an ideal fluid, it reduces to a pressure gradient, thereby specifying that the fluid moves down a compressive pressure gradient, which is Darcy’s Law. The third term is the gradient of the chemical potential, $\mu^\mathrm{f} = e^\mathrm{f} - \theta \eta^\mathrm{f}$, where $e^\mathrm{f}$ is the mass-specific internal energy, $\theta$ is temperature and $\eta^\mathrm{f}$ is the mass-specific entropy. The entropy gradient included in this term results in classical Fickean diffusion if only mixing entropy exists, as discussed in the following section. For a detailed derivation and discussion of , the reader is directed to @growthpaper.

### Saturation and Fickean diffusion of the fluid {#fick}

![Depicted at a microscopic scale, only unsaturated tissues A and B can undergo Fickean diffusion of the fluid. C is saturated.[]{data-label="fick_fig"}](saturation.eps){width="7.50cm"}

As depicted in Figure \[fick\_fig\], only when pores are unsaturated are there multiple configurations available to the fluid molecules at a fixed fluid concentration. This leads to a non-zero mixing entropy. In contrast, if saturated, there is a single available configuration (degeneracy), resulting in zero mixing entropy. Consequently, Fickean diffusion, which arises from the gradient of mixing entropy can exist only in the unsaturated case. However, even a saturated pore structure can demonstrate concentration gradient-dependent mass transport 