Diffusion and Migration Settings
All species, except when the Immobile Species checkbox has been enabled, can be transported by diffusion, and charged species are also transported by migration in the electric field.
For dissociation species you can choose to set the transport parameters to be the Same for all species appearing in the different dissociation steps, or you can use Individual settings for each subspecies. An exception is the Carbonic Acid node that only accounts for Individual subspecies.
The species diffusivity can either be related to the mobility by the Nernst–Einstein relation (default) or user defined values can be set using the No relation alternative.
Note: There are other definitions of the migration transport equations in the literature which use mobilities expressed in m2/(V·s), whereas COMSOL Multiphysics uses s·mol/kg. To convert mobilities expressed in m2/(V·s) to the corresponding values in s·mol/kg, you typically divide by the Faraday constant, F_const (about 96,485 C/mol).