Bubbles to droplets

At the very moment a droplet of water breaks away from a dripping faucet, a singularity is formed. The dynamics leading up to the singularity are governed by the competition between the water’s inertia and its surface tension. (Water’s viscosity is low enough that it does not play a role.) In the reverse setup—an air bubble breaking away from an underwater nozzle—the pinch-off process is driven instead by the difference in pressure between the air and the water. As a result, the bubble and droplet systems differ both in the shapes formed and in the dependence on time. Now, Justin Burton and Peter Taborek of the University of California, Irvine, have observed both bubble-like and droplet-like behavior in a single continuously variable system: xenon bubbles in water over a range of pressures (and hence xenon densities). At low pressures, xenon bubbles behave like air bubbles, as shown in the top row of the figure. At 68 atmospheres, the highest practical pressure for the system, the xenon bubbles are 70% as dense as water and look like upside-down water droplets, as shown in the bottom row. To quantify the behavior of the Xe-water system, the researchers measured the width of the pinch-off region’s neck as a function of time before pinch off. For water droplets, the neck width is proportional to time to the 2/3 power; for air bubbles, it is proportional to time to the 0.57 power. By that standard, the researchers observed a sharp boundary between the bubble-like and droplet-like regimes at a xenon density that is 25% of the density of water. (J. C. Burton, P. Taborek, Phys. Rev. Lett. 101, 214502, 2008 .) — Johanna Miller