A Study On Acoustic Vortices Based On Sector Transducer Array

For the characteristics of strong directivity,long penetration depth and easy to be focused in tissues,ultrasound has been widely studied and applied in imaging,HIFUtherapy and particle manipulation.Especially in the field of object manipulation,acoustic tweezers provide a broad aspect in development.As a special field of acoustic tweezers,the acoustic vortex(AV)has helical wave fronts around itspropagation axis with a null pressure at the core,where the phase is a singularity.In addition,with the orbital angular momentum transfer of AVs,objects can be manipulated in a rotation manner by the rotation torque exerted by the acoustic radiation force.In current studies,the AVs the acoustic pressure of side-AVs generated by the side lobes of planar piston transducers is low in the near-field with a weak capbility of object trapping.While,for the main-AV generated by the main lobes,the location is far away from the source plane with a greatly attenuated pressure,which will reducethe trapping effect.Meanwhile,for the multi-layer circular distribution of the acoustic pressure of AVs,particles will be gathered at the pressure valley to form circular distributions,and cannot come over the circular distribution of pressure peak to thevortex center,based on the principle of the acoustic gradient force(AGF).In order to make full use of the radiation energy of the transducer to improve the capability of particle trapping of the near-field AVs in a large area,AVs generated bycircularly distributed sector transducer array is introduced in this paper.By means of topological charge adjustment and acoustic focusing,the characteristics of several kinds of AV and their applications in object manipulation are studied.(1)By applying the integration of point source radiation,numerical simulations for the acoustic fields generated by the sector transducer array are conducted,and compared with those produced by the circular transducer array.It is proved that strengthened AGFs of near-field multiple AVs with higher peak pressures can be produced by the sector transducer array,and many vortex nodes can be formed to realize the multi-pointparticle trapping along the center axis,which shows potential application prospect inbiomedical practice.(2)In the near field,particles are bounded in the valley region between the two circles of pressure peak,and cannot continue to move to the vortex center.A manipulation approach using AVs with continuously variable topological charges(CVTCs)is used to manipulate particles climb over the peak pressure and move to the vortex center,which is proved to enlarge the range of particle trappingand to improve the aggregation degree of objects in the vortex center.The pressureand gradient force distributions of AVs with different topological charges are analyzedby numerical simulations,and the topological charge regulation process is optimized.The experimental equipment of AV for trapping objects is set up and results show thatthe trapping radius is increased obviously using AVs with CVTCs.Compared with theAV with a single topological charge,the range of particle trapping is increased by 4.14times by the AVs with CVTCs.(3)The concept of power-exponent-phase is introduced into the AV for the first time based on the power-exponent-phase vortextheory,and the vortex fields with power order 2 are generated by the sector transducerarray.A cross-sectional helical potential well is observed,which enables the object to move from a distant position to a center of potential well in a certain spiral trajectory.The experimental and simulation results indicate that the power-exponent-phasevortex expands the trapping range,and has good application potential in directionalcontrol of objects.(4)In order to enhance the acoustic energy and the AGF of AVsgenerated by the plane piston transducer array,the acoustic lens is used to focus energy to realize the higher utilization of acoustic energy and the greater ability of object manipulation.The pressure and AGFs of the focused AV formed by acousticlens and sector transducer array are simulated,which prove that the focused AV with strengthened radial AGFs and ability of trapping objects can be produced in a smaller focal region,and the manipulation characteristics are regulated by several topological charges.The focused AVs is generated by the acoustic lens that is installed on the planar sector transducer array,and the feasibility is verified by the measuring results.The rotation,trapping and movement of the polyethylene particle is achieved successfully by experimental apparatus of the focused AV.The off-target effect can be effectively suppressed by the potential well in the focused AV,showing goodapplication prospect in moving objects.This study solves the problem of circular object aggregation produce by the multi-layer circular pressure distribution,improves the utilization of the acousticenergy excited by the planar sector transducer array,and expands the application scope of the AV.Theoretical analyses and numerical studies on near-field side-AVgenerated by a sector transducer array are conducted,and the corresponding experiments of objects manipulation are also carried out.A new method is proposed for the application of AVs in the manipulation,movement and transportation of objects.