Mechanisms of droplet generation from liquid plug rupture in bronchioles: A numerical study

The rupture of liquid plugs in the lower respiratory tract is a fundamental multiphase flow problem associated with airway reopening and the generation of respiratory droplets and aerosols. This study presents fully three-dimensional numerical simulations of mucus plug rupture within a rigid cylindrical geometry representative of the ninth bronchial generation of an adult lung. We employ a hybrid computational framework coupling a geometric Volume-of-Fluid (VOF) method for resolving mucus rupture with a Lagrangian Particle Tracking (LPT) method for efficient tracking of post-rupture droplets. We investigate the rupture dynamics across a range of dimensionless pressure difference (measured in capillary pressure units, Δ P ∗ ) and Laplace number ( L a ). Two distinct rupture regimes are identified: a capillary-dominated “point contact” regime at low pressure differences ( Δ P ∗ ≤ 2 ) and a “sheet rupture” regime at high pressure differences ( Δ P ∗ ≳ 3 ), where the front meniscus curves inwards to form a thinning liquid sheet. Theoretical analysis using lubrication theory confirms that sheet drainage in the high-pressure difference regime is driven by the viscous timescale. Post-rupture atomization is shown to depend strongly on the imposed pressure difference and Laplace number, which govern the transition from ligament-mediated breakup to shear-dominated droplet generation. We show that the resulting droplet size statistics, as well as key non-dimensional parameters including the Weber number and the total volume of mucus transported by the flow ( V f ), are strongly influenced by Δ P ∗ . The results provide physical insight into how rupture-driven droplet generation mechanisms may contribute to transport of respiratory material from the distal airways.