Entropy Geometry and Augmented Mobility for Reactive Maxwell--Stefan Membrane Transport with Finite Occupancy

Abstract

We study a reactive Maxwell--Stefan-type membrane transport system under a finite-occupancy constraint with explicit vacancies. The admissible state space is $\mathcal{D} = \{u \in (0,1)^n : \varrho(u) := \sum_{i=1}^n u_i < 1\}$, where the vacancy fraction $1-\varrho(u)$ represents the local free volume. This bounded-occupancy geometry induces a Boltzmann--Fermi entropy and a global parametrization by entropy variables. The associated mobility is assumed to split into a composition channel, corresponding to redistribution at fixed total occupancy, and a mass channel, corresponding to variation of the filling fraction. The main structural difficulty is that the unaugmented mobility may lose coercivity in the mass channel. We show that a single rank-one augmentation of the form $γ_0 \mathbf{1} \otimes \mathbf{1}$ restores full coercivity while leaving the composition block unchanged. On this basis, we prove four results: a quantitative channel-wise coercivity estimate for the augmented mobility; global existence of entropy weak solutions via an implicit Rothe scheme in entropy variables; a weak--strong stability estimate in relative entropy with uniqueness in the strong class; and convergence of a fully implicit finite-volume approximation that preserves the bounded-occupancy structure and satisfies a discrete entropy inequality.