Defining diffusivity

Diffusivity defines the diffusion, or movement, of one material through another. The governing equations for mass diffusion are an extension of Fick's equations: they allow for nonuniform solubility of the diffusing substance in the base material and for mass diffusion driven by gradients of temperature and pressure. See the following sections for more information:

  1. From the menu bar in the Edit Material dialog box, select OtherMass DiffusionDiffusivity.

    (For information on displaying the Edit Material dialog box, see Creating or editing a material.)

  2. Click the arrow to the right of the Type field, and specify the directional dependence of the diffusivity.

  3. Select a Law option to specify how you want to define diffusivity behavior:

    • Select General to choose general mass diffusion behavior.

    • Select Fick to choose Fick's diffusion law.

    For more information, see Defining diffusivity.

  4. Toggle on Use temperature-dependent data to define diffusivity data as a function of temperature.

    A column labeled Temp appears in the Data table.

  5. Click the arrows to the right of the Number of field variables field to increase or decrease the number of field variables on which the diffusivity data depend.

  6. In the Data table, enter the applicable data:

    D

    Isotropic diffusivity. (Units of L2T−1.)

    D11, D22, and D33

    Orthotropic diffusivity terms. (Units of L2T−1.)

    D11, D12, D22, D13, D23, D33

    Anisotropic diffusivity terms. (Units of L2T−1.)

    Concentration

    Mass concentration of the diffusing material.

    Temp

    Temperature.

    Field n

    Predefined field variables.

    You may need to expand the dialog box to see all the columns in the Data table. For detailed information on how to enter data, see Entering tabular data.

  7. To describe temperature-driven diffusion, select Soret Effect from the Suboptions menu. (This option is valid only if you selected General in Step 3.) See Defining general temperature-driven mass diffusion” for detailed instructions.

  8. To describe pressure-driven mass diffusion, select Pressure Effect from the Suboptions menu. See Defining pressure-driven mass diffusion” for detailed instructions.

  9. Click OK to close the Edit Material dialog box. Alternatively, you can select another material behavior to define from the menus in the Edit Material dialog box (see Browsing and modifying material behaviors, for more information).