Effect of particle size on acoustic vortex manipulation of Mie particles

An acoustic vortex with orbit angular momentum can manipulate a particle in a single beam and multiple freedom manner, gaining increasing interest in recent years. We studied the performance of acoustic vortex trapping of wavelength-scale Mie particles with a negative acoustic contrast factor. It was found that there was a particle-size dependence of the trapping position in an acoustic vortex field: (1) particles with a radius less than 0.69 λ were trapped in the vortex ring; (2) particles with a radius larger than 0.76 λ were trapped in the vortex center. The physical mechanism originated from the competition between the gradient force induced by an incident acoustic field and the scattering force from scattered waves by the particles, and those high-order scattering terms over the fourth order will play significant roles with the increase of the particle size. The experimental results of two polydimethylsiloxane particles with radii of 500 μm and 1.2 mm validated the prediction under a synthesized vortex beam via a planar ultrasonic array with 256 elements and a working frequency of 1.04 MHz. This investigation will increase the physical insight of the interaction between acoustic vortex beam and matter and pave the way to developing acoustical tweezers to manipulate large objects for drug delivery, tissue engineering, and regenerative medicine.