Surfactant, viscoelasticity, elastoviscoplasticity and two-layer lining in an airway closure model*

The human lung airways are lined by the airway surface liquid (ASL). The ASL is either a layer of mucus or a two-layer liquid film consisting of the serous (outer) and the mucus (inner) layer. Owing to the Plateau-Rayleigh instability, the ASL can lead to airway closure, i.e. the occlusion of the airway due to the formation of a liquid plug. This is typical of distal airways, namely the bronchioles from the 7th generation onwards. In our simulations we focus on the fluid dynamics phenomena occurring during an airway closure, hence we assume the bronchiles walls as rigid. We isolate several elements of complexity to well comprehend their standalone effect on the airway closure. Indeed, we consider the airway closure with: (i) surfactant in a single-layer film, (ii) viscoelasticity and (iii) elastoviscoplasticity of a single-layer liquid, and (iv) two-layer Newtonian film. Apart from the primary instability leading to aiway closure, we focus on the post-coalescence wall stresses and secondary instabilities due to viscoelastic effects. Our future long-term goal is to carry out omni-comprehensive simulations that include the weakly viscoelasticy of the serous layer, the pronuouced  elastoviscoplasticity of the mucus layer and the surfactant dynamics.