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Dynamics of a liquid plug in a capillary tube under cyclic forcing: memory effects and airway reopening
Published online by Cambridge University Press: 12 January 2018
Abstract
In this paper, we investigate both experimentally and theoretically the dynamics of a liquid plug driven by a cyclic periodic forcing inside a cylindrical rigid capillary tube. First, it is shown that, depending on the type of forcing (flow rate or pressure cycle), the dynamics of the liquid plug can either be stable and periodic, or conversely accelerative and eventually leading to plug rupture. In the latter case, we identify the sources of the instability as: (i) the cyclic diminution of the plug viscous resistance to motion due to the decrease in the plug length and (ii) a cyclic reduction of the plug interfacial resistance due to a lubrication effect. Since the flow is quasi-static and the forcing periodic, this cyclic evolution of the resistances relies on the existence of flow memories stored in the length of the plug and the thickness of the trailing film. Second, we show that, contrary to unidirectional pressure forcing, cyclic forcing enables breaking of large plugs in a confined space although it requires longer times. All the experimentally observed tendencies are quantitatively recovered from an analytical model. This study not only reveals the underlying physics but also opens up the prospect for the simulation of ‘breathing’ of liquid plugs in complex geometries and the determination of optimal cycles for obstructed airways reopening.
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- © 2018 Cambridge University Press
References
Signe Mamba et al. supplementary movie 1
Movie showing the temporal evolution of a single liquid plug of initial length $L_0=1.05mm $ pushed with the cyclic flow rate forcing in a cylindrical capillary tube of radius $R_c = 0.47mm$. The movie was shot with a Photron SA3 high speed camera mounted on a Z16 Leica Microscope at a frame rate of $125$ images per second, a trigger time of $1/3000 s$ and a resolution of $1024 \times 64$ pixels. a. Initial state b. Final state.
Signe Mamba et al. supplementary movie 2
Movie showing the temporal evolution of a single liquid plug of initial length $L_0 = 3.39mm $ pushed with the pressure cyclic forcing in a cylindrical capillary tube of radius $R_c = 0.47mm$. The movie was shot with a Photron SA3 high speed camera mounted on a Z16 Leica Microscope at a frame rate of $125$ images per second, a trigger time of $1/3000 s$ and a resolution of $1024 \times 64$ pixels a. Initial state b. End of the cycle there is no more plug and only liquid remains on the walls of the capillary tube.
Signe Mamba et al. supplementary movie 3
Movie showing the temporal evolutions of a single liquid plug of initial length $L1=2.5mm$ pushed with the pressure cyclic forcing in a cylindrical capillary tube of radius $R_c = 0.47mm$. The movie was shot with a Photron SA3 high speed camera mounted on a Z16 Leica Microscope at a frame rate of $125$ images per second, a trigger time of $1/3000 s$ and a resolution of $1024 \times 64$ pixels a. Initial state b. Final state.
Signe Mamba et al. supplementary movie 4
Movie showing the temporal evolutions of a single liquid plug of initial length $L2=2.85mm$ pushed with the pressure cyclic forcing in a cylindrical capillary tube of radius $R_c = 0.47mm$. In the same cycle, the rupture length of the smaller plug in movie S3 is higher that the one for the bigger plug. The movie was shot with a Photron SA3 high speed camera mounted on a Z16 Leica Microscope at a frame rate of $125$ images per second, a trigger time of $1/3000 s$ and a resolution of $1024 imes 64$ pixels a. Initial state b. Final state.
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