Thermoelastic Model for Impulsive Stimulated Scattering Monitoring the Evolution from Capillary to Rayleigh Type Wave Propagation on the Surface of Viscoelastic Materials Throughout the Glass Transition

In recent decades, the impulsive stimulated scattering (ISS) method, which is based on photothermal and photoacoustic phenomena, has been successfully used to simultaneously investigate the thermal and elastic properties in a four-wave mixing configuration, both in transmission in semitransparent materials and on reflecting surfaces of solids. In this report, an extension of the technique is proposed to study a laser-induced thermoelastic response at the free surface of glass-forming liquids. The employed all-optical configuration allows extraction of information about the acoustic shear modulus in the MHz frequency range, and hence is complementary to the classical ISS configuration in the transmission mode, which is suitable to study the relaxation of the longitudinal acoustic modulus, and to another earlier reported ISS configuration, which is exciting and probing laser-induced thermoelastic phenomena at a liquid–solid interface. A theoretical model is presented and numerically illustrated for the glass transition of glycerol, and experimentally validated for water at room temperature.