Observation of Femtosecond Acoustic Anomaly in a Solid Liquid Interface

Here we report a femtosecond anomaly observed at a solid-liquid interface: femtosecond acoustic echo being advanced by the application of a viscous liquid to the solid surface. Through extensive simulation and experiments, we attribute this new discovery to the effect introduced by the surface-adsorbed water layer, switching between properties more viscous than water and more solid-like than ice. Our study further indicates a new technology paradigm: the femtosecond acoustic system to probe the viscoelastic properties of the surface-adsorbed water layer, which is not achievable by any available current technology.