Effects of coughing on a surfactant-laden liquid plug in distal airways

We computationally study the effects of coughing on a surfactant-laden liquid plug in distal airways. The airway is assumed to be a rigid axisymmetric tube lined by a thin liquid film and coughing is mimicked by rapidly increasing the upstream pressure while keeping the downstream pressured constant. The evolution equations of the interfacial and bulk surfactant concentrations are solved fully coupled with the incompressible Navier-Stokes equations using a front-tracking method. Extensive simulations are performed for a range of Laplace numbers and non-dimensional pressure drops to investigate the effects of coughing-induced pressure pulses and surfactant on the mechanical stresses that could be injurious to epithelial cells. Simulations are also performed to examine the cough efficiency defined as bulk volume displacement per unit increase in upstream pressure. It is found that the mechanical stresses are substantially amplified by the coughing while they are significantly reduced with the introduction of surfactants.