Airway reopening mechanics depend on surfactant physicochemical properties. During reopening, the... more Airway reopening mechanics depend on surfactant physicochemical properties. During reopening, the progression of a finger of air down an airway creates an interface that is continually expanding into the bulk fluid. Conventional surfactometers are not capable of evaluating physicochemical behavior under these conditions. To study these aspects, we investigated the pressure required to push a semi-infinite bubble of air down a fluid-filled cylindrical capillary of radius R. The ionic surfactant SDS and pulmonary surfactant analogs L-alpha-dipalmitoylphosphatidylcholine and Infasurf were investigated. We found that the nonequilibrium adsorption of surfactant can create a large nonequilibrium normal stress and a surface shear stress (Marangoni stress) that increase the bubble pressure. The nonphysiological surfactant SDS is capable of eliminating the normal stress and partially reducing the Marangoni stress. The main component of pulmonary surfactant, L-alpha-dipalmitoylphosphatidylcholine, is not capable of reducing either stress, demonstrating slow adsorption properties. The clinically relevant surfactant Infasurf is shown to have intermediate adsorption properties, such that the nonequilibrium normal stress is reduced but the Marangoni stress remains large. Infasurf's behavior suggests that an optimal surfactant solution will have sorption properties that are fast enough to reduce the reopening pressure that may damage airway wall epithelial cells but slow enough to maintain the Marangoni stress that enhances airway stability.
ABSTRACT During pulmonary airway reopening, a finger of air creates an interface that is continua... more ABSTRACT During pulmonary airway reopening, a finger of air creates an interface that is continually expanding into a surfactant filled fluid. The quantity of surfactant transported to this interface affects the local surface tension, γ, and thus the reopening pressure. The goal of this study is to demonstrate how transport barriers can affect the mechanics of this system. We use a combined boundary integral and finite difference technique to solve the governing equations in an axisymmetric geometry. When the quantity of surfactant in the fluid is low, a bulk diffusion barrier exists such that Csγeq, the equilibrium surface tension
ABSTRACT Pulmonary airway closure occurs when the liquid lining layer occludes the airway and obs... more ABSTRACT Pulmonary airway closure occurs when the liquid lining layer occludes the airway and obstructs airflow. Meniscus formation is the result of a surface-tension driven instability within the liquid layer. Airway 'compliant collapse' may result, which leads to tube buckling with airway walls held in apposition. Airway closure is common in premature neonates who do not produce sufficient surfactant and those suffering from emphysema. To model the reopening of a collapsed airway flooded with fluid, we consider the time-dependent motion of an air-bubble driven by a positive bubble pressure Pb through a liquid filled compliant channel. The governing Stokes equations are solved using the boundary element method near the bubble tip, and lubrication theory sufficiently far ahead of the buble where the channel walls have a gentle taper. Results show that for Pb > P_crit, the bubble moves forward and converges to a steady velocity as the airway walls 'peel' open. For Pb < P_crit, no steady solutions are found because fluid continuously accummulates ahead of the bubble tip. This result validates the stability analysis of the previously steady wall peeling solution branch. The impact of the flow field on transport of surfactant and the applied shear and normal stresses on the wall as they relate to pulmonary reopening are also discussed.
The displacement of a viscous fluid by a semi-infinite air bubble models the continual interfacia... more The displacement of a viscous fluid by a semi-infinite air bubble models the continual interfacial expansion aspects of opening collapsed pulmonary airways. The mechanics of this system, especially the interfacial pressure drop Delta P, is affected by the local surface tension gamma. Surfactant in the bulk fluid can be transported to the interface, where it adsorbs with a concentration Gamma
Airway reopening mechanics depend on surfactant physicochemical properties. During reopening, the... more Airway reopening mechanics depend on surfactant physicochemical properties. During reopening, the progression of a finger of air down an airway creates an interface that is continually expanding into the bulk fluid. Conventional surfactometers are not capable of evaluating physicochemical behavior under these conditions. To study these aspects, we investigated the pressure required to push a semi-infinite bubble of air down a fluid-filled cylindrical capillary of radius R. The ionic surfactant SDS and pulmonary surfactant analogs L-alpha-dipalmitoylphosphatidylcholine and Infasurf were investigated. We found that the nonequilibrium adsorption of surfactant can create a large nonequilibrium normal stress and a surface shear stress (Marangoni stress) that increase the bubble pressure. The nonphysiological surfactant SDS is capable of eliminating the normal stress and partially reducing the Marangoni stress. The main component of pulmonary surfactant, L-alpha-dipalmitoylphosphatidylcholine, is not capable of reducing either stress, demonstrating slow adsorption properties. The clinically relevant surfactant Infasurf is shown to have intermediate adsorption properties, such that the nonequilibrium normal stress is reduced but the Marangoni stress remains large. Infasurf's behavior suggests that an optimal surfactant solution will have sorption properties that are fast enough to reduce the reopening pressure that may damage airway wall epithelial cells but slow enough to maintain the Marangoni stress that enhances airway stability.
ABSTRACT During pulmonary airway reopening, a finger of air creates an interface that is continua... more ABSTRACT During pulmonary airway reopening, a finger of air creates an interface that is continually expanding into a surfactant filled fluid. The quantity of surfactant transported to this interface affects the local surface tension, γ, and thus the reopening pressure. The goal of this study is to demonstrate how transport barriers can affect the mechanics of this system. We use a combined boundary integral and finite difference technique to solve the governing equations in an axisymmetric geometry. When the quantity of surfactant in the fluid is low, a bulk diffusion barrier exists such that Csγeq, the equilibrium surface tension
ABSTRACT Pulmonary airway closure occurs when the liquid lining layer occludes the airway and obs... more ABSTRACT Pulmonary airway closure occurs when the liquid lining layer occludes the airway and obstructs airflow. Meniscus formation is the result of a surface-tension driven instability within the liquid layer. Airway 'compliant collapse' may result, which leads to tube buckling with airway walls held in apposition. Airway closure is common in premature neonates who do not produce sufficient surfactant and those suffering from emphysema. To model the reopening of a collapsed airway flooded with fluid, we consider the time-dependent motion of an air-bubble driven by a positive bubble pressure Pb through a liquid filled compliant channel. The governing Stokes equations are solved using the boundary element method near the bubble tip, and lubrication theory sufficiently far ahead of the buble where the channel walls have a gentle taper. Results show that for Pb > P_crit, the bubble moves forward and converges to a steady velocity as the airway walls 'peel' open. For Pb < P_crit, no steady solutions are found because fluid continuously accummulates ahead of the bubble tip. This result validates the stability analysis of the previously steady wall peeling solution branch. The impact of the flow field on transport of surfactant and the applied shear and normal stresses on the wall as they relate to pulmonary reopening are also discussed.
The displacement of a viscous fluid by a semi-infinite air bubble models the continual interfacia... more The displacement of a viscous fluid by a semi-infinite air bubble models the continual interfacial expansion aspects of opening collapsed pulmonary airways. The mechanics of this system, especially the interfacial pressure drop Delta P, is affected by the local surface tension gamma. Surfactant in the bulk fluid can be transported to the interface, where it adsorbs with a concentration Gamma
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Papers by D. Gaver