How a leak can stop itself

Small fluid leaks are a common - and often troublesome - occurrence 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 somewhat different questions: how might a leak stop itself? What governing physics determines the moment and mechanism of this spontaneous arrest? We report experiments that quantify the initiation and spontaneous self-arrest of leaking fluid flows emerging from a small hole in a vertical tube as a function of hole size and surface hydrophobicity. We use high-speed imaging to capture the mechanism of flow-stop, and observe that the leaking fluid undergoes a Rayleigh-Plateau-like rivulet instability leading to the creation of a final, static "capping droplet" whose surface curvature prevents further leakage. We compute the potential energy landscape of such capping droplets as a function of volume, hole size, and material properties, and combine this with existing rivulet stability theory to predict when a flow-stop transition will occur.