There are several mechanisms which may cause airway closure in the lung. In this paper, we focus on airway closure due to the capillary instability [1]. In Gauglitz and Radke’s study [2], they showed that once the ratio of film thickness to tube radius is larger than 0.12, airway closure could occur. The induced interfacial deformation creates a driving pressure which forces more liquid into a growing bulge. The interface will then deform rapidly towards the end of the closure process due to the presence of large curvatures, which create strong driving pressures and will eventually lead to the formation of a liquid plug. Due to the instability, stresses, including normal and shear stresses on the airway wall, will be induced. The epithelial cells on the inner wall of the airway may be injured by these induced stresses. The purpose of this study is to calculate the wall stresses for a two-layer system and determine if the magnitude of the stresses are sufficient to injure the epithelial cells.

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