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We demonstrate that the acoustic spin of a first-order Bessel beam can be transferred to a subwavelength (prolate) spheroidal particle at the beam axis in a viscous fluid. The induced radiation torque is proportional to the acoustic spin, which scales with the beam energy density. The analysis of the particle rotational dynamics in a Stokes' flow regime reveals that its angular velocity varies linearly with the acoustic spin. Asymptotic expressions of the radiation torque and angular velocity are obtained for a quasispherical and infinitely thin particle. Excellent agreement is found between the theoretical results of radiation torque and finite element simulations. The induced particle spin is predicted and analyzed using the typical parameter values of the acoustical vortex tweezer and levitation devices. We discuss how the beam energy density and fluid viscosity can be assessed by measuring the induced spin of the particle.

The acoustic radiation force produced by ultrasonic waves is the “workhorse” of particle manipulation in acoustofluidics. Nonspherical particles are also subjected to a mean torque known as the acoustic radiation torque. Together they constitute the mean acoustic fields exerted on the particle. Analytical methods alone cannot calculate these fields on arbitrarily shaped particles in actual fluids and are no longer fit for purpose. Here, a semi-analytical approach is introduced for handling subwavelength axisymmetric particles immersed in an isotropic Newtonian fluid. The obtained mean acoustic fields depend on the scattering coefficients that reflect the monopole and dipole modes. These coefficients are determined by numerically solving the scattering problem. Our method is benchmarked by comparison with the exact result for a subwavelength rigid sphere in water. Besides, a more realistic case of a red blood cell immersed in blood plasma under a standing ultrasonic wave is investigated with our methodology.

Acoustofluidics technology can be used to trap live cells (and also micro/nanoparticles) in microenvironments suitable for cell assays. Herein, a cheap and easyto-fabricate device is proposed that works with Raman spectroscopy for biosensing applications. The device comprises a 3D-printed microchamber working as a halfwavelength acoustic resonator. By tuning the resonance frequency with a low voltage (%4 V), cells or particles are aggregated and levitated in seconds by the action of the acoustic radiation force. Based on finite element simulations, the radiation force field produced inside the device is described. In the cellular enrichment (aggregation) process, a metastable honeycomb lattice is formed mostly due to the cell-to-cell attraction caused by the secondary acoustic radiation force. Orderly and metastable levitating aggregates provide an excellent arrangement for Raman spectroscopy to investigate cells individually. Polystyrene particles are used for the device characterization and Raman acquisition process. Biosensing applications are showcased with live murine macrophages J774.A1, which are used in infection assay of leishmaniasis disease. The unique features of the device, e.g., simple fabrication process with cheap materials, simple operation, fast time response, and formation of metastable cellular aggregates; hold a noteworthy potential for applications in life sciences and biotechnology involving cell assays.

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