Fig. 1.

Acoustically levitated particles with tunable interactions. (A) Schematic of the experimental setup. A single-axis transducer consisting of a piezoelectric driving element coupled to an aluminum horn generates ultrasonic standing waves in the gap (adjusted to half the resonant sound wavelength λ₀) between the bottom of the horn and a reflecting, optically transparent surface. Particles are levitated at the pressure nodal plane in the middle of the gap and imaged with a high-speed video camera from below. (B–G) Images of levitated particle rafts (Top) and zoomed-in views (Bottom). The particles have average diameters D = 180 μm (B and C) and D = 40 μm (D–G) and are levitated with frequency ω/(2π)=34.7 kHz (λ = 9.9 mm) (B, C, F, and G) and ω/(2π)=64.3 kHz (λ = 5.3 mm) (D and E). (All scale bars are 100 μm.)