Effects of elastoviscoplastic properties of mucus on airway closure in healthy and pathological conditions

Airway mucus is a complex material with both viscoelastic and viscoplastic properties that vary with healthy and pathological conditions of the lung. 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. The parameters for the model are obtained for the mucus in healthy, asthma, chronic obstructive pulmonary disease (COPD), and cystic fibrosis (CF) conditions by fitting the rheological model to the experimental data. Then the liquid plug formation is studied by varying the Laplace number and undisturbed liquid film thickness. Airway closure is a surface-tension-driven phenomenon that occurs when the ratio of the pulmonary liquid layer thickness to the airway radius exceeds a certain threshold. In previous studies, it has been found that airway epithelial cells can be lethally or sublethally damaged due to the high peak of the wall stresses and stress gradients during the liquid plug formation. Here we demonstrate that these stresses are also related to the EVP features of the liquid layer. Yielded zones of the liquid layer are investigated for the different mucus conditions, and it is found that the liquid layer is in a chiefly unyielded state before the closure, which indicates that this phase is dominated by the elastic behavior and solvent viscosity. This is further confirmed by showing that the elastic coefficient is one of the most critical parameters determining whether the closure occurs. This parameter also largely affects the closure time. The wall stresses are also investigated for the pathological and healthy cases. Their peaks for COPD and CF are found to be the highest due to the viscoelastic extra stress contribution. Contrary to the Newtonian case, the wall stresses for COPD and CF do not smoothly relax after closure, as they rather remain effectively almost as high as the Newtonian peak. Moreover, the local normal wall stress gradients are smaller for the COPD and CF liquid layer due to their higher stiffness causing a smaller curvature at the capillary wave. The local tangential wall stress gradients are also shown to be smaller for these cases because of the slower accumulation of the liquid at the bulge

Coherent structures in the turbulent channel flow of an elastoviscoplastic fluid

In this numerical and theoretical work, we study the turbulent channel flow of Newtonian and elastoviscoplastic fluids. The coherent structures in these flows are identified by means of higher order dynamic mode decomposition (HODMD), applied to a set of data non-equidistant in time, to reveal the role of the near-wall streaks and their breakdown, and the interplay between turbulent dynamics and non-Newtonian effects. HODMD identifies six different high-amplitude modes, which either describe the yielded flow or the yielded-unyielded flow interaction. The structure of the low- and high-frequency modes suggests that the interaction between high- and low-speed streamwise velocity structures is one of the mechanisms triggering the streak breakdown, dominant in Newtonian turbulence where we observe shorter near-wall streaks and a more chaotic dynamics. As the influence of elasticity and plasticity increases, the flow becomes more correlated in the streamwise direction, with long streaks disrupted for short times by localised perturbations, reflected in reduced drag. Finally, we present streamwise-periodic dynamic mode decomposition modes as a viable tool to describe the highly complex turbulent flows, and identify simple well-organised groups of travelling waves

Suppressing prompt splash with polymer additives

Splash suppression during drop impact continues to be a grand challenge. To date, only a few techniques for the complete suppression of splash exist. Reducing the ambient pressure and using complex surfaces (microstructured and/or soft) are two of the recently discovered ones which may not be very practical in many technological processes. The idea of using additives directly into the liquid used to produce the drops, to inhibit this undesirable phenomenon, is, therefore, desired. Prompt splash is a type of splashing that releases diminutive droplets at high speeds from the tip of the lamella at the spreading liquid-substrate contact line immediately after the impact (within the first 10 μm), without generating the typical thin-sheet or corona. Prompt splash remained hidden for many years until high-speed imaging allowed for its visualisation. Here, we demonstrate that by adding very low amounts of polymer (around 0.01 wt%) into normally splashing water droplets a reduction and even a complete suppression of the prompt splash is observed. In this work, a systematic experimental study of the impact of viscoelastic drops, by varying size, impact velocity, and the “degree” of viscoelasticity, is conducted. When capillary forces are insufficient to maintain the integrity of the drop, elastic forces seem to pull the attached small droplets/fingers back to the lamella preventing their ejection and, therefore, inhibiting prompt splash. However, surprisingly, larger quantities of the polymer additive lead to a secondary transition, in which another, more common, type of splash is induced: corona splash