If you want to specify the concentration of a subset of the partaking species, this can be done by using the Concentration node instead.

For the Electroanalysis interface, this node is available when you select the Convection check box on the physics interface Settings window.

For the concentration of each species c0,c (SI unit: mol/m3), enter a value or expression.

This section in the settings is only available for some products. Search for “Inflow” on the page: http://www.comsol.com/products/specifications/ for more details on availability.

The option Concentration constraint constrains the concentration values on the boundary by the use of pointwise constraints. The other option, Flux (Danckwerts) can be more stable and fast to solve when high reaction rates are anticipated in the vicinity of the inlet. Oscillations on the solutions can also be avoided in such cases. The latter condition uses a flux boundary condition based on the velocity across the boundary and the concentration values. See further details in the theory section.

To display this section, click the Show button () and select Advanced Physics Options. You can find details about the different constraint settings in the section Constraint Reaction Terms.