Cassie-Baxter and Wenzel States and the Effect of Interfaces on Transport Properties across Membranes.

Michael T Rauter,Sondre K Schnell,Signe Kjelstrup

doi:10.1021/acs.jpcb.1c07931

Michael T Rauter, Sondre K Schnell + Show 1 more

Open Access

https://doi.org/10.1021/acs.jpcb.1c07931

Copy DOI

Abstract

Mass transfer across a liquid-repelling gas permeable membrane is influenced by the state(s) of the liquid–vapor interface(s) on the surface of the membrane, the pore geometry, and the solid–fluid interactions inside the membrane. By tuning the different local contributions, it is possible to enhance the temperature difference-driven mass flux across the membrane for a constant driving force. Non-equilibrium molecular dynamics simulations were used to simulate a temperature difference-driven mass flux through a gas permeable membrane with the evaporating liquid on one side and the condensing liquid on the other. Both sides were simulated for Wenzel- and Cassie–Baxter-like states. The interaction between the fluid and the solid inside the gas permeable membrane varied between the wetting angles of θ = 125° and θ = 103°. For a constant driving force, the Cassie–Baxter state led to an increased mass flux of almost 40% in comparison to the Wenzel state (given a small pore resistance). This difference was caused by an insufficient supply of vapor particles at the pore entrance in the Wenzel state. The difference between the Wenzel and Cassie–Baxter states decreased with increasing resistance of the pore. The condensing liquid–vapor interface area contributed in the same manner to the overall transport resistance as the evaporating liquid–vapor interface area. A higher repulsion between the fluid and the solid inside the membrane decreased the overall resistance.

Highlights

Gas permeable liquid-repelling membranes have been studied for a long time and are of interest in many different applications such as outdoor-clothing,[1] biochemical transport systems,[2] wastewater treatment,[3,4] or medical devices.[5]
This work has reproduced the effects of the Wenzel and Cassie− Baxter states on permeate fluxes through a gas permeable membrane observed by others,[17,41] using non-equilibrium molecular dynamics simulations
It was demonstrated that the Cassie−Baxter state leads for the same driving force to a larger permeate flux than the Wenzel state, when the resistance of the pore is not dominating the transport

Summary

Introduction

Gas permeable liquid-repelling membranes have been studied for a long time and are of interest in many different applications such as outdoor-clothing,[1] biochemical transport systems,[2] wastewater treatment,[3,4] or medical devices.[5] In the presence of a temperature difference across the membrane, they can further be used for seawater desalination,[6] waste-heat to energy conversion,[7] or both.[8] When the membrane is in contact with the liquid on both sides and a temperature difference is applied, the fluid passes the membrane only in the vapor phase by evaporating on one side and condensing on the other. Much work has been done on the lab-scale, there is still a lack of developed membranes and modules for vapor transport through hydrophobic membranes in the presence of evaporating and condensing interfaces.[6,10,11] A key point for further development and design is the understanding of the physical phenomena involved. The purpose is always to increase mass transport and limit energy dissipation

Objectives

Results

Conclusion