Manipulation of an acoustically levitated object using externally excited standing waves

Ultrasonic standing waves can be used to manipulate the position and control the movement of levitated objects through acoustic radiation forces. Within this context, the theory of the Gor'kov potential function and its acoustic contrast factor are revisited, considering the scenario of a harmonic disturbance to the standing wave and its influence on the levitated spherical object. This disturbance causes a levitated object-trapped within a standing, plane ultrasonic wave field in an ideal fluid-to undergo oscillations in sympathy with the resulting motion of the wave field. In this paper, we determine how the acoustic contrast factor depends on the properties of the object, the fluid and the external excitation, in combination. We show that positive, negative, and zero acoustic radiation forces can be achieved, causing the object to be pushed towards the nearest pressure or velocity node. We experimentally verify-through external excitation of an ultrasonic standing wave generator-that the disturbance vibration frequency and amplitude are transmitted to the object. The dependence on the external excitation amplitude and force reversal are novel features that can be employed in acoustic manipulation for non-contact dynamic characterization of small objects.