Oscillatory Shear Stress Induced Stabilization of Thin Film Instabilities

Abstract

The airways of the lungs are coated with a thin viscous film. Often, especially in disease, the liquid film can form a plug that obstructs airflow. The formation of the liquid plug is due to capillary driven instabilities that can arise in the lining, causing the lining to close up [1–4]. Airflow can also be obstructed if the airway wall collapses. This occurs when the elastic forces of the tube are not large enough to sustain the negative fluid pressures inside the tube. The phenomenon of airway closure occurs in many normal and pathological situations. It can occur in premature neonates who do not produce enough surfactant to keep the surface-tension sufficiently low. A clinical treatment is to deliver exogenous surfactant to these neonates in the form of a liquid bolus. In order to facilitate breathing they are also placed on mechanical ventilators. The amplitude (tidal volume) and frequency settings are two key parameters at the disposal of clinicians. Frequencies may range from normal breathing to high frequency ventilation at 5 Hz or more [6–7]. Airway obstruction and hypercapnea (build up of carbon dioxide in the lungs) are two examples of the manifestation of chronic obstructive pulmonary disease that may also occur in adults. In order to reduce breathlessness in patients with obstructive airway disease, chest wall vibrators are placed around the patients rib cage, which oscillate at frequencies up to 100 Hz [8]. Airway closure is also an issue in the microgravity environment of outer space since there is marked decrease in ventilatory inhomogeneity. It has been shown using the single nitrogen breath washout test that even though closing volumes are not affected by microgravity, there is considerable variability [9]. Their investigations suggest that some airways may close above residual volume, and that other factors besides gravity, such as the mechanical properties of the airway walls, are important factors on whether airway closure occurs.