**Figure 2.20. Diffusion Infinite Extent Couple**.
This diagram shows an exact solution to the diffusion equation given by Crank (1975, equation 2.14) for diffusion across a boundary between two phases of infinite extent. Phase 1 on the left initially has a uniform high concentration (C_{1}) of the diffusing component. Phase 2 on the right initially has a low concentration (C_{2}) of the diffusing component. The red line shows normalized concentrations (C-C_{2})/(C_{1}-C_{2}). The same diffusion coefficient D (m^{2}/s) is used for both phases.

**Move the Time Slider** to show changes of the normalized concentrations with time. Use the radio buttons to change the scale if you wish. To see the effect of the diffusion coefficient, **use the slider to change log D**. Alternatively, you may specify D by entering the parameters for the Arrhenius equation D = Do Exp(-Ea/(R*T)). Then you may use the sliders to see how the parameters Ea (activation energy), Do (constant), and temperature (T) affect the results.

Notice that the parameter (4DT)^{½} gives a reasonable estimate of the distance on either side of the phase boundary over which there is significant diffusion in time t. Notice that the Time Slider is in logarthmic units.