Study On Acoustic Scattering Characteristics Of Objects In Bessel Beams And The Related Radiation Force And Torque

Nondiffracting Bessel beam could propagate with orbital angular momentum and is characterized by the properties of non-diffraction and self-reconstruction.Both in academic field and engineering applications,Bessel beams are of great importance due to the peculiar wave shapes and physical properties,which have broad applications in the fields of acoustic manipulations,such as particle trapping,cell selection,lab-on-a-chip technique and surface chemistry,and is attracting more and more attentions all over the world.Up to date,studies of acoustic scattering and manipulations from Bessel beams have mainly focused on theoretical and experimental models of simple objects,whereas the related investigations on complicated models are not enough.Hence,it is necessary and urgent to establish and develop complex models of acoustic scattering and manipulatiosn from Bessel beams.The present work developed the scattering model of spherical and non-spherical objects illuminate by Bessel beams with arbitrary order and location to investigate the scattering characteristics and related physical mechanisms.The theoretical solutions of the incident beam shape coefficients are derived for the on-axis incidence based on the spherical expansion of cylindrical wave function and for the off-axis incidence on the multipole expansion method.Based on the above derivation,a semi-analytical and semi-numerical method,the T-matrix method,is introduce to study the scattering from an arbitrary shape in Bessel beams.It is found that the elastic resonance contribution will be suppressed with a selected half cone angle of Bessel beams when the object is placed on the axis of the beam,whereas the resonance suppression fails for the off-axis incidence.For a rigid non-spherical shape in Bessel beams,a geometrical ray model is proposed to predict the intervals of backscattering form function versus dimensionless frequency and reveal the physical mechanism,which help us to better understand the beam structure and validate the effectiveness of our homemade programmings.The highlight model is introduced to qualitatively explain the scattering phenomena of rigid non-spherical shapes in Bessel beams.Moreover,the angle exchange symmetry of the scattering field from a rotationally symmetrical object placed on the axis of Bessel beams is observed and interpreted with the theoretical derivations.The interactions of the Bessel beams and objects will induce the transfer of the linear and angular momenta,which leads to the radiation forces and torques on the objects from the acoustic fields and is of great importance in the field of particle dynamics.According to conservations of linear and angular momenta in the ideal medium,the integral of scattering over the near-field surface of objects could be equivalent to that of far-field scattering over an arbitrary spherical surface,which simplifies the theoretical derivations and numerical computations.Based on the radiation stress tensor approach,the three dimensional radiation forces and torques in Cartesian coordinates on an object exerted by arbitrary acoustic fields are derived theoretically,which are given with closed-form expressions in terms of the incident and scattered beam shape coefficients.The unknown scattered coefficients could be calculated by the partial-wave series method and T-matrix method with the known incident coefficients of Bessel beams.Unlike in the ordinary plane waves,the radiation force and torque in Bessel beams may reverse under certain circumstances and the corresponding physical mechanisms are revealed,which is very potential in the field of particle separation.For example,considering the particles with same properties but different sizes,part of them may be induced negative radiation force to move close to the source,while part may be induced the positive force and move far away from the source,resulting in the separation of the particles with different sizes.Based on the proposed scattering models of Bessel beams,the present work has derived the theoretical formulas of acoustic scattering and radiation force(torque)under the illumination of the incident beam with arbitrary order and location.The numerical computations and theoretical derivations reveal the corresponding physical phenomena and mechanics.The present study suggests that acoustic Bessel beams have greatly potential applications in the field of particle manipulations,which could help design the general three-dimensional acoustic tweezers numerical toolbox.Meanwhile,this work is useful to related theoretical and experimental researches as guidance and reference.