Experimental Results for Diffusion

Chapman s approximate formula for the coefficient of diffusion may be rewritten as 

This equation predicts a diffusion constant which is independent of composition, and that seems to be the experimental finding. The maxi- 

The inverse variation of D with pressure Nt has also been verified, while the temperature dependence (like that shown by viscosity) seems to be greater than that predicted. 

An idea of the agreement of the formula with the facts may be obtained by a comparison of the mean diameters 712 calculated from this equation and those computed from measurements of the viscosity of mixtures. A list of such values is given in Table VIII.3, which is taken from Chapman and Cowling.  

Mean collision diam from viscosity data 

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Atomic jumps in random walk diffusion of closely bound atomic clusters on the W (110) surface cannot be seen. A diatomic cluster always lines up in either one of the two (111) surface channel directions. But even in such cases, theoretical models of the atomic jumps can be proposed and can be compared with experimental results. For diffusion of diatomic clusters on the W (110) surface, a two-jump mechanism has been proposed by Bassett151 and by Cowan.152 Experimental studies are reported by Bassett and by Tsong Casanova.153 Bassett measured the probability of cluster orientation changes as a function of the mean square displacement, and compared the data with those derived with a Monte Carlo simulation based on the two-jump mechanism. The two results agree well only for very small displacements. Tsong Casanova, on the other hand, measured two-dimensional displacement distributions. They also introduced a correlation factor for these two atomic jumps, which resulted in an excellent agreement between their experimental and simulated results. We now discuss briefly this latter study. 

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