The Concentrated Electrolyte Transport (cet) (), interface, found under the Electrochemistry branch () when adding a physics interface, is used to compute the electrolyte potential and concentration distribution for an electrolyte mixture comprising charged (ions) and neutral species. The interface is suitable for modeling electrolytes featuring large concentration gradients of the charge-carrying ions, where at the same time the concentrations of the charge-carrying ions are significant in relation to the total concentration of all species in the electrolyte mixture.

The interface defines the individual transport of electrolyte species using the Onsager-Stefan-Maxwell equations, which account for the binary interactions between all modeled species in the mixture. In the Species section (see below), the active species (cations, anions and neutral species) of the electrolyte mixture are defined. A minimum of one cation and one anion needs to be defined. Internally, various matrix operations are then performed in order to render an equation system defining an electrolyte phase potential (default variable name: cet.phil) and n − 2 (where n is the total number of electrolyte species) neutral electrolyte component fraction (cet.y_XXX) dependent variables.

The names (XXX) of the electrolyte component fractions are generated automatically, and are based on the species names and charges. For instance, the species set consisting of the cation Fe (set to charge +2), the anion OH (charge -1) and the neutral species H2O, will render the two electrolyte components Fe_OH_2 and H2O. Note that the main purpose of defining the neutral electrolyte components is to generate an electroneutral base for the dependent variables of the equation system, and the resulting chemical formulas for the neutral salts may have no correspondence to for instance solid salt compounds in nature.

Due to the constraint of local electroneutrality, a system of n species results in n-1 electrolyte components. Since it is also assumed all major electrolyte species are included in the formulation, the equation system can be reduced further to include only n-2 electrolyte component dependent variables.

Throughout the user interface, symbols related to species are generally shown as Pi, with P being the parameter or variable name, and i the species index, whereas symbols related to electrolyte components are shown as , i.e. with a tilde (~) above the parameter/variable symbol, with i referring to the electrolyte component index.

The interface adds one Electrolyte node and one Reference Electrode as default domain nodes. Both nodes are singletons and cannot be removed.

The Label is the default physics interface name.

The Name 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 Name field. The first character must be a letter.

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

Specify the names and molar masses of the species in the Cations, Anions and Neutral species tables. Only letters and numbers are permitted in the Name field. For the Cations and Anions tables, also define the species charges. As described above in the Overview section, this will result in a set of n-1 electrolyte components.

The Electrolyte component from molar-fraction constraint setting defines which electrolyte component to be eliminated when defining what n-2 electrolyte component fractions to use as dependent variables.

To display this sections, click the Show More Options button () and select Stabilization in the Show More Options dialog.

Streamline diffusion (enabled by default) adds weak terms to the equation system that may improve the quality of results for convection-dominated problems on coarse meshes. When the Crosswind diffusion checkbox is selected, additional weak terms that reduce spurious oscillations are added to the transport equations.

To display the settings available in this section, click the Show More Options button () and select Advanced Physics Options in the Show More Options dialog.

The same discretization is used for all dependent variables in the interface. Quadratic elements are used by default. See Overview above regarding what dependent variables are solved for, and naming conventions.

The Compute boundary fluxes checkbox is activated by default. When this option is checked, the solver computes variables storing accurate boundary fluxes from each boundary into the adjacent domain. If the checkbox is cleared, the boundary flux variables are computed from the dependent variables using extrapolation, which is less accurate in postprocessing results. This setting does however not create extra dependent variables on the boundaries for the fluxes.

Also the Apply smoothing to boundary fluxes checkbox is available if the previous checkbox is checked. 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 Value type when using splitting of complex variables, see Splitting Complex-Valued Variables in the COMSOL Multiphysics Reference Manual.