Resistance of mucus-lined tubes to steady and oscillatory airflow

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Abstract
We examined the effects of quantity and physical properties of mucus on resistance to steady and oscillatory flows in a circular tube. Gels with similar rheological properties to canine tracheal mucus were prepared from hog gastric mucin or locust bean gum cross-linked with Na2B4O7. A horizontal straight tube (D 1.85 cm) was lined with these mucus simulants to depths ranging from 0.3 to 1.0 mm. The pressure difference over a 50-cm portion of the tube and the volumetric flow rate were determined simultaneously. Low-amplitude oscillatory flow were generated with a modified Harvard pump. For steady flow, the resistance at low Reynolds number (Re) increased with increasing gel depth only to the extent expected for simple constriction of the tube cross-sectional area. The same was true for oscillatory (0.25--6 Hz) flow resistance at low flow amplitude (corresponding to Re less than 4,000). No effect of gel cross-link density at low Re was observed. At high steady-flow rates, and for high-amplitude oscillatory flow, resistance increased beyond that predicted for simple constriction. Plots of friction factor (f) vs. Re showed a critical point (Recrit) of the order of 1.5 x 10(4), at which f increased sharply. Recrit, which corresponded to the onset of wave formation in the lining layer, was insensitive to changes in gel depth. However, gel cross-link density did affect the onset of wave formation: in oscillatory flow Recrit was shifted to higher Re, and the rise in f in steady flow was blunted with high degrees of cross-linking. The existence of Recrit and its association with wave formation are consistent with predictions based on two-phase flow theory.