The drip, drip, drip of a leaky faucet might drive most people batty, but it doesn't bother physicist Song-I Han. Along with her colleagues at the Institute of Technology of the State of Nordrhein-Westfalen in Aachen, Germany, Han has taken new snapshots that reveal the elusive swirling vortices within a falling drop of water.
Scientists have long studied how liquids flow under different conditions, but problems are evasive, such as the flow within a falling drop of water. Each technique for imaging drops has its drawbacks. For instance, one can suspend a drop in a cushion of air to get a good picture, but, of course, that means the drop is no longer falling. Since falling droplets play an important part in atmospheric chemistry and other chemical processes, Han's team decided to try to capture the patterns in a moving drop.
The team used nuclear magnetic resonance (NMR), even though few scientists thought that the technology was up to the task--including Han herself. Taking a high-resolution picture with NMR is so slow compared to a plummeting drop, it takes about 30,000 exposures to make a single image, and each exposure is taken of a different drop. Han thought that the differences from drop to drop would blur the image beyond usefulness. But "it turned out that the pattern ... was always the same," she says. Thanks to the invariance of the drops, Han was able to combine the 30,000 individual snapshots into a single clear, composite picture, the team reports in the 1 October issue of Physical Review Letters. The result: an unprecedented image of the convection currents within a falling drop of water.
"I was just flabbergasted" that the strategy succeeded, says Eiichi Fukushima, a physicist at New Mexico Resonance, a nonprofit laboratory in Albuquerque. Han hopes that imaging the convection patterns in dripping water can help scientists understand chemical processes that involve drops of fluid or aerosols--and other drip-related problems that keep scientists up at night.