Human whole saliva consists of a physically-associated network of mucin molecules. The interchain physical associations control the rheological characteristics of the solution and in particular, the lifetime of filaments and threads. Measurements show that the shear rheology of salivary mucin solutions (as measured by steady shear viscosity and small amplitude oscillatory shear) is quite insensitive to sample age over a 24 h period following sample collection. By contrast, the filament thinning dynamics vary dramatically, with the characteristic relaxation time of the saliva and the breakup time of a fluid thread decreasing significantly with sample age. We interpret our results within the framework of a sticky finitely extensible network model which respects the known physical dimensions and properties of the mucin molecules in saliva and models them as a network of physically associating and finitely extensible polymer chains. The model predicts an initially strain-hardening response in the transient extensional rheology, followed by a sudden extensional thinning and filament rupture as the chains approach their maximum extensibility and the tension between junction points increases rapidly. We show that the model can accurately capture the changes observed in the filament thinning dynamics with sample age by incorporating a steady decrease in the molecular weight of the supramolecular aggregates of mucin. These experimental observations highlight the importance of considering sample age and enzymatic degradation when reporting extensional rheological measurements of saliva.
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