Surface-induced near-field scaling in the Knudsen layer of a rarefied gas

TL;DR

This study investigates the near-field behavior of a rarefied gas in the Knudsen layer using a nano-electro-mechanical string probe, revealing temperature-dependent surface effects and deviations from equilibrium distribution.

Contribution

It demonstrates that surface-induced near-field scaling in the Knudsen layer depends on temperature and surface interactions, providing new insights into rarefaction phenomena at cryogenic temperatures.

Findings

Reduction in damping force is independent of geometry.

The effect is dependent on temperature.

Adsorbed atoms influence the rarefaction process.

Abstract

We report on experiments performed within the Knudsen boundary layer of a low-pressure gas. The non-invasive probe we use is a suspended nano-electro-mechanical string (NEMS), which interacts with $^4$He gas at cryogenic temperatures. When the pressure $P$ is decreased, a reduction of the damping force below molecular friction $\propto P$ had been first reported in Phys. Rev. Lett. Vol 113, 136101 (2014) and never reproduced since. We demonstrate that this effect is independent of geometry, but dependent on temperature. Within the framework of kinetic theory, this reduction is interpreted as a rarefaction phenomenon, carried through the boundary layer by a deviation from the usual Maxwell-Boltzmann equilibrium distribution induced by surface scattering. Adsorbed atoms are shown to play a key role in the process, which explains why room temperature data fail to reproduce it.