Air-driven dynamics of viscoplastic liquid layers

TL;DR

This study models and experimentally investigates how turbulent air flow can induce wave formation and blow-out events in viscoplastic liquid layers, shedding light on mucus transport mechanisms in lungs.

Contribution

It introduces a combined theoretical and experimental analysis of air-driven viscoplastic liquid films, highlighting yield-stress effects on wave dynamics and blow-out phenomena.

Findings

Rapid wave growth occurs when the fluid ahead is unyielded.

Blow-out events are caused by static fluid consumption during wave propagation.

Newtonian films exhibit multiple waves, with blow-out only in thicker layers.

Abstract

Airway clearance by coughing is a key mechanism for mucus transport, particularly in obstructive lung diseases associated with altered mucus rheology. We investigate the dynamics of a viscoplastic liquid film driven by flow in a turbulent air layer, which is a model for air-driven mucus transport that incorporates yield-stress effects. Our theoretical analysis is based on a long-wave model for the liquid film flow, and we complement this with experiments, in which layers of Newtonian and yield-stress liquids are exposed to air flow in a rectangular duct. We demonstrate how perturbations to the layer depth can lead to localised yielding and wave generation. Rapid wave growth occurs when the fluid ahead of the oncoming wave is unyielded, so that as the wave propagates, it consumes this static fluid while depositing a much thinner film behind. This mechanism causes dramatic "blow-out" events in experiments, where liquid hits the roof of the tank. By contrast, in Newtonian thin films, multiple surface waves typically form, and blow-out only occurs in experiments when a Newtonian film is sufficiently thick.