Modelling surface tension effects on mucus transport

Abstract

Mucociliary clearance (MC) protects the lung from pollutants by removing mucus from the lung. Pathologies can affect MC and alter its efficiency. Changes in mucus physical properties can play an important role, but remain not well understood. More particularly, the role of surface tension (ST) has never been studied. Bronchial mucus is in contact with air and the air-mucus interface is submitted to a physical force due to ST. Its amplitude depends on the local properties of mucus, such as the layer thickness and curvature. Our goal was to estimate if these properties are able to affect mucus transport through ST. We used modelling tools based on our previous work (Mauroy, B. et al. Frontiers 2015; 6:214) and on thin layer theory to compute the forces induced by ST on mucus in an idealised lung. We analysed the conditions for induced shear stress to overcome mucus yield stress, computed the resulting mucus velocity and compared it with estimation of MC. The main parameters used were a ST of 0.03 N/m, mucus yield stress of 0.1 Pa and mucus viscosity of 1 Pa.s. No ST effects are observed for healthy mucus thicknesses of 5 - 10 microns. However, for thicker mucus layers, more than 50 microns, our results indicate that ST effects are able to motion mucus in distal lung bifurcations. In these cases, ST effects tend to always oppose MC and to reduce notably its efficiency. For thicknesses larger than 80 microns, ST effects in the most distal bifurcations can overcome MC and motion mucus in the opposite direction, reaching mean velocities of about 35 microns/s. Hence, our model indicates that distal lungs bifurcations might be prone to mucus accumulation in pathologies inducing mucus hypersecretion, because of ST effects.