Thermal Conductivity at Solid/Fluid interfaces: From Adsorption to Phonon Scattering through the Rattle Effect

Abstract

Of relevance to adsorption, separation and catalysis, fluids in nanoporous materials display intriguing phenomena arising from their underlying structural, dynamical, and thermodynamic behavior. While many effects at the solid/fluid interface in nanoporous materials are well-documented, the thermal behavior of such solids subjected to fluid adsorption remains to be deciphered. Among exotic mechanisms, the so-called \textit{rattle} effect, which corresponds to the decrease in the solid thermal conductivity induced by phonon scattering at the solid/fluid interface, has received only little attention. This phenomenon, which challenges existing mixing rules and effective medium approaches, has been identified in nanoporous materials (\textit{e.g.} zeolite). Considering that this nanoscale effect is necessarily restricted to a small region near the solid/fluid interface, its impact for less finely divided materials (\textit{i.e.} with thicker solid/fluid domains) remains to be established. Here, we address this question by employing a molecular simulation strategy to investigate thermal transport in a prototypical nanoporous silica material filled with a simple fluid. As a result, while the conventional behavior predicting an overall thermal conductivity increase upon fluid addition is qualitatively recovered in many cases, we also observe some situations where the overall thermal conductivity $\kappa$ in such hybrid systems is mostly unchanged as conductivity through fluid is counterbalanced by the rattle effect at the solid surface. Understanding such intrinsic relations and the \textit{rattle} effect that govern the thermal conductivity in solid/fluid systems paves the way for the design of novel materials to harness thermal processes in practical applications.