The surface velocity can sometimes be assumed to be independent of any particle transport, if the process is reaction-limited, where enough reactants are always on the surface. This is generally the case for wet processes for which the diffusive transport to the surface can be neglected. The simplest case is isotropic etching with a constant etch rate.
The description of anisotropic wet etching is more complex, since the etch rate can depend on the crystallographic direction [96]. In this case the surface velocity depends only on the surface orientation described by the normal direction
