Rarefied gas flow in functionalized microchannels

TL;DR

This study investigates how surface functionalization affects rarefied gas flow in microchannels across a wide range of Knudsen numbers, finding minimal influence of functionalization at the tested scale.

Contribution

It provides experimental data and analysis showing that surface functionalization has negligible impact on gas flow in microchannels at certain scales, contrasting with smaller-scale studies.

Findings

No significant difference between plain and functionalized channels.

Surface-to-volume ratio influences the effect of functionalization.

Experimental results align with analytical and numerical models.

Abstract

The interaction of rarefied gases with functionalized surfaces is of great importance in technical applications such as gas separation membranes and catalysis. To investigate the influence of functionalization and rarefaction on gas flow rate in a defined geometry, pressure-driven gas flow experiments with helium and carbon dioxide through plain and alkyl-functionalized microchannels are performed. The experiments cover Knudsen numbers from 0.01 to 200 and therefore the slip flow regime up to free molecular flow. To minimize the experimental uncertainty which is prevalent in micro flow experiments, a methodology is developed to make optimal use of the measurement data. The results are compared to an analytical model predicting rarefied gas flow in straight channels and to numerical simulations of the S-model and BGK equations. The experimental data shows no significant difference between plain and functionalized channels. This stands in contrast to previous measurements in smaller geometries and demonstrates that the surface-to-volume ratio seems to be too small for the functionalization to have an influence and highlights the importance of geometric scale for surface effects. These results also shed light on the molecular reflection characteristics described by the TMAC.