4.8 Viscosity of Gases at Low Pressures
The equations for the coefficient of viscosity of gases developed so far are found to be independent of pressure. This conclusion is also verified by experiments over wide ranges of pressure. Every expression for η is found to involve the mean free path λ. As the pressure of the gas is reduced, the collision between molecules becomes less frequent and the mean free path increases in length. When the value of λ is comparable with the dimensions of the containing vessel, the mathematical method of derivation of the formula necessarily fails.
The experimental methods of determination of η are generally based on Poisseuille's equation of flow of gas through a tube. In deriving this equation, it is assumed that ...
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