Rainbow trapping of spoof acoustic surface waves in a defective closed surface

In this paper, we demonstrate that rainbow trapping of spoof acoustic surface waves (SASWs) can be realised by introducing structured defects on a textured closed surface in the subwavelength scale. To analyse the basic mechanism of the trapping of SASWs, we first introduce a defective zigzag groove in a closed surface and illustrate that the SASWs can be effectively trapped in the defect when the frequency exceeds the cut-off frequency of the periodic grooves on both sides. Subsequently, we experimentally demonstrate the spatial-spectral separation effect of SASWs by introducing three defective zigzag grooves with different curl numbers in the closed surface. Finally, we design a structure with gradient defective zigzag grooves to show the rainbow trapping of SASWs and spatial separation of different frequency components at different positions. Our results may have potential applications in miniaturised acoustic devices.