Lagrangian analysis of surfactant transport processes in experimental and computational models of pulsatile airway reopening
Description
Disease states such as acute respiratory distress syndrome and acute lung injury are characterized by pulmonary airway fluid occlusion and pulmonary surfactant insufficiency. Mechanical ventilation is used to restore airway patency, although this may result in ventilator induced lung injury. The goal of this dissertation is to investigate pulsatile ventilation waveforms in an idealized model of pulmonary airway reopening wherein the migrating semi-infinite bubble periodically retracts during reopening, termed retrograde motion. Such waveforms may redistribute surfactant along the air-liquid interface and reduce damage during airway reopening. We use computational and experimental techniques to explore the physicochemical hydrodynamic interactions that occur in this system We utilize the boundary element method to computationally investigate a model of airway reopening wherein a semi-infinite bubble migrates with both mean and sinusoidal velocity components through a fluid-occluded rigid tube. We find that the temporally- and spatially-dependent pressure gradient ∂tau n/∂z, the mechanical stimulus implicated in airway epithelial damage, increases significantly during retrograde bubble motion. During moderate oscillations large ∂tau n/∂z is confined to localized wall regions. However, at high frequencies and amplitudes a greater wall area is exposed to large ∂taun/∂z due to repeated passages of the bubble tip We experimentally investigate this system using micro-particle image velocimetry and shadowgraphy to measure the time-dependent velocity fields and bubble shapes in capillary tubes. A computer-controlled translating microscope stage and a suite of post-processing algorithms allow us to compute the ensemble-averaged bubble shape and velocity field at many points in the pulsatile cycle. We observe that Infasurf, a clinical pulmonary surfactant, blunts the bubble tip during pulsatile flows and significantly modifies the microscale flow field in the region of the tip due to physicochemical interactions Finally, we combine experimental and computational investigations to elucidate transport processes using Finite-time Lyapunov exponent (FTLE) analysis. In both cases we find that temporally-dependent Lagrangian coherent structures (LCS) demarcate the boundary between fluid advected upstream into the residual film and fluid advected downstream of the bubble tip. FTLE analysis of the measured velocities show increased mobility of the LCS in the presence of Infasurf, and may therefore elucidate the increase in surfactant transport during pulsatile ventilation