  Airway mucus is a complex material with both viscoelastic and viscoplastic
properties that vary with healthy and pathological lung conditions. In this
study, the effects of these conditions on airway closure are examined in a
model problem, where an elastoviscoplastic (EVP) single liquid layer lines the
inner wall of a rigid pipe and surrounds the air core. The EVP liquid layer is
modelled using the Saramito-HB model. Model parameters are obtained for the
mucus in healthy, asthma, chronic obstructive pulmonary disease (COPD) and
cystic fibrosis (CF) conditions by fitting the rheological model to
experimental data. Then, liquid plug formation is studied by varying the
Laplace number and undisturbed liquid film thickness. According to previous
studies, airway epithelial cells can be damaged by high peak of the wall
stresses and stress gradients during liquid plug formation. Here, we show that
these stresses are also related to the EVP features of the liquid layer.
Yielded zones of the liquid layer are examined for different mucus conditions.
Before closure, liquid layer is found to be unyielded, indicating that this
phase is dominated by the elastic behaviour and solvent viscosity. This is
further confirmed by showing that the elastic coefficient is important in
determining whether closure occurs or not. Wall stresses are also investigated
for pathological and healthy cases. Their peaks are higher in COPD and CF due
to the viscoelastic extra stress contribution. Unlike the Newtonian case, COPD
and CF wall stresses do not relax after closure but remain approximately as
high as the Newtonian peak. Moreover, the local normal wall stress gradients
are smaller in COPD and CF liquid layers since their higher stiffness causes
smaller curvature at the capillary wave. The local tangential wall stress
gradients are also lower in these cases due to slower accumulation of the
liquid at the bulge.
