  We present an experimentally guided, bi-phasic, multi-species ionic gel model
to compare and make quantitative predictions on the viscoelastic properties of
healthy mucus versus Cystic-Fibrosis infected mucus. The mixture theory
consists of the mucus (polymer phase) and water (solvent phase) as well as
several different ions: H$^+$, Na$^+$, Cl$^-$ and Ca$^{2+}$. The model is
calibrated with the experimental data of mucus diffusivities. Numerical studies
reveal that the Donnan forces are the dominating mechanism driving the mucus
swelling/deswelling transition. However, these transitions could be continuous
or discontinuous, depending on the complex interplay between ionic bath
concentrations, pH, polymer mass in the solvent and the average charge per
monomer.
