrylamide) (PNIPAM)-based hydrogels. The introduction of high-k nanoparticles enables the hydrogel to have an RF response, thus making a composite with highly variable elastic properties susceptible to RF light. The non-contact mode of applied RF results in a broadening and shift of the transmission spectra resulting in the realization of novel ultrasonic filters and modulators. The RF field also eliminates hybridization and resonance features in the spectra. The metamaterial exhibits tuning of ultrasound waves in both water and air medium.