Retardation by diffusion

For example, if the bulk concentration of oxygen is low (or simply lower than its steady-state value), the movement along the phase trajectory towards the increasing concentrations of surface oxygen is retarded by diffusion. As one goes towards the decreasing concentrations, the movement is... 

Thiele was one of the first to use the concept of an effectiveness factor [5]. To calculate this factor he introduced a dimensionless number, nowadays called the Thiele modulus. Thiele defined the effectiveness factor as the amount of a certain component that is actually converted, divided by the amount that could have been converted if retardation by diffusion had not occured. The dimensionless modulus he used was defined as... 

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