Congratulations to Colin Bousige from LMI, who, with his colleagues Benoît Coasne (LiPhy, Grenoble) and Pierre Levitz (PHENIX, Paris), has just published an article in the journal “Nature Communications”, entitled: “ Bridging scales in disordered porous media by mapping molecular dynamics onto intermittent Brownian motion”.

The diffusion of fluids in complex nanoporous geometries represents a challenge for modelling approaches. Here, the authors describe the macroscopic diffusivity of a simple fluid in disordered nanoporous materials by bridging microscopic and mesoscopic dynamics with parameters obtained from simple physical laws.

Abstract

Owing to their complex morphology and surface, disordered nanoporous media possess a rich diffusion landscape leading to specific transport phenomena. The unique diffusion mechanisms in such solids stem from restricted pore relocation and ill-defined surface boundaries. While diffusion fundamentals in simple geometries are well-established, fluids in complex materials challenge existing frameworks. Here, we invoke the intermittent surface/pore diffusion formalism to map molecular dynamics onto random walk in disordered media. Our hierarchical strategy allows bridging microscopic/mesoscopic dynamics with parameters obtained from simple laws. The residence and relocation times – $t_A$, $t_B$ – are shown to derive from pore size $d$ and temperature-rescaled surface interaction $\varepsilon/k_{\textrm{B}}T$. $t_A$ obeys a transition state theory with a barrier $\sim \varepsilon/k_{\textrm{B}}T$ and a prefactor $\sim 10^{-12}$ s corrected for pore diameter $d$. $t_B$ scales with $d$ which is rationalized through a cutoff in the relocation first passage distribution. This approach provides a formalism to predict any fluid diffusion in complex media using parameters available to simple experiments.