Acoustic levitation dynamics of spherical objects manipulated by modulated confronting sound waves

The dynamic characteristics of acoustically levitated spheres were studied within a single-axis double emitters levitator. Through experimental validation and numerical simulation, this work investigated the impacts of synthesized field frequency and phase on levitated samples. When a subtle frequency difference existed in two confronting sound waves, the levitated sample vibrated vertically at a frequency identical to the frequency difference. As the frequency difference increased, the temporal evolution of sphere position progressively converged to a sinusoidal function, and its vibration amplitude declined simultaneously, which accorded with the calculated radiation force. In the case of identical frequencies, the confronting waves counteracted each other at zero phase difference but strengthened at π phase difference under odd multiples of half-wavelength spacing. The effects of superposition were opposite under even multiples of half-wavelength spacing. An 8 mm diameter titanium ball and a 7 mm diameter pure lead ball were levitated at either the middle pressure node or the lower pressure node at the fifth resonant mode with π phase difference. The maximum acoustic radiation force measured adjacent to two nodes achieved 22.9 and 24 mN, respectively, much greater than the 15.8 and 9.3 mN produced with single emitter.