The Formation And Manipulation Of Off-axis Multi-vortex Sound Field

As a kind of special acoustic field,the helical wavefront of an acoustic vortex(AV)beam is demonstrated to have a pressure zero with phase singularity at the center in the transverse plane.The orbital angular momentum of AVs can be applied to the field of particle manipulation,which attracts more and more attention in acoustic researches.In this paper,by using the simplified circular array of point sources,dual coaxial AV beams are excited by the even-and odd-numbered sources with the topological charges of lE and lO based on the phase-coded approach,and the composite acoustic field with multiple off-axis AVs can be generated by the superimposition of the AV beams.The proposed approach breaks through the limit of only one on-axis AV with a single topological charge along the beam axis,and also provides the feasibility of off-axis particle trapping with multiple AVs in object manipulation,helps to increase particle capture and accuracy.In the theoretical and numerical simulations,the number of vortexes,topological charges and two-dimensional spatial distributions in a multi-vortex acoustic field with circular even distributions are analyzed.The topological charge,the influence of the propagation distance,the acoustic pressure ratio and the initial phase difference of the sources are helpful to further understanding the characteristics of the generation of off-axis multiple AVs.In addition,the special cases of the strengthened AV beam,the phase edge dislocation and multiple off-axis AVs with a centered quasi-plane wave formed by two AV beams with the topological charges of the same and opposite as well as one zero are discussed.The composite acoustic field provides a richer structures of AV distribution,which plays an important role for further study on the properties of the superimposition of acoustic beams.The distributions of acoustic pressure and phase of off-axis multiple AVs with a centered quasi-plane wave with the topological charge of lE=1-3 and lO=0 are experimentally measured using a 16-source AV system,and generation of the centered quasi-plane wave are proved by the equiphase distribution in the center region.The research on off-axis multiple AVs has broken the limitation of the traditional AV beam with only a vortex trapping along the central axis and the lack of particle agglomeration.The circularly distributed multiple AVs can be applied to off-axis multi-point particle capture.The central AV or the centered quasi-plane wave can be used to achieve the aggregation of particles or a uniform distribution of particles in a limited area,realizing the topological energy transfer from the center to the surrounding area.It has the dual effects of off-axis multi-point particle capture and particle dispersion in the central region,which can be used to achieve particle accumulation and uniform distribution with reduced agglomeration of particles in the center area.In conclusion,the proposed acoustic field of off-axis multiple AVs in this study can be applied to potential applications in the fields of sonochemistry,particle dispersion,multi-targeted drug delivery and three-dimensional object manipulation.