Forced wetting transition
B. Zhao, A. A. Pahlavan, L. Cueto-Felgueroso, and R. Juanes. Forced wetting transition and bubble pinch-off in a capillary tube. Physical Review Letters, 120:084501, 2018 (pdf).
When two fluids co-exist in a porous medium, from water and air in a kitchen sponge to water and oil in a rock, the solid will prefer to be coated by one of the two fluids rather than the other. This preference is known as wettability. Wettability is important whenever two fluids interact with a solid surface, and it is particularly important in porous media because of the large contact area between the fluids and the solid.
While wettability is strictly dependent on the properties of the fluids and the surrounding solid, the apparent wettability of the system can change due to flow. That is, a weakly wetting fluid can be "forced" to appear more wetting in the presence of flow. Here, we study the displacement of a viscous fluid by a less viscous fluid in a circular capillary tube. We show that in the partial wetting regime (i.e. contact angle greater than zero), the presence of a moving contact line induces a wetting transition at a critical capillary number (Ca) 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 de-wetting 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.
The results of this work could have important applications in many natural and industrial settings, including water infiltration into soil, ink-jet printing, and microfluidics.