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• An impact of the hydrophilic wetting on solid phonon dynamics is revealed by IXS. • Twofold impact: a depth-dependent hardening of acoustic phonons and an internal stress relaxation. • Therefore, a dynamic liquid/solid coupling is revealed, echoing mesoscopic liquid shear elasticity. We examine the influence at room temperature of the deposit of a water layer on the phonon dynamics of a solid. It is shown that the water wetting at the surface of an Alumina monocrystal has deep effects on acoustic phonons, propagating over several hundred µm distance and taking place on a relatively long time scale. The effect of the wetting at the boundary is two-fold: a hardening of both transverse and longitudinal acoustic phonons is observed as well as a relaxation of internal stresses. These acoustic phonon energy changes were observed by inelastic X-ray scattering up to 40 meV energy loss, allowing us to probe the solid at different depths from the surface. [ABSTRACT FROM AUTHOR]

• An impact of the hydrophilic wetting on solid phonon dynamics is revealed by IXS. • Twofold impact: a depth-dependent hardening of acoustic phonons and an internal stress relaxation. • Therefore, a dynamic liquid/solid coupling is revealed, echoing mesoscopic liquid shear elasticity. We examine the influence at room temperature of the deposit of a water layer on the phonon dynamics of a solid. It is shown that the water wetting at the surface of an Alumina monocrystal has deep effects on acoustic phonons, propagating over several hundred µm distance and taking place on a relatively long time scale. The effect of the wetting at the boundary is two-fold: a hardening of both transverse and longitudinal acoustic phonons is observed as well as a relaxation of internal stresses. These acoustic phonon energy changes were observed by inelastic X-ray scattering up to 40 meV energy loss, allowing us to probe the solid at different depths from the surface. [ABSTRACT FROM AUTHOR]