COMSOL 6.4 - The Electrophoretic Transport Interface

The interface (

), found under the branch (

), is used to solve for the electrophoretic transport of an arbitrarily number of species in water-based system, subject to potential gradients. The species transported can be any combination of weak and strong acids and bases, ampholytes, and uncharged species. The transport of masses and charge is based on the Nernst–Planck equations for molecular transport, in combination with electroneutrality, dissociation equilibria for weak acids, bases, and ampholytes as well as the water autoionization reaction.

The physics interface can simulate most forms of electrophoresis modes, such as zone electrophoresis, isotachophoresis, isoelectric focusing, and moving boundary electrophoresis.

Gel electrophoresis can be simulated by the inclusion of immobile charged species.

The interface supports simulation in 1D, 2D, and 3D as well as for axisymmetric components in 1D and 2D.

The dependent variables are the electrolyte potential, and the molar concentrations of the included species, added individual by each species node in the model tree.


	Zone Electrophoresis: Application Library path Chemical_Reaction_Engineering_Module/Electrokinetic_Effects/zone_electrophoresis

Settings

The is the default physics interface name.

The is used primarily as a scope prefix for variables defined by the physics interface. Refer to such physics interface variables in expressions using the pattern <name>.<variable_name>. In order to distinguish between variables belonging to different physics interfaces, the name string must be unique. Only letters, numbers, and underscores (_) are permitted in the field. The first character must be a letter.

The default (for the first physics interface in the model) is el.

Domain Selection

If any part of the model geometry should not partake in the mass transfer model, remove that part from the selection list.

Out-of-Plane Thickness


	For 2D components, enter a value or expression for the out-of-plane Thickness d (SI unit: m). The value of d determines the size of the domain perpendicular to the modeled 2D cross section. This value yields, for example, the correct total current when the current density is obtained from a 2D simulation.

Cross-Sectional Area


	For 1D components, enter a Cross-sectional area Ac (SI unit: m2) to define a parameter for the area of the geometry perpendicular to the 1D component. The value of this parameter is used, among other things, to automatically calculate the total current from the current density vector. The analogy is valid for other fluxes. The default is 1 m2.

Transport Mechanisms

Mass transport due to diffusion and migration is always included. Use the checkboxes available under to control other transport mechanisms.

•

By default, the checkbox is selected. Clear the checkbox to disable convective transport.

•

The checkbox activates functionality specific to species transport in porous media. When selected, the Porous Matrix Properties node can be added to a domain to specify the electrolyte volume fraction and tortuosity, and the Effective Transport Parameter Correction sections are enabled in the species nodes.

Consistent Stabilization

To display this section, click the button (

) and select in the dialog.

•

When the checkbox is selected, a weak term that reduces spurious oscillations is added to the transport equation. The resulting equation system is always nonlinear.

•

For both and , select an . is the default and means that derivatives of the diffusivity are neglected. This setting is usually accurate enough and is computationally faster. If required, select instead.

Inconsistent Stabilization

To display this section, click the button (

) and select in the dialog. By default, the checkbox is not selected because this type of stabilization adds artificial diffusion and affects the accuracy of the original problem. However, this option can be used to get a good initial guess for under-resolved problems.

Advanced Settings

To display this section, click the button (

) and select in the dialog. Normally these settings do not need to be changed. Select a — (the default) or . Use the conservative formulation for compressible flow.

Discretization

To display all settings available in this section, click the button (

) and select in the dialog.

The concentration variables are set to use Linear elements by default.

The potential variable is set to use Quadratic elements by default.

The checkbox is activated by default so that COMSOL Multiphysics computes predefined accurate boundary flux variables. When this option is selected, the solver computes variables storing accurate boundary fluxes from each boundary into the adjacent domain.

If the checkbox is cleared, the COMSOL Multiphysics software instead computes the flux variables from the dependent variables using extrapolation, which is less accurate in postprocessing results but does not create extra dependent variables on the boundaries for the fluxes.

The flux variables affected in the interface are:

•

<name>.nIl, where <name> is the name of the interface (default is el), set on the interface top node. This is the normal electrolyte current density.

•

<name>.ntflux_<species_name> is the (see Common Settings for the Species Nodes in the Electrophoretic Transport Interface below). This is the normal total flux for each species.

Also the checkbox is available if the previous checkbox is selected. The smoothing can provide a more well-behaved flux value close to singularities.

For details about the boundary fluxes settings, see Computing Accurate Fluxes in the COMSOL Multiphysics Reference Manual.

Regarding the , see Splitting Complex-Valued Variables in the COMSOL Multiphysics Reference Manual.

Dependent Variables

The dependent variable name for the electrolyte potential variable is phil by default.

The name of the concentration dependent variables are named as el.xxx, where el is the name of the interface as set above, and the xxx string is controlled by the setting on the individual species nodes.