  The tractor behavior of a zero-order Bessel acoustic beam acting on a fluid
sphere, and emanating from a finite circular aperture (as opposed to waves of
infinite extent) is demonstrated theoretically. Conditions for an attractive
force acting in opposite direction of the radiating waves, determined by the
choice of the beam's half-cone angle, the size of the radiator, and its
distance from a fluid sphere, are established and discussed. Numerical
predictions for the radiation force function, which is the radiation force per
unit energy density and cross-sectional surface, are provided using a
partial-wave expansion method stemming from the acoustic scattering. The
results suggest a simple and reliable analysis for the design of Bessel beam
acoustical tweezers and tractor beam devices.

Classical Physics