Morphology of mucus films in lung airways: secretion and ciliary evacuation
Lung airways are lined by a film of mucus which protects the epithelium from inhaled particles. To maintain a uniform coating, the mucus that is secreted into airways must be distributed into a film by wall-attached cilia, which constantly convey mucus along the airway. However, the film's natural tendency is to accumulate into humps and plugs, due to the Rayleigh-Plateau instability. To understand the behaviour of the film amidst these competing factors, we perform simulations of an idealized tubular airway using a reduced-order thin-film model. The axial boundaries are nonperiodic, allowing for cilia-driven inflow and outflow; a tangential velocity along the tubular wall models ciliary transport, while a localized source at the wall accounts for secretion. On increasing the mucus input rate, we find three distinct film morphologies: (i) uniform flat films; (ii) nonuniform films that are composed of travelling unduloid-shaped humps, separated by mucus-depleted zones; and (iii) films that form an occluding plug. The flat-film regime, absent in closed periodic domains, emerges as a consequence of the convective nature of the Rayleigh-Plateau instability in the presence of ciliary transport. Higher secretion rates increase the mean film-thickness and induce a convective-to-absolute transition, which manifests in the appearance of travelling humps. In the plug-forming regime, capillary forces dominate and drive the incoming mucus into a single hump, which resists ciliary translation and remains near the mucus source. Our results show how low-level baseline secretion sustains a protective, uniform mucus film, and how hypersecretion -- stimulated, for example, by inhaled allergens -- produces mucus plugs.