Simultaneous trapping and pulling by acoustical Bessel beams as stable tractor beams

Abstract

A stable tractor beam for long range pulling of particles by acoustic Bessel beams requires a simultaneous trapping in the lateral direction. Trapping force acting on a sphere centered on the axis of Bessel beams is examined to guide the selection of material and beam parameters. It is found that a heavy and rigid sphere in the Rayleigh regime can be trapped at central pressure maximum of zero-order Bessel beam, which is contrary to the repelling behavior at the pressure anti-nodes of one- or two-dimensional standing waves. The trapping results from the three-dimensional features of the velocity fields and the momentum projection. The projection leads the trapping of an elastic sphere or a droplet at the axis of ordinary and vortex Bessel beams in the Rayleigh regime to be reversed when reducing the conical angle of the beam at the situation when there is a strong contrast of mass density between the particle and the surrounding fluid. Ranges of conical angle and material parameters for simultaneous trapping and pulling a spherical object are identified.A stable tractor beam for long range pulling of particles by acoustic Bessel beams requires a simultaneous trapping in the lateral direction. Trapping force acting on a sphere centered on the axis of Bessel beams is examined to guide the selection of material and beam parameters. It is found that a heavy and rigid sphere in the Rayleigh regime can be trapped at central pressure maximum of zero-order Bessel beam, which is contrary to the repelling behavior at the pressure anti-nodes of one- or two-dimensional standing waves. The trapping results from the three-dimensional features of the velocity fields and the momentum projection. The projection leads the trapping of an elastic sphere or a droplet at the axis of ordinary and vortex Bessel beams in the Rayleigh regime to be reversed when reducing the conical angle of the beam at the situation when there is a strong contrast of mass density between the particle and the surrounding fluid. Ranges of conical angle and material parameters for simultaneous trapp...