Experiment on gas-liquid displacement in a capillary

毛细管中气液驱替现象的实验研究

It has been reported in recent experimental and numerical studies that the forced gas-liquid displacement in a partially wettable capillary tube can give rise to entrainment of a liquid film and generation of Taylor bubbles at a large displacement rate.However,simulations and hydrodynamic theory of Gao et al.(J.Fluid Meeh.,2019,vol 859:308-320)predicted an abrupt drop of the contact line velocity Ca_(cl)at the onset of wetting transition,which conflicted with the experiments of Zhao et al.(Phys.Rev.Lett.,2018,vol 120:084501),who suggested a continuous variation of Ca_(cl).To resolve this discrepancy,we performed experiments of gas-liquid displacement,focusing on the contact line velocity close to the threshold.It is found that Ca_(cl) is indeed discontinuous,confirming the validity of the hydrodynamic description of the moving contact line.After an abrupt drop,Ca_(cl)increases to a constant for a slight raise of the displacement rate.The failure of detecting this discontinuity in previous experiments is due to the low resolution of the displacement rate.The propagating velocity and the length of the generated bubbles are presented and agree well with the existing theory.Variation of the liquid slug length with the displacement rate is also presented.

近期关于气液驱替的实验和数值研究表明,在不完全浸润和大驱替速度条件下可在毛细管壁面上产生液膜并最终演化为泰勒气泡.基于数值模拟和流体动力学理论,Gao等人证明接触线速度Ca_(cl)在这种润湿状态转换的临界条件附近会出现突降.这种突降在Zhao等人的实验中并未被发现,因此他们认为Ca_(cl)是连续变化的.为了解决这种矛盾,我们重新开展了毛细管中的气液驱替实验,重点关注临界条件附近的接触线速度变化.我们发现Ca_(cl)确实具有不连续性,证实了采用流体动力学理论描述移动接触线的有效性.在突降发生后,Ca_(cl)会随着驱替速度的增加迅速上升到一个常值,在前人实验中,由于驱替速度的分辨率较低,未能检测到这种不连续性.我们给出了气泡运动速度和长度的变化规律,发现与现有理论非常吻合.此外,还研究了气泡间的液塞长度随驱替速度的变化.