Stability and size restrictions of trapped objects in acoustic levitation

High intensity ultrasonic waves confined in an acoustic resonator can be used to trap objects a fraction of the wavelength. Objects are completely free from contact with a rigid surface only maintaining contact with the surrounding fluid medium, in this case air. However, trapped objects suffer from spontaneous oscillations in both the longitudinal and transverse directions of the resonator. The tendency of objects to oscillate becomes more pronounced as the object size decreases, thus setting a lower limit on object size. Stability presents an extreme problem for beam-based experiments, i.e. laser or x-ray probe, since the motion of the trapped object couples to measurement noise. We will discuss a sequence of oscillation or ‘jitter’ measurements to characterize the relative noise as a function of object size. A critical parameter to exploit here is the shape of the resonator mode that determines the peak sound intensity as well as the acoustic force gradient on trapped objects. We will discuss our efforts to accurately model and control the acoustic profile in an ultrasonic resonator. Modeling and simulation work has been carried out in COMSOL Multiphysics platform and focusing schemes have been used to optimize the resonator mode for trapping stability.