The flexibility of COMSOL means that a reaction can result in nearly any reaction rate contribution. But a reactant, with mass fraction ωi, can physically only be consumed if ωi > 0. Correspondingly a product can only be formed when ωi < 1. The Reaction feature in Transport of Concentrated Species Interface contains a way to regularize the rate expression and comply with these restrictions.

When selected, the original reaction rate contribution, Ric, is replaced by

If the mass fraction for a reactant species ωi becomes smaller than its Damping limit, ωidl, the rate expression is reduced linearly. If ωi ≤  0 for a reactant species, the reaction rate contribution to that species is completely removed. Similarly, if the mass fraction for a product species ωj becomes larger than 1 − ωjdl, the rate expression added is damped linearly. If ωj ≥  1 for a product species, the reaction rate contribution to that species is completely removed.

The default value for the Damping limit, ωidl, is 10−6, which is appropriate for most applications, but can require adjustment when working with for example catalytic trace species.