# Transient flows in active porous media

## TL;DR

This paper models the coupled advective and diffusive transport in a channel with stimulus-responsive permeability, revealing damped oscillations in steady state and discussing experimental observability.

## Contribution

It provides an exact analytical solution for coupled transport in a stimulus-responsive porous medium, highlighting oscillatory behavior in steady state.

## Key findings

- Existence of damped oscillations in flow reaching equilibrium.
- Exact solution to the coupled transport problem.
- Feasibility of experimental observation discussed.

## Abstract

Stimuli-responsive materials that modify their shape in response to changes in environmental conditions -- such as solute concentration, temperature, pH, and stress -- are widespread in nature and technology. Applications include micro- and nanoporous materials used in filtration and flow control. The physiochemical mechanisms that induce internal volume modifications have been widely studies. The coupling between induced volume changes and solute transport through porous materials, however, is not well understood. Here, we consider advective and diffusive transport through a small channel linking two large reservoirs. A section of stimulus-responsive material regulates the channel permeability, which is a function of the local solute concentration. We derive an exact solution to the coupled transport problem and demonstrate the existence of a flow regime in which the steady state is reached via a damped oscillation around the equilibrium concentration value. Finally, the feasibility of an experimental observation of the phenomena is discussed. Please note that this version of the paper has not been formally peer reviewed, revised or accepted by a journal.