Fundamentals of Diffusion in Gases

Free molecular diffusion is governed by First Pick s law of diffusion  

A molecule will diffuse differently in different media, hence D is a property of the pair of diffusing and media molecules. Substituting the definition of /m into Equation 1.1, we find that the velocity of diffusive flow, vq, is proportional to D  

The quantity in Equation 1.3 is the first-order binary collision integral of above pair—the first of an infinite number of collision integrals defined in the transport theory. The molecular thermal motion is manifested not only in random translation. 

FIGURE 1.1 Four random orientations of C36 monocyclic ring. 

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