✍ Scribed by J.L. García Fierro; A.M. Alvarez García
No coin nor oath required. For personal study only.
Determination of micromass changes of solid samples in a vacuum and controlled gaseous environments at different temperatures is a powerful method for studying surfaces. The weighing at low gas pressure is highly susceptible to temperature inhomogeneities which cause spurious mass changes, the phenomenon being called the thermomolecular flow effect. This effect was explained for some time on the basis of transverse and longitudinal Knudsen forces, both related to gradient pressure, which at low pressure accompany a thermal gradient. The problem is analyzed by the use of a BGK model and diffuse scattering at the surface of the inner and outer cylinders. Experimental results for helium, krypton, hydrogen oxygen and carbon dioxide are reported. In all cases the Knudsen forces are seen to exhibit a maximum (volcano plots) in the transition regime. For several values of the Knudsen number (Kn) the BGK model theory is found to be in good qualitative, as well as quantitative, agreement with the experimental data.
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