Steerable radiation forces with frequency-detuned acoustic metasurfaces

This paper is a contribution to the Physical Review Applied collection titledPhononics and Metamaterials. We demonstrate that acoustic waves can induce controlled translation and rotation of macroscopic objects through small, but deliberate, detuning of the driving wave frequency. When an object is patterned with a suitably designed acoustic metasurface, small changes in the incident frequency <ani:inline-formula><ani:math display="inline"><ani:mi>ω</ani:mi><ani:mo>±</ani:mo><ani:mi>δ</ani:mi><ani:mi>ω</ani:mi></ani:math></ani:inline-formula> are converted into directional radiation forces and torques, enabling steerable motion even for objects much larger than the acoustic wavelength. We present the concept of a force-optimal metasurface topology and show that it enables fully reversible forces in real time: the object is moved in one direction for positively detuned incident frequency <ani:inline-formula><ani:math display="inline"><ani:mi>ω</ani:mi><ani:mo>+</ani:mo><ani:mi>δ</ani:mi><ani:mi>ω</ani:mi></ani:math></ani:inline-formula> and in the opposite direction for negatively detuned frequency <ani:inline-formula><ani:math display="inline"><ani:mi>ω</ani:mi><ani:mo>−</ani:mo><ani:mi>δ</ani:mi><ani:mi>ω</ani:mi></ani:math></ani:inline-formula>, where <ani:inline-formula><ani:math display="inline"><ani:mi>ω</ani:mi><ani:mo>=</ani:mo><ani:mn>22.5</ani:mn><ani:mrow><ani:mtext> </ani:mtext><ani:mi>kHz</ani:mi></ani:mrow></ani:math></ani:inline-formula> and <ani:inline-formula><ani:math display="inline"><ani:mi>δ</ani:mi><ani:mi>ω</ani:mi><ani:mo>=</ani:mo><ani:mn>2.5</ani:mn><ani:mrow><ani:mtext> </ani:mtext><ani:mi>kHz</ani:mi></ani:mrow></ani:math> </ani:inline-formula> for a proof of concept at inaudible frequencies. This mechanism is demonstrated experimentally at ultrasonic frequencies with 3D-printed metasurfaces. The proposed concept is scalable across frequencies and materials, offering a building block for realizing complex, remote-controlled, dynamical behaviors that can be programed by reconfiguring material surface patterns.