Observation of Scholte-like waves on the liquid-loaded surfaces of periodic structures

The observation of Scholte-like ultrasonic waves travelling along the water-loaded surfaces of solids with periodically varying properties is reported. Results are presented for two 2D superlattices that intersect the surface normally: a laminated solid of alternating 0.5 mm thick layers of aluminium and a polymer, and a hexagonal array of polymer rods of lattice spacing 1 mm in an aluminium matrix. The surface waves are generated and detected by line-focus acoustic lenses aligned parallel to each other, and separated by varying distances. For homogeneous solids, phase matching constraints do not allow the Scholte wave to be coupled into with an experimental configuration of this type, and this is demonstrated with results on a uni-directional carbon-fibre/epoxy composite. These constraints are relaxed for a periodic solid, where coupling takes place through Umklapp processes. In our experiments, the source pulse is fairly broadband, extending up to about 6 MHz, whereas the spectrum of the observed Scholte arrival is peaked at around 4 MHz. We attribute this to a resonance in the surface response of the solid associated with the superlattice structure. On rotating the solid about its surface normal, the Scholte wave displays a characteristic variation in phase arrival time and, to a lesser extent, also group arrival time. This variation is well accounted for with a model that incorporates Umklapp processes in the solid's surface response.