Sustainable pulling motion of an active scatterer

In this paper, we concern the generation of attractive net motion with respect to the location of external wave source for active spherical carriers. Here we recall that the exerted acoustic radiation force in an acoustic field resulting from an incident wave, a scattered wave, and the radiated wave from the active carrier, can be positive, negative, or zero based on the location of the object in the field. Thus, in a general case, sustainable pulling motion is not guaranteed. In this work, by considering the point that the average of the radiation force for an object active in a single mode, over a complete wavelength, is equal to the radiation force applied to a passive object (which is always positive), we put forward a technique to generate acoustic radiation force with a negative average over a complete wavelength to ensure that the net motion is attractive. The idea here is to simultaneously excite of the object in two modes of breathing and first, with a difference in the phase of excitations. We will also show that by controlling the phase difference, the average of the force exerted on the object can be positive, negative or zero. Moreover, we show that for specific phase differences not only the average of the force is negative but also the force itself never experiences a positive value in a whole wavelength at those phase differences, which this can be a desired state to achieve a perfect negative net motion. The formulation of the paper is developed based on the velocity distribution over the surface of the sphere, but an implementation method with piezoelectric actuators is suggested. Here, by setting the zero-radiation force state (i.e., radiation force function cancellation) as a reasonable criterion for determination of the required amplitude for surface velocity or the operation voltage, we calculate the averaged force vs the frequency and the phase difference. The current work, with the hope of achieving complete maneuverability extends the concept of manipulation of smart carriers and reinforces the literature on single beam acoustic handling techniques. (c) 2023 Elsevier B.V. All rights reserved.