Discontinuous liquid rise in capillaries with nonuniform cross-sections

Abstract

We consider theoretically liquid rise against gravity in capillaries with height-dependent cross-section. For a conical capillary made from a hydrophobic surface and dipped in a liquid reservoir, the equilibrium liquid height depends on the cone opening angle $α$, the Young-Dupré contact angle $θ$, the cone radius at the reservoir's level $R_0$ and the capillary length $κ^{-1}$. As $α$ is increased from zero, the meniscus' position changes continuously until, when $α$ attains a critical value, the meniscus jumps to the bottom of the capillary. For hydrophilic surfaces the meniscus jumps to the top. The same liquid height discontinuuity can be achieved with electrowetting with no mechanical motion. Essentially the same behavior is found for two tilted surfaces. We further consider capillaries with periodic radius modulations, and find that there are few competing minima for the meniscus location. A transition from one to another can be performed by the use of electrowetting. The phenomenon discussed here may find uses in microfluidic applications requiring the transport small amounts of water ``quanta'' (volume$<1$ nL) in a regular fashion.