Size-controlled non-Fickian diffusion in a combined micro- and mesoporous material

Abstract

In the porous media, the diffusion is affected by the pore size. Particularly, the diffusion coefficient is treated as a function of the pore size. In this letter, we have studied the mass transfer of the diffusing substances with different molecule size through the media characterized by the presence of the micropores and the mesopores. The diffusant with the molecule size larger than the micropore size demonstrates the standard Fickian behavior, whereas the diffusant with the molecule size smaller than the micropore size gives the non-Fickian diffusion kinetics. It reveals that the pore size has a specific impact on the mass transfer deviation from the classical Fickian law. It also affects the expression of the conservation law for the diffusion flux. The obtained findings demonstrate the size-controllability of the diffusion regime, contrary to the existing diffusion models based on the dependence of the diffusion coefficient on the pore size only.