The settings in this node are dependent on the check boxes selected under Transport Mechanisms on the Settings window for the Transport of Diluted Species interface. It includes only the sections required by the activated transport mechanisms. It has all the equations defining transport of diluted species as well as inputs for the material properties.

The temperature model input is always available. Select the source of the Temperature. For User defined, enter a value or expression for the temperature in the input field. This input option is always available.

You can also select the temperature solved for by a Heat Transfer interface added to the model component. These physics interfaces are available for selection in the Temperature list.

If transport by convection is active, the velocity field of the solvent needs to be specified. Select the source of the Velocity field. For User defined, enter values or expressions for the velocity components in the input fields. This input option is always available.

You can also select the velocity field solved for by a Fluid Flow interface added to the model component. These physics interfaces are available for selection in the Velocity field list.

Use the Source list to select to pick up diffusion coefficients defined in a material or a Chemistry interface.

Select Material, and a Fluid material to use a diffusion coefficient in a material available in the model. User-defined property groups including a diffusion coefficient output property are available for selection in the Fluid diffusion coefficient list.

Select Chemistry to use a diffusion coefficient from a Chemistry interface added to model component. Diffusion coefficients are automatically defined when Calculate transport properties is selected on the interface level of a Chemistry interface. All defined diffusion coefficients are available for selection in the Fluid diffusion coefficient list.

For User defined, enter a value or expression for the Fluid diffusion coefficient Dc of each species in the corresponding input field. This can be a scalar value for isotropic diffusion or a tensor describing anisotropic diffusion. Select the appropriate tensor type — Isotropic, Diagonal, Symmetric, or Full that describes the diffusion transport, and then enter the values in the corresponding element (one value for each species).

Note that multiple species, as well as Migration in Electric fields (described below) is only available for certain COMSOL Multiphysics add-on products. See details: https://www.comsol.com/products/specifications/.

This section is available when the Migration in electric field check box is selected. From the Electric potential list, select the source of the electric field.

By default the Mobility is set to be calculated based on the species diffusivity and the temperature using the Nernst-Einstein relation. For User defined, and under Mobility, select the appropriate scalar or tensor type — Isotropic, Diagonal, Symmetric, or Full — and type in the value of expression of the mobility um,c.

Enter the Charge number zc (dimensionless, but requires a plus or minus sign) for each species.

The temperature (if you are using mobilities based on the Nernst–Einstein relation) is taken from Model Inputs section.

Note that the migration in electric fields feature is only available in some COMSOL products. See details: https://www.comsol.com/products/specifications/.