Equilibrium distributions under advection-diffusion in laminar channel flow with partially absorbing boundaries

Advective-diffusive transport in Poiseuille flow through a channel with partially absorbing walls is a classical problem with applications to a broad range of natural and engineered scenarios, ranging from solute and heat transport in porous and fractured media to absorption in biological systems and chromatography. We study this problem from the perspective of transverse distributions of surviving mass and velocity, which are a central ingredient of recent stochastic models of transport based on the sampling of local flow velocities along trajectories. We show that these distributions tend to asymptotic equilibria for large times and travel distances, and derive rigorous explicit expressions for arbitrary reaction rate. We find that the equality of flux-weighted and breakthrough distributions that holds for conservative transport breaks in the presence of reaction, and that the average velocity of the scalar plume is no longer fully characterized by the transverse distribution of flow velocities sampled at a given time.