Large-scale particle trapping by acoustic vortices with a continuously variable topological charge

Strengthened directivity with higher-order side lobes can be generated by the transducer with a larger radius at a higher frequency. The multi-annular pressure distributions are displayed in the cross-section of the acoustic vortices(AVs)which are formed by side lobes. In the near field, particles can be trapped in the valley region between the two annuli of the pressure peak, and cannot be moved to the vortex center. In this paper, a trapping method based on a sector transducer array is proposed, which is characterized by the continuously variable topological charge(CVTC). This acoustic field can not only enlarge the range of particle trapping but also improve the aggregation degree of the trapped particles. In the experiments, polyethylene particles with a diameter of 0.2 mm are trapped into the multi-annular valleys by the AV with a fixed topological charge. Nevertheless, by applying the CVTC, particles outside the radius of the AV can cross the pressure peak successfully and move to the vortex center. Theoretical studies are also verified by the experimental particles trapping using the AV with the continuous variation of three topological charges, and suggest the potential application of large-scale particle trapping in biomedical engineering.

Helical particle manipulation based on power-exponent-phase acoustic vortices generated by a sector transducer array

The characteristics of traditional acoustic vortices(AVs) were verified by the cross-sectional axisymmetric pressure distributions with perfect phase spirals around the center pressure null. In order to generate a non-axisymmetric pressure distribution, the concept of power-exponent-phase was first introduced into the formation of AV beams, named “powerexponent-phase acoustic vortices(PAVs)” in this paper. Based on a ring-array of sector transducers, the helical distribution of the low-pressure valley in cross-sections of PAVs, which enables particles to move from a distant position to the center low-potential well along a certain spiral passageway, was proved theoretically. The particle manipulation behavior for PAVs with a power order of 2 is numerically modeled and experimentally confirmed. The results show that PAVs with a nonaxisymmetric spiral pressure distribution can be used to realize the directional transport of particles in an enlarged scope,suggesting prospective application potential in biomedical engineering.

Beam alignments based on the spectrum decomposition of orbital angular momentums for acoustic-vortex communications

Given the enhanced channel capacity of wave chirality,acoustic communications based on the orbital angular momentum(OAM)of acoustic-vortex(AV)beams are of significant interest for underwater data transmissions.However,the stringent beam alignment is required for the coaxial arrangement of transceiver arrays to ensure the accuracy and reliability of OAM decoding.To avoid the required multiple measurements of the traditional orthogonality based algorithm,the beam alignment algorithm based on the OAM spectrum decomposition is proposed for AV communications by using simplified ring-arrays.Numerical studies of the single-OAM and OAM-multiplexed AV beams show that the error of the OAM spectrum increases with the translation distance and the deflection angle of the transceiver arrays.To achieve an ideal arrangement,two methods of the single-array translation alignment and the dual-array deflection alignment are developed based on the least standard deviation of the OAM spectrum(SD-OAM).By decreasing the SD-OAM towards zero using transceiver arrays of 16 transmitters and 16 receivers,accurate beam alignments are accomplished by multiple adjustments in three dimensions.The proposed method is also demonstrated by experimental measurements of the OAM dispersion and the SD-OAM for misaligned beams.The results demonstrate the feasibility of the rapid beam alignment based on the OAM spectrum decomposition by using simplified transceiver ring-arrays,and suggest more application potentials for acoustic communications.

Pulling force of acoustic-vortex beams on centered elastic spheres based on the annular transducer model

To solve the difficulty of generating an ideal Bessel beam,an simplified annular transducer model is proposed to study the axial acoustic radiation force(ARF)and the corresponding negative ARF(pulling force)exerted on centered elastic spheres for acoustic-vortex(AV)beams of arbitrary orders.Based on the theory of acoustic scattering,the axial distributions of the velocity potential and the ARF for AV beams of different orders generated by the annular transducers with different physical sizes are simulated.It is proved that the pulling force can be generated by AV beams of arbitrary orders with multiple axial regions.The pulling force is more likely to exert on the sphere with a smaller k0a(product of the wave number and the radius)for the AV beam with a bigger topological charge due to the strengthened off-axis acoustic scattering.The pulling force decreases with the increase of the axial distance for the sphere with a bigger k0a.More pulling force areas with wider axial regions can be formed by AV beams using a bigger-sized annular transducer.The theoretical results demonstrate the feasibility of generating the pulling force along the axes of AV beams using the experimentally applicable circular array of planar transducers,and suggest application potentials for multi-position stable object manipulations in biomedical engineering.