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The discovery of the transverse spin of acoustic waves in structural acoustic fields and acoustic structural surface waves in recent years has expanded our knowledge of the basic characteristics of acoustic waves and opened up new avenues for their manipulation. On structured surfaces, however, the distribution of acoustic surface waves often exhibits a uniform distribution, which restricts the local modification of acoustic spin angular momentum and particle manipulation capabilities. In this study, we develop acoustic waveguides with gradients that are flat, up-convex, and down-concave in order to manipulate the lateral spin distribution of acoustic surface waves. We verify the direction-locking of the near-field acoustic spin-momentum, explore the pressure field distribution and the spin angular momentum density distribution of a spin acoustic source excited in the three gradient structures, and show how to manipulate the spin intensity distribution of acoustic surface waves in the gradient waveguide through theoretical analysis and numerical simulation. The results of numerical calculations show that the spin angular momentum density is positive on the upper surface of the structured waveguide and negative on the lower surface when the acoustic surface wave is excited by a clockwise rotating spin source and propagates along the leftward side of the waveguide. The spin angular momentum distribution and the direction of propagation of acoustic waves are entirely altered when the spin source is rotated counterclockwise. Specifically, an unequal distribution of acoustic spin angular momentum is produced by the upper convex-type and bottom concave-type waveguides when we convert the flat-type acoustic structure waveguide to a gradient-type waveguide. According to the computational results, the down-concave type waveguide exhibits a stronger density of acoustic spin angular momentum at the ends and gathers the acoustic surface waves at the ends of the constructed waveguide. On the other hand, the waveguide collects acoustic waves close to the structure's center when it is an up-convex structural waveguide. The findings could lead to new avenues for the manipulation of particles using acoustic waves by offering a means of controlling the acoustic spin angular momentum density and improving our grasp of symmetry in acoustic near-field physics.
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Keywords:
- acoustic metamaterials /
- acoustic spin angular momentum /
- unidirectional sound propagation /
- gradient acoustic waveguides
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