Theory of acoustic scattering by supported ridges at a solid-liquid interface.

We combine a general Green's function formalism and an approach due to Nyborg [W. L. Nyborg, in Acoustic Streaming, Physical Acoustics, edited by W. P. Mason (Academic, London, 1965), Vol. II B, Chap. 11] to calculate the first-order pressure and second-order pressure gradient fields in the vicinity of solid inhomogeneities at a solid/liquid interface. We treat the problem of scattering of an incident acoustic plane wave by a single ridge and two parallel ridges separated by a trench on a planar substrate. The calculated vibrational density of states shows the existence of resonances at low frequencies, especially in the case of a trench. Excitation of a trench resonant vibrational mode enhances the magnitude of the first-order pressure and of the second-order pressure gradient. The resonant frequencies of a trench decrease and the pressure enhancement increases with increasing aspect ratio of the ridges (height to width).