A Non-equilibrium Statistical Mechanics-Based Diffusion Theory: A Unified Model for Diffusion in Porous Media (II)

Starting from the linear response theory of non-equilibrium statistical mechanics, this study establishes a unified model that comprehensively describes bulk diffusion, Knudsen diffusion, and interface diffusion. The diffusion process within porous media is decoupled into a bulk–Knudsen diffusion process and an interface diffusion process, and the flux expressions corresponding to each process are derived separately. The influence of the geometric structure and physicochemical properties of porous media on the Onsager phenomenological coefficients is analyzed, and the variation of the ratio of the chemical potential gradient to the concentration gradient is revealed. On this basis, the ratio of interface diffusion to bulk–Knudsen diffusion is derived and proposed as a criterion for the transition of diffusion mechanisms. The influence of the geometric structure and physicochemical properties of porous media on this criterion is further discussed. Finally, the relationships between this model and the classical theories are analyzed. The proposed model captures the constitutive relationship between the geometric structure and physicochemical properties of porous media and their diffusion capacity, providing a new theoretical paradigm for the construction of diffusion models in porous media.