Research On Focused Ultrasonic Vortex And Object Manipulation Based On Acoustic Lens

Due to the annular pressure distribution and helical wave front,the acoustic vortex(AV)can form a low-pressure potential well and carry the orbital angular momentum(OAM),which can realize non-contact object trapping in a rotation manner and has great potential applications in biomedical and industrial fields.However,the divergent AV beams formed by point source or planar-type transducer have less energy involved in object manipulation,resulting in low acoustic radiation force.In order to enhance energy utilization with improved flexibility and capability for object manipulation,based on a circular array of planar sector transducer,two kinds of acoustic lens are designed to construct a focused acoustic vortex(FAV)which realizes precise positioning and object manipulation of the AV tweezers in local area.In this paper,based on the phase-coded approach,an FAV with controllable focus position and adjustable field characteristics is devised by installing a spherical acoustic lens on a circular array of planar sector transducers.Based on the acoustic refraction of a concave spherical acoustic lens,numerical simulations show that an FAV with considerable pressure gain and strengthened acoustic gradient force(AGF)can be produced by the effective concentration of acoustic waves.The performance of rotational object trapping is shown by the axial and radial distributions of the AGF as well as the cross-sectional acoustic intensity for FAVs of different orders.Objects of nanometer,micrometer,millimeter,and even larger-than-wavelength size can be precisely and stably captured with the trapping radius and trapping force determined by the topological charge.Secondly,since the traditional spherical acoustic lens cannot completely concentrate acoustic beam on the focal point,an aspherical concave acoustic lens is devised by Fermat's principle,which achieves an ideal focused acoustic field.The designed aspherical concave acoustic lens improves the focus performance and further enhances the energy utilization of the transducer as well as the object manipulation performance of the FAV.With the deduced hyperboloid equation,determined by the refractive index ? and focal length F,of the aspherical concave acoustic lens,the equation of the ideal focused acoustic vortex(IFAV)is constructed.By numerical simulations,it is proved that compared with the focused acoustic field generated by a spherical acoustic lens,the axial length and radial radius of the focal region generated by the hyperboloidal acoustic lens are reduced by 37% and 13%,respectively,with the peak acoustic pressure increasing by 22%.Based on the phase-coded approach,an IFAV with more concentrated energy is produced by the combination of a circular array of planar sector transducers and a hyperboloidal acoustic lens,obtaining larger acoustic pressure and smaller vortex radius.Compared with the FAV generated by a spherical acoustic lens,the radial AGF peak of the IFAV increases by 59% when l = 1,which achieves improved performances of particle capture and rotation manipulation,enabling more significant applications in the area of particle manipulation.Finally,with 3D-printing technology,spherical acoustic lens and hyperboloidal acoustic lens are prepared.With a circular array of 16 planar sector transducers,driven by the established 16-channel phase-controlled experimental system,FAVs and IFAVs of different orders under two kinds of acoustic lens are constructed in water,respectively.The axial and radial distributions of the FAV are obtained by the scanning measurement of the focal area,and the feasibility of FAV generated by the combination of a spherical or hyperboloidal acoustic lens and a circular array of planar sector transducers is verified by their clear pressure circles and phase spirals consistent with simulation.At the same time,it is experimentally proved that the IFAV formed by a hyperboloidal acoustic lens has smaller focal region and more concentrated energy with improved flexibility and capability for object manipulation than that formed by a spherical acoustic lens.Finally,the FAVs with different topological charges are constructed by spherical acoustic lens,and the manipulation experiment of polyethylene particles of different sizes suspended on the water surface is carried out,which proves that FAV can accurately and stably capture the rotational objects smaller than capture radius at the focal point.In this thesis,with the combination of the active phase-controlled AV method and the passive acoustic-focusing technology,an FAV is devised by installing an acoustic lens on a circular array of planar sector transducers.This approach simplifies the traditional phased array with complex structure and circuit when constructing an FAV in the previous and ensures the flexibility of field regulation at the same time.Also,the FAVs and IFAVs could offer an efficient means of high-intensity focused ultrasound therapy to improve the therapeutic effect,which have potential application in the areas of high precision acoustic field regulation,ultrasonic therapy,ultrasonic particle manipulation and targeted drug delivery,thereby promoting more practical applications of acoustic tweezers technology in biomedical engineering.