3 results
The influence of surfactant on the propagation of a semi-infinite bubble through a liquid-filled compliant channel
- David Halpern, Donald P. Gaver III
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- Journal:
- Journal of Fluid Mechanics / Volume 698 / 10 May 2012
- Published online by Cambridge University Press:
- 30 March 2012, pp. 125-159
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We investigate the influence of a soluble surfactant on the steady-state motion of a finger of air through a compliant channel. This study provides a basic model from which to understand the fluid–structure interactions and physicochemical hydrodynamics of pulmonary airway reopening. Airway closure occurs in lung diseases such as respiratory distress syndrome and acute respiratory distress syndrome as a result of fluid accumulation and surfactant insufficiency. This results in ‘compliant collapse’ with the airway walls buckled and held in apposition by a liquid occlusion that blocks the passage of air. Airway reopening is essential to the recovery of adequate ventilation, but has been associated with ventilator-induced lung injury because of the exposure of airway epithelial cells to large interfacial flow-induced pressure gradients. Surfactant replacement is helpful in modulating this deleterious mechanical stimulus, but is limited in its effectiveness owing to slow surfactant adsorption. We investigate the effect of surfactant on micro-scale models of reopening by computationally modelling the steady two-dimensional motion of a semi-infinite bubble propagating through a liquid-filled compliant channel doped with soluble surfactant. Many dimensionless parameters affect reopening, but we primarily investigate how the reopening pressure depends upon the capillary number (the ratio of viscous to surface tension forces), the adsorption depth parameter (a bulk concentration parameter) and the bulk Péclet number (the ratio of bulk convection to diffusion). These studies demonstrate a dependence of on , and suggest that a critical bulk concentration must be exceeded to operate as a low-surface-tension system. Normal and tangential stress gradients remain largely unaffected by physicochemical interactions – for this reason, further biological studies are suggested that will clarify the role of wall flexibility and surfactant on the protection of the lung from atelectrauma.
Lagrangian transport properties of pulmonary interfacial flows
- Bradford J. Smith, Sarah Lukens, Eiichiro Yamaguchi, Donald P. Gaver III
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- Journal:
- Journal of Fluid Mechanics / Volume 705 / 25 August 2012
- Published online by Cambridge University Press:
- 09 November 2011, pp. 234-257
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Disease states characterized by airway fluid occlusion and pulmonary surfactant insufficiency, such as respiratory distress syndrome, have a high mortality rate. Understanding the mechanics of airway reopening, particularly involving surfactant transport, may provide an avenue to increase patient survival via optimized mechanical ventilation waveforms. We model the occluded airway as a liquid-filled rigid tube with the fluid phase displaced by a finger of air that propagates with both mean and sinusoidal velocity components. Finite-time Lyapunov exponent (FTLE) fields are employed to analyse the convective transport characteristics, taking note of Lagrangian coherent structures (LCSs) and their effects on transport. The Lagrangian perspective of these techniques reveals flow characteristics that are not readily apparent by observing Eulerian measures. These analysis techniques are applied to surfactant-free velocity fields determined computationally, with the boundary element method, and measured experimentally with micro particle image velocimetry (-PIV). We find that the LCS divides the fluid into two regimes, one advected upstream (into the thin residual film) and the other downstream ahead of the advancing bubble. At higher oscillatory frequencies particles originating immediately inside the LCS experience long residence times at the air–liquid interface, which may be conducive to surfactant transport. At high frequencies a well-mixed attractor region is identified; this volume of fluid cyclically travels along the interface and into the bulk fluid. The Lagrangian analysis is applied to velocity data measured with 0.01 mg ml−1 of the clinical pulmonary surfactant Infasurf in the bulk fluid, demonstrating flow field modifications with respect to the surfactant-free system that were not visible in the Eulerian frame.
The pulsatile propagation of a finger of air within a fluid-occluded cylindrical tube
- BRADFORD J. SMITH, DONALD P. GAVER III
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- Journal:
- Journal of Fluid Mechanics / Volume 601 / 25 April 2008
- Published online by Cambridge University Press:
- 25 April 2008, pp. 1-23
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We computationally investigate the unsteady pulsatile propagation of a finger of air through a liquid-filled cylindrical rigid tube. The flow field is governed by the unsteady capillary number CaQ(t)=μQ*(t*)/πR2γ, where R is the tube radius, Q* is the dimensional flow rate, t* is the dimensional time, μ is the viscosity, and γ is the surface tension. Pulsatility is imposed by CaQ(t) consisting of both mean (CaM) and oscillatory (CaΩ components such that CaQ(t)=CaM+CaΩ sin(Ωt). Dimensionless frequency and amplitude parameters are defined, respectively, as Ω=μωR/γ and A=2CaΩ/Ω, with Ω epresenting the frequency of oscillation. The system is accurately described by steady-state behaviour if CaΩ<CaM; however, when CaΩ>CaM, reverse flow exists during a portion of the cycle, leading to an unsteady regime. In this unsteady regime, converging and diverging surface stagnation points translate dynamically along the interface throughout the cycle and may temporarily separate to create internal stagnation points at high Ω. For CaΩ<10CaM, the bubble tip pressure drop ΔPtip may be estimated accurately from the pressure measured downstream of the bubble tip when corrections for the downstream viscous component of the pressure drop are applied. The normal stress gradient at the tube wall ∂τn/∂z is examined in detail, because this has been shown to be the primary factor responsible for mechanical damage to epithelial cells during pulmonary airway reopening (Bilek, Dee & Gaver III 2003; Kay et al. 2004). In the unsteady regime, local film-thinning produces high ∂τn/∂z at low CaΩ; however, film thickening at moderate Ca protects the tube wall from large ∂τn/∂z. This stress field is highly dynamic and exhibits intriguing spatial and temporal characteristics that may be used to reduce ventilator-induced lung injury.