Acoustic response of molecular adsorption and sound propagation in nanoporous materials

Molecular simulation and statistical mechanics are used to unravel the microscopic mechanisms through which fluid adsorption impacts sound propagation and attenuation in nanoporous materials. By considering different fluids, temperatures, and fluid-solid interaction strengths, the authors first derive a simple model that predicts the decay in the sound velocity upon increasing the fluid mass density. They also show that sound attenuation increases with the amount of fluid adsorbed and with the solid-fluid interaction strength due to phonon scattering at the fluid-solid interface. The authors establish that all data can be quantitatively rationalized by considering the change in the phonon lifetime through an additional relaxation time arising from the interaction between fluid molecules and the atoms of the nanoporous solid.