Effect of viscoelasticity on mucus clearance in the pulmonary airways

The clearance of mucus from the human tracheobronchial tree is ensured by two mechanisms. Firstly, mucociliary clearance, i.e. via the coordinated beating of cilia covering the epithelium. Secondly, via cough, i.e. a sudden increase in air flow rate, which produces a strong enough viscous stress and pressure gradient at the surface of the mucus film.
In both situations, the role of mucus rheology, and in particular its viscoelasticity, remains largely unknown, although its crucial character is recognized in the case of respiratory pathologies such as cystic fibrosis.

Reliable predictive models are essential, however, the complexity of the fluid-structure interaction between cilia and mucus as well as the three-dimensional airway geometry means that available numerical tools remain limited.

Via the CNRS-funded MUCUS project team, which involves researchers from FAST, LISN, ESPCI, Tokyo University of Science, and IIT Bombay, we investigate the hydrodynamics of both clearance mechanisms. In the current project, we will focus on the dynamics of liquid plugs resulting from airway occlusion events under different breathing scenarios (normal breathing, assisted ventilation, cough):

• Numerical axis: Here, we will employ the academic solver Basilisk to perform numerical simulations of liquid plugs subject to a time-constant or time-varying pressure difference. We will focus on the role of visco-elasticity in the rupture of liquid plugs. Preliminary results by Iris Fambart suggest that viscoelasticity precipitates plug rupture, which may be the result of spatio-temporal oscillations within the residual film. Simulations will be performed on axisymmetric or fully three-dimensional computational domains, using the supercomputer RUCHE.
• Experimental axis: We aim to study the problem of occlusion events, which are of medical relevance, using a microfluidic model. The focus is on the dynamics and rupture of liquid plugs driven by a pressure gradient in viscoelastic fluids. Preliminary results obtained by the intern Benjamin Thomas (M1) highlighted the influence of viscoelasticity on the plug’s shape and dynamics , an effect that remains to be further explored. In addition, the impact of a cough-like airflow will be investigated.