Theory Manual Version 3.4
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Subsection 5.8.1: Constant Isotropic Diffusivity Up Section 5.8: Solute Diffusivity Subsection 5.8.3: Referentially Isotropic Diffusivity

### 5.8.2 Constant Orthotropic Diffusivity

When the permeability is orthotropic, where are orthonormal vectors normal to the planes of symmetry. For this material model, the are constant. Therefore this model should be used only when strains are small. Note that the user must specify , where is the solute diffusivity in free solution, since a solute cannot diffuse through the biphasic-solute mixture faster than in free solution.
Subsection 5.8.1: Constant Isotropic Diffusivity Up Section 5.8: Solute Diffusivity Subsection 5.8.3: Referentially Isotropic Diffusivity