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When the permeability is isotropic, $$\begin{equation} \mathbf{d}=d\,\mathbf{I}\,.\label{eq510} \end{equation}$$ For this material model, $d$ is constant. This assumption is only true when strains are small. Note that the user must specify $d\leqslant d_{0}$ , where $d_{0}$ is the solute diffusivity in free solution, since a solute cannot diffuse through the biphasic-solute mixture faster than in free solution.