Figure 1.

Ray acoustics model of acoustic trapping. Two parallel rays (P1 and P2) are incident upon a sphere in a sound field of Gaussian intensity distribution, and their corresponding exiting rays are denoted as P1′ and P2′. The momentum transfers from the rays to the sphere are defined by dP1 and dP2, and redirect the sound propagation at the sphere. Resultant forces acted on the rays are determined by the rate of the momentum change. According to Newton’s third law, the sphere experiences an equal and opposite force that leads to an acoustic radiation force (a dotted arrow). Consequently the sphere is attracted to the beam axis. A thick arrow at the sphere’s center indicates the trapping direction.