Use this node to specify the mass transfer in the mobile liquid solvent present in the pores of the Unsaturated Porous Medium.

The temperature model input is always available. By default, the Temperature model input is set to Common model input, and the temperature is controlled from Default Model Inputs under Global Definitions or by a locally defined Model Input. If a Heat Transfer interface is included in the component, it controls the temperature Common model input. Alternatively, the temperature field can be selected from another physics interface. All physics interfaces have their own tags (Name). For example, if a Heat Transfer in Fluids interface is included in the component, the Temperature (ht) option is available for T.

You can also select User defined from the Temperature model input in order to manually prescribe T.

Select Saturation or Liquid volume fraction from the list.

For Saturation, enter a value for s (dimensionless) between 0 and 1. The liquid volume fraction is then computed from the saturation and porosity as θl = εps.

For Liquid volume fraction, enter a value for θl (dimensionless) between 0 and the value of the porosity.

Select a Liquid fraction time change: Liquid fraction constant in time (the default), Time change in liquid fraction, or Time change in pressure head.

If transport by convection is active, the velocity field of the solvent needs to be specified. 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 Liquid 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 Liquid 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 Liquid diffusion coefficient list.

For User defined, enter a value or expression for the Liquid diffusion coefficient DL,i of each species in the corresponding input field.

In a porous medium the diffusivity is reduced due to the fact that the solid grains impede Brownian motion. Select an Effective diffusivity model, liquid to account for the reduced diffusivity in the liquid. The available models are Millington and Quirk model (the default), Bruggeman model, Tortuosity model, or No correction. For Tortuosity model, enter a value for the tortuosity τL,i (dimensionless).

Note that multiple species, as well as Migration in Electric fields (described below) is only available for certain COMSOL Multiphysics add-on products. For details see:  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 effective diffusivity and the temperature using the Nernst-Einstein relation. For User defined, select the appropriate scalar or tensor type — Isotropic, Diagonal, Symmetric, or Full — and type in the value or expression of the effective mobility ume,i.

Enter the Charge number zc for each species.