Transport Properties

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.

When the Convection check box is selected, the Initial Values subnode is available from the context menu as well as from the Physics toolbar, Attributes menu. Note that this feature is only available in some COMSOL products. See details: http://www.comsol.com/products/specifications/.

Model Inputs

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

You can also select the velocity field solved by a Fluid Flow interface that been added to the model component. These physics interfaces are available for selection from the Velocity field list if they are active in the domains. A list of the variable names related to the Fluid Flow interface will be visible in a table.

When the Migration in electric field check box is selected on the Settings window for Transport of Diluted Species, select the source of the electric potential field and, optionally, temperature.

•	Enter values or expressions for the Electric potential V, which is User defined; this input option is always available.

•	Select the electromagnetic field solved by an AC/DC-based interface that has also been added to the model. This works like the velocity field model input described above.

•	The default mobility model is the Nernst-Einstein relation. This also requires a temperature input, which will be exposed in the same manner as the electric potential described above.

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

Diffusion

Select an option from the Material list. This selection list can only be used if a material has been added in the Materials node and if that material has a diffusion coefficient defined. Else, you need to type in the diffusivity in the User Defined edit field.

Enter the Diffusion coefficient Dc for each species. This can be a scalar value for isotropic diffusion or a tensor describing anisotropic diffusion. Select the appropriate tensor type —Isotropic, Diagonal, Symmetric, or Anisotropic 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 add-on products. See details: http://www.comsol.com/products/specifications/.

Migration in Electric Field

This section is available when the Migration in electric field check box is selected. 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 Anisotropic—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.

Specify the temperature (if you are using mobilities based on the Nernst-Einstein relation) and electric field in the Model Inputs section.

Example Models

Separation Through Dialysis: Application Library path Chemical_Reaction_Engineering_Module/Separation_Processes/dialysis Web link: http://www.comsol.com/model/separation-through-dialysis-258

Transport in an Electrokinetic Valve: Application Library path Microfluidics_Module/Fluid_Flow/electrokinetic_valve Web link: http://www.comsol.com/model/electrokinetic-valve-603