Water molecular bridge undermines thermal insulation of Nano-porous silica aerogels.

• Adsorptive water exists as clusters rather than liquid assumed widely. • Water adsorption can penetrate into the contact interface of silica particles. • A cross-scale model of heat transfer in silica aerogels is proposed. • Linearity dependence is illustrated between thermal conductivity and nanoscale adsorption. It is challenging to quantitatively predict the insulation degradation of porous materials caused by moisture absorption. A multiscale solution is proposed in this work. The adsorption intensity on connecting nanoparticles is evaluated at the nanoscale by defining an effective contact radius, which is detected and measured numerically. The thermal conductivity of connecting nanoparticles with adsorptive water is assessed by molecular statistics as the thermal conductivity of backbones. The up-scaling from nanoparticles to porous materials is built by mathematical modeling based on geometry structures. A simulation-based theoretical model is proposed for the thermal conductivity of silica aerogels with moisture. The results predicted by the present model agree well with experimental data. An in-depth analysis of experimental data using the proposed model demonstrates why a simple linear dependence is observed between the effective thermal conductivity and water fraction. In addition, the linearity can be attributed to the adsorbed water effect on the heat transfer through the silica-silica interface. The proposed model can deserve more credits considering that all the parameters are introduced explicitly and/or measured reliably by experimental measurement, mathematical derivations, or molecular dynamics simulations. It can demonstrate the molecular origin of the degeneration of thermal insulators under moisture. The present methodology may stimulate more cross-scale modeling and promote more understandings of heat transfer levels in nanoparticle-based materials. [ABSTRACT FROM AUTHOR]