Molecular gas transport properties in anisotropic fiber structures

• Molecular gas transport properties in anisotropic fiber structures is investigated. • Orientation metrics predict anisotropy in mean free path of gas molecules. • Path decomposition links mean free path to chord length statistics. • Geometry-based framework enables predictive design of porous media. We numerically investigate molecular gas transport in anisotropic fiber structures, with particular focus on linking geometrical descriptors to the mean free path in the free-molecular flow regime, which is a key for determining diffusion coefficient. The fiber structures are modeled by infinitely long, overlapping cylinders, where anisotropy is introduced through a tunable orientation parameter. The mean free path is evaluated by mean-square displacement of gas molecules. The results show that increasing anisotropy enhances the mean free path in the in-plane directions while suppressing it in the through-plane direction, yielding an almost constant direction-averaged value. We demonstrate that the anisotropy of the mean free path can be predicted from orientation metrics, which act as purely geometrical descriptors. In addition, path decomposition analysis yields an analytical expression for the mean free path based on chord length statistics. While the original formulation overestimates the simulations due to path correlation approximations, a simple porosity-based correction provides quantitative agreement. Our findings establish a framework that enables prediction of isotropic and anisotropic gas transport in fibrous porous media based solely on structural features, offering a geometrical foundation for rational material design.