How a leak can stop itself*

Small fluid leaks are common—and often troublesome—occurrences in everyday life.  We often consider what action is required to stop a leak, or to prevent one from starting in the first place, but here we consider a somewhat different question: how can a leak stop itself? We perform simple experiments characterizing the leakage of fluid from a small circular hole in a vertical tube. We observe flow transitions from jetting to rivulet flow to dripping to spontaneous arrest with decreasing driving pressure. Using high-speed imaging, we capture the mechanism of flow-stop, and observe that the leaking fluid undergoes a series of breakup instabilities leading to the creation of a final, static "capping droplet" whose surface curvature prevents further leakage. Measurements of attempted capping events reveal that the equilibrating droplets behave as lightly damped harmonic oscillators. Combining our experiments with an energetic analysis, we find that a leak can stop itself by generating a spherical cap of fluid whose total energy is less than the potential barrier at that driving pressure and fluid contact geometry.