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Acoustic waves propagating in fluids like gas and liquid can carry angular momentum, similar to the angular momentum of light. Acoustic vortex beams with rotating helical phase front possess orbital angular momentum (OAM), whose radiation torque was applied for the control of particle rotations. The orthogonality between acoustic OAM with different topological charges was exploited to increase information capacity and communication speed of a single pulse. Meanwhile, the circular energy flux obtained in several phononic crystals forms another type of acoustic OAM named pseudospin was implemented for the realization of one-way topological edge states. In acoustic helical waveguides, the geometric phase induced by the curvature of the propagation paths was also observed. The properties of the acoustic OAM are similar to their optical counterparts. Besides OAM, optical waves can also carry spin angular momentum (SAM) induced by the rotation of optical polarization characterized by local electric vector like circular polarized light. The optical spin is an intrinsic quantity that induces many intriguing wave physics including optical spin Hall effect and spin-orbit coupling of light. On the other hand, the existence of acoustic spin is an important question that is still under debating. Because of the longitudinal nature of acoustic waves propagating in fluids like airborne sound, the acoustic waves are usually characterized by scalar pressure fields and are considered to be spinless. While most of acoustic phenomena can be characterized by the scalar pressure field, the local particle velocity vector field is a crucial quantity for the understanding of the complete acoustic physics. In this work, we analytically derive a density quantity that characterize the acoustic spin if exist. The acoustic spin can exist in waves whose local particle velocity vector is rotating about itself. This acoustic spin is experimentally observed in the interference of two beams propagating perpendicular to each other. The spin induced torque is measured by a design acoustic dipole particle that interacts with the local particle velocity field. The acoustic spin also exist in evanescent waves propagating along a periodic groove waveguide, where acoustic spin-momentum locking is observed experimentally.
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