Generation of negative radiation forces for pulling spheres

Negative radiation forces have been reported on objects smaller than the wavelength λ; however, for larger objects, the negative forces have been feasible under limited scenarios. Here, we use a multi-element array to generate various acoustic beams that employ non-gradient forces to impart negative forces on solid targets larger than λ. The array operates at 1.5 MHz and creates a null region that surrounds target spheres with zero on-axis pressure. The generated wavefield produces forward scattering while minimizing backscattering, thus enabling the generation of negative forces. Simulations and measurements show a distinct transition between gradient and non-gradient forces. The negative forces generated are distal to the transducer. Calculations and measurements of the field show agreement for various acoustic traps for spheres up to 2λ. Although the acoustic exposure levels exceed current regulatory limits for in vivo applications, this technology has the potential utility in the development of acoustic tweezers for non-contact transport applications. [This work was supported by NIH K25-DK132416.]