Effects of two-layer coating on plug propagation and rupture in an elastoviscoplastic airway reopening model

Abstract

The two-layer coating plug propagation and rupture are studied computationally as a model for airway reopening for the eighth-to-tenth generations of a typical adult lung. The computational model incorporates the bi-layer structure of the serous–mucus liquid film lining the rigid tube, where the outer serous layer is treated as a Newtonian fluid, while the inner mucus layer is modelled as an elastoviscoplastic fluid governed by the Saramito–Herschel–Bulkley model. Compared with the one-layer plugs: (i) the two-layer plugs necessitate a higher driving pressure for rupture and exhibit a longer propagation distance, both of which increase the risk of failed airway reopening; (ii) both the wall shear stress and the wall shear stress derivative exhibit a significant reduction of approximately 75% in the two-layer plugs; (iii) the two-layer liquid film cannot be modelled using a one-layer plug model by simply applying the Navier boundary conditions. The critical mechanism due to dynamic elastic stretching (Hao et al. J. Fluid Mech. 1023, 2025, A14) persists for the two-layer plug. The serous layer appears to limit the transmission of elastic resonance to the airway wall, which underscores the protective role of the serous layer. The shear stress and shear stress derivative increase with increasing Weissenberg number. At low Weissenberg numbers, rupture time increases as a result of an increase in the Bingham number and a decrease in the power-law index; at high Weissenberg numbers, viscoelastic effects dominate the elastoviscoplastic airway reopening. These distinct two-layer phenomena offer crucial insights for developing more physiologically accurate models of airway fluid mechanics.