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A large family of plasmas has collisional mean-free-path much longer than the non-neutral sheath width, which scales with the plasma Debye length. The plasmas, particularly the electrons, assume strong temperature anisotropy in the sheath. The temperature in the sheath flow direction ($T_{e\parallel}$) is lower and drops towards the wall as a result of the decompressional cooling by the accelerating sheath flow. The electron temperature in the transverse direction of the flow field ($T_{e\perp}$) not only is higher but also spikes up in the sheath. This abnormal behavior of $T_{e\perp}$ spike is found to be the result of a negative gradient of the parallel heat flux of transverse degrees of freedom ($q_{es}$) in the sheath. The non-zero heat flux $q_{es}$ is induced by pitch-angle scattering of electrons via either their interaction with self-excited electromagnetic waves in a nearly collisionless plasma or Coulomb collision in a collisional plasma, or both in the intermediate regime of plasma collisionality.

The plasma exit flow speed at the sheath entrance is constrained by the Bohm criterion. The so-called Bohm speed regulates the plasma particle and power exhaust fluxes to the wall, and it is commonly deployed as a boundary condition to exclude the sheath region in quasi-neutral plasma modeling. Here the Bohm criterion analysis is performed in the intermediate plasma regime away from the previously known limiting cases of adiabatic laws and the asymptotic limit of infinitesimal Debye length in a finite-size system, using the transport equations of an anisotropic plasma. The resulting Bohm speed has explicit dependence on local plasma heat flux, temperature isotropization, and thermal force. Comparison with kinetic simulations demonstrates its accuracy over the plasma-sheath transition region in which quasineutrality is weakly perturbed and Bohm criterion applies.

Based on the analyses of the wave front and the wave vector of an acoustic-vortex (AV) spanner generated by a circular array of point source, the principle of object rotation is investigated through the calculation of the orbital angular momentum (OAM) and the radiation torque for AVs with various topological charges. It is demonstrated theoretically that the rotation of the axisymmetric disk centered on an AV spanner is mainly driven by the exerted radiation torque without the contribution of the OAM transfer. The radiation torque on a small-radius object is inversely associated with the topological charge in the center of the AV spanner, and it is enhanced significantly for a larger AV with a higher topological charge. The special case of the radiation torque proportional to the topological charge might be realized when the disk radius is much larger than the wavelength, in which case the acoustic power of the AV spanner can be absorbed as much as possible. With the established sixteen-source experimental setup, the radial pressure distributions of AVs with different topological charges measured at the frequency of 1.3 kHz in air agree well with the simulations. The topological-charge dependence of the radiation torque for AVs is also verified by the quantitative laser-displacement (angle) measurements for disks with different radii. The favorable results demonstrate that, for the object covering the vortex center of an AV spanner, the total OAM transfer might be 0 and the object rotation is contributed by the acting moments. Thus, the radiation torque of an AV spanner is more applicable than the OAM for describing the driving capability of object rotation, and it can be used as an effective tool in clinical applications to noninvasively manipulate objects with a feature size at the wavelength-scale (e.g. kidney stone in lithotripsy) inside body., Comment: 28 pages, 6 figures