B. J. Mullins

The reopening of an occluded airway can lead to the formation of droplets and aerosols, which can be released during exhalation, providing a possible mechanism of disease transmission. In this study, the flow behavior of airway occlusions (“plugs”) close to their point of rupture is examined using a free-surface model (volume of fluid), such that factors influencing the formation of droplets during their reopening can be identified. The propagation of airway occlusions is highly influenced by recirculating flow at the edge of the front interface, where significant fluctuations of wall shear stresses occur. The resulting drag force causes the rear interface to advance at a greater rate, destabilizing the plug. As the plug thickness decreases, a thin film with uniform thickness forms, resulting in a disk-like structure around the centerline. Rupture occurs around the disk formation largely due to surface tension instability. At lower pressures, smaller disks form causing the rupture to occur through a puncture point (forming no droplets); at higher pressures, a larger disk forms, with rupture occurring along the disk edge and at the center (forming multiple droplets). Upon reopening, a jet of air is produced, causing a temporary increase in shear stress along the wall. However, the magnitude and duration of this increase do not scale directly to the applied pressure, as the formation of droplets and irregularities in airway lining were found to disrupt the flow field and the shear stresses at the wall.