Zhao Benzhong, Alizadeh Pahlavan Amir, Cueto-Felgueroso Luis, Juanes Ruben
Massachusetts Institute of Technology, 77 Massachusetts Avenue, Cambridge, Massachusetts 02139, USA.
Technical University of Madrid, Calle del Profesor Aranguren 3, 28040 Madrid, Spain.
Phys Rev Lett. 2018 Feb 23;120(8):084501. doi: 10.1103/PhysRevLett.120.084501.
Immiscible fluid-fluid displacement in partial wetting continues to challenge our microscopic and macroscopic descriptions. Here, we study the displacement of a viscous fluid by a less viscous fluid in a circular capillary tube in the partial wetting regime. In contrast with the classic results for complete wetting, we show that the presence of a moving contact line induces a wetting transition at a critical capillary number that is contact angle dependent. At small displacement rates, the fluid-fluid interface deforms slightly from its equilibrium state and moves downstream at a constant velocity, without changing its shape. As the displacement rate increases, however, a wetting transition occurs: the interface becomes unstable and forms a finger that advances along the axis of the tube, leaving the contact line behind, separated from the meniscus by a macroscopic film of the viscous fluid on the tube wall. We describe the dewetting of the entrained film, and show that it universally leads to bubble pinch-off, therefore demonstrating that the hydrodynamics of contact line motion generate bubbles in microfluidic devices, even in the absence of geometric constraints.
在部分润湿情况下，不混溶的流体 - 流体驱替仍然对我们的微观和宏观描述构成挑战。在此，我们研究在部分润湿状态下，低粘性流体在圆形毛细管中对粘性流体的驱替。与完全润湿的经典结果不同，我们表明移动接触线的存在会在与接触角相关的临界毛细管数处引发润湿转变。在小驱替速率下，流体 - 流体界面从其平衡状态稍有变形，并以恒定速度向下游移动，而不改变其形状。然而，随着驱替速率增加，会发生润湿转变：界面变得不稳定并形成一个指状物，该指状物沿管轴推进，将接触线留在后面，与弯月面由管壁上的粘性流体宏观薄膜隔开。我们描述了夹带薄膜的去湿过程，并表明它普遍导致气泡 pinch - off，因此证明了接触线运动的流体动力学在微流体装置中产生气泡，即使在没有几何约束的情况下也是如此。
