The propagation of air fingers into an elastic branching network

TL;DR

This study experimentally investigates how air fingers propagate through an elastic Y-shaped channel, revealing complex behaviors influenced by initial collapse, flow rate, and elastic forces, with implications for airway reopening models.

Contribution

It provides new insights into the dynamics of air finger propagation in elastic branching networks, highlighting the effects of initial collapse and flow conditions on steady state recovery.

Findings

Steady propagation maps onto main channel behavior at low collapse

Multiple reopening states occur at high initial collapse

Recovery length and time depend on flow rate and initial collapse

Abstract

We study experimentally the propagation of an air finger through the Y-bifurcation of an elastic, liquid-filled Hele-Shaw channel, as a benchtop model of airway reopening. With channel compliance provided by an elastic upper boundary, we can impose collapsed channel configurations into which we inject air with constant volume-flux. We typically observe steady finger propagation in the main channel, which is lost ahead of the Y-bifurcation but subsequently recovered in the daughter channels. At low levels of initial collapse, steady finger shapes and bubble pressure in the daughter channels map onto those in the main channel, despite small differences in initial collapse in different parts of the Y-channel. However, at higher levels of initial collapse where the elastic sheet almost touches the bottom boundary of the channel, experimentally indistinguishable fingers in the main channel can lead to multiple states of reopening of the daughter channels. The downstream distance at which steady propagation is recovered in the daughter channels also varies considerably with injection flow rate and initial collapse because of a transition in the mechanics regulating finger propagation. We find that the characteristic time and length-scales of this recovery are largest in the regime where viscous and surface tension forces dominate at low flow rate and/or low initial collapse, and that they decrease towards a constant plateau reached in the limit where elastic and surface tension forces balance at high flow rate and/or high initial collapse. Our findings suggest that practical networks are unlikely to comprise long enough channels for steady state propagation to remain established.