Three-dimensional spiral motion of microparticles by a binary-phase logarithmic-spiral zone plate

Acoustic vortex beams, which have both linear and angular momentum, can be used to make precise acoustic tweezers. Limited by the symmetry of a normal vortex beam, these tweezers are usually used for trapping or rotating particles in two dimensions. Here, the three-dimensional spiral motion of two soft particles of different sizes was realized using a vortex beam with a twisted focus, which was synthesized by a silicone binary-phase logarithmic-spiral zone plate. Numerical simulations and experimental measurements demonstrated that the beam had anisotropic focuses of crescent transverse intensity profiles and a screw phase dislocation with a singularity at the center. Experiments showed that a small particle (k₀r ≈ 1.3) can follow the twisted intensity of the beam, but a large particle (k₀r ≈ 4.7) spirals up away from the twisted field pattern. This is attributed to the dominant gradient force for the small particle, whereas the scattering effect induced a scattering force combined with a gradient force for the large particle. This focused twisted beam, which was generated with a structured silicone plate, and the three-dimensional spiral motion of microparticles, advance the development of simple, compact, and disposable acoustic devices for the precise and diverse manipulation of microparticles.