Air Finger Research Articles

Oscillatory bubbles induced by geometrical constraint

We show that a simple change in pore geometry can radically alter the behavior of a fluid-displacing air finger, indicating that models based on idealized pore geometries fail to capture key features of complex practical flows. In particular, partial occlusion of a rectangular cross section can force a transition from a steadily propagating centered finger to a state that exhibits spatial oscillations formed by periodic sideways motion of the interface at a fixed distance behind the moving finger tip. We characterize the dynamics of the oscillations, which suggest that they arise from a global homoclinic connection between the stable and unstable manifolds of a steady, symmetry-broken solution.

Lagrangian transport properties of pulmonary interfacial flows

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 (μ-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.

Epithelium damage and protection during reopening of occluded airways in a physiologic microfluidic pulmonary airway model

Airways of the peripheral lung are prone to closure at low lung volumes. Deficiency or dysfunction of pulmonary surfactant during various lung diseases compounds this event by destabilizing the liquid lining of small airways and giving rise to occluding liquid plugs in airways. Propagation of liquid plugs in airways during inflation of the lung exerts large mechanical forces on airway cells. We describe a microfluidic model of small airways of the lung that mimics airway architecture, recreates physiologic levels of pulmonary pressures, and allows studying cellular response to repeated liquid plug propagation events. Substantial cellular injury happens due to the propagation of liquid plugs devoid of surfactant. We show that addition of a physiologic concentration of a clinical surfactant, Survanta, to propagating liquid plugs protects the epithelium and significantly reduces cell death. Although the protective role of surfactants has been demonstrated in models of a propagating air finger in liquid-filled airways, this is the first time to study the protective role of surfactants in liquid plugs where fluid mechanical stresses are expected to be higher than in air fingers. Our parallel computational simulations revealed a significant decrease in mechanical forces in the presence of surfactant, confirming the experimental observations. The results support the practice of providing exogenous surfactant to patients in certain clinical settings as a protective mechanism against pathologic flows. More importantly, this platform provides a useful model to investigate various surface tension-mediated lung diseases at the cellular level.

Front Self-Correction for In-Situ Combustion

Summary Since it was discovered in the early 1920s, several improvements have been suggested for in-situ combustion (ISC) technology. Top-down combustion displacement is a process proposed for application with horizontal well technologies. In this configuration, air is injected near the top of the reservoir, close to the overburden, through a horizontal well or a number of vertical wells. Fluids are produced from the reservoir through horizontal wells placed near the bottom of the reservoir. In this configuration, the combustion front moves downward with gravity and displaces fluids to the production well. Combustion front formation and its stability are major concerns in this arrangement. While the short distance between injector and producer addresses the cold oil blocking problem in the reservoir, it has the potential to create air fingers, which could prevent stable advance of the combustion zone in a horizontal plane. Front self-correction must occur to prevent oxygen from fingering to the production wells. To prove this idea, experiments at reservoir pressure were carried in a 2D SAGD model with Athabasca bitumen. Air was injected into the SAGD steam chamber from the top, and fluids were produced from the bottom. Front advancing directions were monitored by a series of thermocouples located inside the model. It was found that the combustion zone starts spreading horizontally in the model until it reaches a limiting point, then it starts moving downward as a horizontal front. Despite heterogeneity of the oil saturation and relative permeability in the steam chamber and unswept zones, the combustion front moved almost as a horizontal plane and no sharp fingers were recorded in the experiments. These experiments show that stable top-down combustion is feasible.

When smokers move out and non-smokers move in: residential thirdhand smoke pollution and exposure.

This study examined whether thirdhand smoke (THS) persists in smokers' homes after they move out and non-smokers move in, and whether new non-smoking residents are exposed to THS in these homes. The homes of 100 smokers and 50 non-smokers were visited before the residents moved out. Dust, surfaces, air and participants' fingers were measured for nicotine and children's urine samples were analysed for cotinine. The new residents who moved into these homes were recruited if they were non-smokers. Dust, surfaces, air and new residents' fingers were examined for nicotine in 25 former smoker and 16 former non-smoker homes. A urine sample was collected from the youngest resident. Smoker homes' dust, surface and air nicotine levels decreased after the change of occupancy (p<0.001); however dust and surfaces showed higher contamination levels in former smoker homes than former non-smoker homes (p<0.05). Non-smoking participants' finger nicotine was higher in former smoker homes compared to former non-smoker homes (p<0.05). Finger nicotine levels among non-smokers living in former smoker homes were significantly correlated with dust and surface nicotine and urine cotinine. These findings indicate that THS accumulates in smokers' homes and persists when smokers move out even after homes remain vacant for 2 months and are cleaned and prepared for new residents. When non-smokers move into homes formerly occupied by smokers, they encounter indoor environments with THS polluted surfaces and dust. Results suggest that non-smokers living in former smoker homes are exposed to THS in dust and on surfaces.

Measurement of the receding contact angle at the interface between a viscoelastic material and a rigid surface

We present the first quantitative measurement of the receding contact angle between a soft viscoelastic material and a solid surface during the debonding of the material. We developed a new method of observation, based on the technique pioneered by Yamaguchi et al. (T. Yamaguchi, K. Koike, and M. Doi, Euro. Phys. Lett., 2007, 77, 64002). In a probe-tack geometry, we obtained a 3D-like view of the debonding pattern as seen through the adhesive/probe interface, providing thus an excellent view of the details of the contact geometry near the interface. Combining visual and force-displacement information in a probe tack test, we identified three different categories of mechanisms and contact geometries as a function of the material parameters. For very weakly crosslinked viscoelastic materials around the gel point, air fingers form in the bulk of the layer. For higher degrees of crosslinking, the air fingers form and propagate at the interface, with a large deformation of the bulk of the layer. A well defined and reproducible receding contact angle φ < 90° depending on the viscoelastic properties of the layer was observed at the leading edge of the moving finger. Finally for well crosslinked samples the failure occurs by interfacial crack propagation with little deformation of the layer; the contact angle at the leading edge then was close to 90° as expected from elastic fracture mechanics.

Microbial contamination of syringes during preparation: The direct influence of environmental cleanliness and risk manipulations on end-product quality

The direct influence of environmental cleanliness and risk manipulations on prepared syringes was evaluated. Media-fill testing was used to estimate potential microbial contamination. Syringes were prepared in three different environments using four different uncontrolled high-risk manipulations. The three environments included an International Organization for Standardization (ISO) class 5 horizontal laminar-airflow hood in an ISO class 6 cleanroom (in accordance with United States Pharmacopeia [USP] chapter 797), an ISO class 7 drug preparation area of an operating room, and an uncontrolled decentralized pharmacy in a ward. For each combination of environment and manipulation, 100 syringes were filled by a single operator. The four high-risk manipulations used included simple filling of syringes with trypticase soy broth, three-second contact by the ungloved fingers of the operator with the hub of the syringe, three-second contact between an object and the hub of the syringe, and exposure of the filled syringes to ambient air for 10 minutes. Of the 1500 syringes prepared in three different environments, none produced within the cleanroom contained microorganisms, 6% were contaminated in the operating room, and 16% were contaminated in the ward (p < 0.0001). Certain high-risk manipulations were associated with a significant increase in the contamination of the surrogate syringes, including exposure to nonsterile ambient air and nonsterile objects or fingers (p < 0.0001). High contamination rates were measured when the hub of syringes touched nonsterile environmental surfaces and fingers, whereas the drawn-air manipulation was associated with a lower risk of contamination. Working within a properly operating unidirectional airflow primary engineering control in an ISO class 5 cleanroom in accordance with USP chapter 797 requirements was demonstrated to be the best way to avoid bacterial or fungal contamination of injectable drugs directly resulting in patient infections.

Physicochemical Effects Enhance Surfactant Transport in Pulsatile Motion of a Semi-Infinite Bubble

In this study, we investigate the sorption of pulmonary surfactant (Infasurf, Ony, Buffalo, NY) occurring at the air-liquid interface of a semi-infinite finger of air as it oscillates and progresses along a small rigid tube ( 1 mm inner diameter ) occluded with a surfactant-doped solution of concentrations C = 0.1 , 0.05 , or 0.01 mg/mL . This simple experimental model of pulmonary airway reopening is designed to examine how altering the fluid flow field may lower reopening pressures and lead to a reduction in airway wall damage that is associated with the mechanical ventilation of an obstructed pulmonary system in airways of the deep lung with depleted endogenous and little exogenous surfactant. We analyzed a range of pulsatile flow scenarios by varying the oscillation frequency ( 0 ≤ f ≤ 1 Hz ), the oscillation flow waveform, and the steady flow rate ( Q steady = 0.1 or 0.01 mL / min ). These experimental studies indicate that a high frequency (1 Hz, amplitude = 5 mm), fast-forward oscillation waveform superimposed onto a fast steady flow ( 0.1 mL / min ) substantially reduces mean reopening pressures (31%) as a consequence of the modified flow field and the commensurate increase in surfactant transport and adsorption. This result suggests that imposing high frequency, low amplitude oscillations during airway reopening will help to diminish ventilator-induced lung injury.

The steady propagation of an air finger into a rectangular tube

The steady propagation of an air finger into a fluid-filled tube of uniform rectangular cross-section is investigated. This paper is primarily focused on the influence of the aspect ratio, α, on the flow properties, but the effects of a transverse gravitational field are also considered. The three-dimensional interfacial problem is solved numerically using the object-oriented multi-physics finite-element library oomph-lib and the results agree with our previous experimental results (de Lózar et al. Phys. Rev. Lett. vol. 99, 2007, article 234501) to within the ±1% experimental error.At a fixed capillary number Ca (ratio of viscous to surface-tension forces) the pressure drops across the finger tip and relative finger widths decrease with increasing α. The dependence of the wet fraction m (the relative quantity of liquid that remains on the tube walls after the propagation of the finger) is more complicated: m decreases with increasing α for low Ca but it increases with α at high Ca. Our results also indicate that the system is approximately quasi-two-dimensional for α ≥ 8, when we obtain quantitative agreement with McLean &amp; Saffman's two-dimensional model for the relative finger width as a function of the governing parameter 1/B = 12α2Ca. The action of gravity causes an increase in the pressure drops, finger widths and wet fractions at fixed capillary number. In particular, when the Bond number (ratio of gravitational to surface-tension forces) is greater than one the finger lifts off the bottom wall of the tube leading to dramatic increases in the finger width and wet fraction at a given Ca.For α ≥ 3 a previously unobserved flow regime has been identified in which a small recirculation flow is situated in front of the finger tip, shielding it from any contaminants in the flow. In addition, for α ≳ 2 the capillary number, Cac, above which global recirculation flows disappear has been observed to follow the simple empirical law: Cac2/3α = 1.21.

The influence of gravity on the steady propagation of a semi-infinite bubble into a flexible channel

Motivated by discrepancies between recent bench-top experiments [A. Juel and A. Heap, J. Fluid Mech. 572, 287 (2007)] and numerical simulations [A. L. Hazel and M. Heil, ASME J. Biomech. Eng. 128, 573 (2006)] we employ computational methods to examine the effects of transverse gravity on the steady propagation of a semi-infinite, inviscid air finger into a two-dimensional elastic channel filled with a Newtonian fluid. The special case of propagation in a rigid channel is also discussed in Appendix B. The coupled free-surface, fluid-structure-interaction problem is solved numerically using the object-oriented multiphysics finite-element library OOMPH-LIB. In the absence of gravity the relationship between the applied pressure and the propagation speed of the finger is nonmonotonic, with a turning point at small values of the propagation speed. We demonstrate that the turning point disappears when a modest gravitational force is applied and conjecture that it is this effect of gravity rather than any instability of the zero-gravity solution, as postulated in previous studies, that explains why the turning point has never been observed in experiments. At large propagation speeds, the presence of transverse gravity is shown to increase the pressure required to drive the air finger at a given speed, which is consistent with the observed discrepancies between previous zero-gravity simulations and the experimental results. Finally, we briefly discuss the possible implications of our results for the physiological problem of pulmonary airway reopening.

Anomalous bubble propagation in elastic tubes

Airway reopening is an important physiological event, as exemplified by the first breath of an infant that inflates highly collapsed airways by driving a finger of air through its fluid-filled lungs. Whereas fundamental models of airway reopening predict the steady propagation of only one type of bubble with a characteristic rounded tip, our experiments reveal a surprising selection of novel bubbles with counterintuitive shapes that reopen strongly collapsed, liquid-filled elastic tubes. Our multiple bubbles are associated with a discontinuous relationship between bubble pressure and speed that sets exciting challenges for modelers.

The pulsatile propagation of a finger of air within a fluid-occluded cylindrical tube

We computationally investigate the unsteady pulsatile propagation of a finger of air through a liquid-filled cylindrical rigid tube using a combined boundary element method and lubrication theory approach. The flow-field is governed by the dimensionless parameters Ca(Q)(t) = Ca(M) + Ca(Omega) sin(Omegat) = muQ*(t*)/piR(2)gamma, Omega = muomegaR/gamma and A = 2Ca(Omega)/Omega. Here, Ca(Q)(t) consists of both mean (Ca(M)) and oscillatory (Ca(Omega)) components. It is shown that the behavior of this system is appropriately described by steady-state responses until the onset of reverse flow, wherein the system operates in the unsteady regime (Ca(Omega) > Ca(M)). When flows in this regime are considered, converging and diverging stagnation points move dynamically throughout the cycle and may temporarily separate from the interface at high Omega. We have also found that for Ca(Omega) < 10Ca(M) the bubble tip pressure drop DeltaP(tip) may be estimated accurately from the pressure measured downstream of the bubble tip when corrections for the pressure drop due to Poiseuille flow are applied. The normal stress gradient at the tube wall ( partial differentialtau(n)/ partial differentialz) is discussed in detail, as this is believed to be the primary factor in airway epithelial cell damage (Bilek et al 2003). In the unsteady regime we find that local film-thinning produces high partial differentialtau(n)/ partial differentialz at low Ca(Omega). Film thickening at moderate Ca(Omega) in the unsteady regime protects the tube wall from the large gradients near the bubble tip, therefore reducing partial differentialtau(n)/ partial differentialz. We find that the stress field is highly dynamic and exhibits intriguing spatial and temporal characteristics that may be of interest to our field of study, pulmonary airway reopening.

The reopening of a collapsed fluid-filled elastic tube

We present an experimental study of the reopening mechanics of a collapsed liquid-filled elastic tube. The experiment is a simple mechanical model of pulmonary airway reopening and aims to assess the robustness of existing theoretical models. A metre-long horizontal elastic tube of inner radius Ri=4.88 ± 0.14mm is filled with silicone oil and is carefully collapsed mechanically. The injection of nitrogen at a constant flow rate results in the steady propagation of an air finger, after the decay of initial transients. This behaviour is observed over the realizable range of the capillary numbers Ca, which measures the ratio of viscous and capillary forces. With increasing Ca, the transition region between the collapsed and reopened sections of the tube shortens, and the height of the tube behind the bubble tip increases. We also find that air fingers can propagate in partially reopened tubes, in which the transmural pressure is negative far behind the finger tip.The effect of viscosity on the reopening dynamics was explored by performing experiments using three different grades of silicone oil, with kinematic viscosities of 1000cS, 200cS and 100cS. A direct comparison between the experimental pressure dependence on Ca and numerical simulations of the zero-gravity three-dimensional airway-reopening model of Hazel &amp; Heil (Trans. ASME: J. Biomech. Engng, vol. 128, 2006, p. 473) highlights some significant differences. Within the experimental parameter range, gravity profoundly influences the reopening mechanics in several ways. The reopening tube is supported by a rigid base, which induces an asymmetry about the horizontal mid-plane of the collapsed tube, resulting in distinct phases of reopening as Ca increases. In addition, buoyancy forces act on the air finger, which is observed to propagate near the top of the cross-section of the tube, leaving a thicker fluid-lining below. In the limit of small Ca, the height of the reopened tube increases significantly with viscosity. Experimental evidence suggests that this increase in viscosity leads to significant changes in the film configuration behind the propagating finger, caused by the increased contribution of buoyancy forces. The altered film configuration changes the mechanical load on the tube walls and, hence, the shape of the reopened tube.

Finite-Reynolds-Number Effects in Steady, Three-Dimensional Airway Reopening

Motivated by the physiological problem of pulmonary airway reopening, we study the steady propagation of an air finger into a buckled elastic tube, initially filled with viscous fluid. The system is modeled using geometrically non-linear, Kirchhoff-Love shell theory, coupled to the free-surface Navier-Stokes equations. The resulting three-dimensional, fluid-structure-interaction problem is solved numerically by a fully coupled finite element method. Our study focuses on the effects of fluid inertia, which has been neglected in most previous studies. The importance of inertial forces is characterized by the ratio of the Reynolds and capillary numbers, ReCa, a material parameter. Fluid inertia has a significant effect on the system's behavior, even at relatively small values of ReCa. In particular, compared to the case of zero Reynolds number, fluid inertia causes a significant increase in the pressure required to drive the air finger at a given speed.

The steady propagation of an air finger into a partially collapsed, fluid-filled, elastic tube

Es soll herausgearbeitet werden, wie mechanische Krafteinwirkung, der der gesamte Organismus ausgesetzt ist, auf individuelle Zellen fortgeleitet und in eine biomechanische Reaktion umgewandelt wird.Es werden fundamentale Aufbauprinzipien beschrieben, die der Stabilisierung des muskuloskelettalen Systems in vielen Größenordnungen dienen, und es soll gezeigt werden, dass diese Kon-struktionsmerkmale in einer bestimmten Architekturform verankert sind, die als Tensegrity bekannt ist.Tensegrity-Strukturen sind durch kontinuierliche Zugkräfte und lokale Kompression gekennzeichnet; Architektur, Vorspannung (interne Krafteinwirkung vor Anlage einer externen Kraft) und Triangulierung spielen für die Bestimmung der mechanischen Stabilität die entscheidende Rolle. In lebenden Organismen wird mittels einer Hierarchie von Tensegrity-Netzwerken sowohl die strukturelle Effizienz optimiert als auch ein Mechanismus bereitgestellt, um die Teile mit dem Ganzen zu verknüpfen: Mechanische Krafteinwirkung auf der Makroebene resultiert in strukturellen Neuanordnungen auf zellulärer und molekularer Ebene.Aufgrund der Tensegrity-Architektur wird mechanische Krafteinwirkung auf signalumwandelnde Moleküle konzentriert und umgewandelt, die physikalisch mit Zelloberflächenmolekülen, welche Zellen an der extrazellulären Matrix verankern – z.B. Integrine – sowie mit Last tragenden Elementen im inneren Zellskelett und Nukleus assoziiert sind. Die mechanochemische Umwandlung könnte demnach über lokale, spannungsabhängige Veränderungen der molekularen Mechanik, Thermodynamik und Kinetik innerhalb der Zelle ablaufen. Auf diese Weise könnte die gesamte zelluläre Reaktion auf die Krafteinwirkung durch Abänderung der Vorspannung in der Zelle orchestriert und abgestimmt werden, so wie auch eine Veränderung des Muskeltonus die mechanische Stabilität und strukturelle Koordination im gesamten muskuloskelettalen System verändern kann.To elucidate how mechanical stresses that are applied to the whole organism are transmitted to individual cells and transduced into a biochemical response.In this article, we describe fundamental design principles that are used to stabilize the musculoskeletal system at many different size scales and show that these design features are embodied in one particular form of architecture that is known as tensegrity.Tensegrity structures are characterized by use of continuous tension and local compression; architecture, prestress (internal stress prior to application of external force), and triangulation play the most critical roles in terms of determining their mechanical stability. In living organisms, use of a hierarchy of tensegrity networks both optimizes structural efficiency and provides a mechanism to mechanically couple the parts with the whole: mechanical stresses applied at the macroscale result in structural rearrangements at the cell and molecular level.Due to use of tensegrity architecture, mechanical stress is concentrated and focused on signal transducing molecules that physically associate with cell surface molecules that anchor cells to extracellular matrix, such as integrins, and with load-bearing elements within the internal cytoskeleton and nucleus. Mechanochemical transduction may then proceed through local stress-dependent changes in molecular mechanics, thermodynamics, and kinetics within the cell. In this manner, the entire cellular response to stress may be orchestrated and tuned by altering the prestress in the cell, just as changing muscular tone can alter mechanical stability and structural coordination throughout the whole musculoskeletal system.

Airway reopening: oscillating air finger propagation through a liquid-filled tube

Airway reopening: oscillating air finger propagation through a liquid-filled tube

Quasi-static liquid–air drainage in narrow channels with variations in the gap

This paper studies the shape of an air bubble quasi-statically flowing in the longitudinal direction of narrow channels. Two bottom topographies are treated, i.e., linear and quadratic variations of the gap along the transverse direction. This work analyses the main characteristics of the gas–liquid interface with respect to the wedge aspect ratio. From the convergence of asymptotic, numerical and experimental analyses, we found simple dependences for the finger width and total curvature as a function of channel aspect ratio. These results provide simple and general expressions for the pressure drop needed to overcome capillary forces and push the air finger inside the channel.

Stretch flow of thin layers of Newtonian liquids: Fingering patterns and lifting forces

We study the stretch flow of a thin layer of Newtonian liquid constrained between two circular plates. The evolution of the interface of the originally circular bubble is studied when lifting one of the plates at a constant velocity and the observed pattern is related to the measured lifting force. By comparing experimental results to numerical simulations using a Darcy’s law model we can account for the fully nonlinear evolution of the observed fingering pattern. One observes an initial destabilization of the interface by growth of air fingers due to a Saffman–Taylor-like instability and then a coarsening of the pattern toward a circular interface until complete debonding of the two plates occurs. Numerical simulations reveal that when relating the observed patterns to the lifting force not only the number of fingers but also the amplitude of the fingering growth has to be taken into account. This is consistent with the experimental observations.

Optimization of an air cushion vehicle bag and finger skirt using genetic algorithms

The air cushion vehicle (ACV) is the advanced marine vehicle that provides not only excellent performance on rough surfaces but also the high speed that other conventional marine vehicles could not achieve. Hence, a large number of ACVs have been utilized for various purposes and missions. At the present the flexible skirt system, the bag and finger skirt, is recognized as the most updated and advanced skirt system. In this paper, the ACV's bag and finger skirt system geometry is optimized to modify an undesirable heave response for the better ride quality by using the Genetic Algorithm (GA). The pure heave motion of a two dimensional section of the bag and finger skirt system is investigated using a mathematical model. Two configurations representative of a 37 ton vehicle used by Canadian Coast Guard (CCG) and 150 ton vehicle used by US Navy show a resonance associated with the skirt mass at frequencies at which humans are most sensitive. New skirt geometry obtained by GA considerably ameliorates this effect, and the ride comfort of the optimized ACV can be greatly improved compare to the original configuration within the given constraints such as the required vehicle's hard structure dimensions and weight.

Three-dimensional airway reopening: the steady propagation of a semi-infinite bubble into a buckled elastic tube

We consider the steady propagation of an air finger into a buckled elastic tube initially filled with viscous fluid. This study is motivated by the physiological problem of pulmonary airway reopening. The system is modelled using geometrically nonlinear Kirchhoff–Love shell theory coupled to the free-surface Stokes equations. The resulting three-dimensional fluid–structure-interaction problem is solved numerically by a fully coupled finite element method.The system is governed by three dimensionless parameters: (i) the capillary number, Ca=μU/σ*, represents the ratio of viscous to surface-tension forces, where μ is the fluid viscosity, U is the finger's propagation speed and σ* is the surface tension at the air–liquid interface; (ii) σ=σ*/(RK) represents the ratio of surface tension to elastic forces, where R is the undeformed radius of the tube and K its bending modulus; and (iii) A∞=A*∞/(4R2), characterizes the initial degree of tube collapse, where A*∞ is the cross-sectional area of the tube far ahead of the bubble.The generic behaviour of the system is found to be very similar to that observed in previous two-dimensional models (Gaver et al. 1996; Heil 2000). In particular, we find a two-branch behaviour in the relationship between dimensionless propagation speed, Ca, and dimensionless bubble pressure, p*b/(σ*/R). At low Ca, a decrease in p*b is required to increase the propagation speed. We present a simple model that explains this behaviour and why it occurs in both two and three dimensions. At high Ca, p*b increases monotonically with propagation speed and p*b/(σ*/R) ∝ Ca for sufficiently large values of σ and Ca. In a frame of reference moving with the finger velocity, an open vortex develops ahead of the bubble tip at low Ca, but as Ca increases, the flow topology changes and the vortex disappears.An increase in dimensional surface tension, σ*, causes an increase in the bubble pressure required to drive the air finger at a given speed; p*b also increases with A*∞ and higher bubble pressures are required to open less strongly buckled tubes. This unexpected finding could have important physiological ramifications. If σ* is sufficiently small, steady airway reopening can occur when the bubble pressure is lower than the external (pleural) pressure, in which case the airway remains buckled (non-axisymmetric) after the passage of the air finger. Furthermore, we find that the maximum wall shear stresses exerted on the airways during reopening may be large enough to damage the lung tissue.