Free molecular flow
= Free molecular flow
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In <free molecular flow>, gas–gas <collisions> are negligible over the trajectories relevant to the observable. Molecules still encounter walls, whose scattering of <momentum> and <energy> must be modeled. In a long narrow tube, $\ell_{\mathrm{mfp}}$ exceeding the diameter does not by itself prove that every complete transmission path is collision-free: repeated wall reflections can create much longer paths. The approximation must be checked against those paths as well as the width. https://farside.ph.utexas.edu/teaching/355/Surveyhtml/node223.html[Fitzpatrick, Molecular Flow, wall encounters in a long tube]{ref}