Researchers at MIT recently found an elegant solution to a sticky scientific problem in basic fluid mechanics: why water doesn’t soak into soil at an even rate, but instead forms what look like fingers of fluid flowing downward. Scientists call these rivulets “gravity fingers,” and the explanation for their formation has to do with the surface tension where the water—or any liquid—meets the soil (or other medium). Knowing how to account for this phenomenon mathematically will have wide-ranging impact on science problems and engineering applications, including the recovery of oil from reservoirs and the sequestration of carbon underground. The solution reported in the Dec. 12 issue of Physical Review Letters by co-authors Luis Cueto-Felgueroso and Ruben Juanes involves borrowing a mathematical phrase, if you will, from the mathematical description of a similar problem, a solution both simple and elegant that had escaped the notice of many researchers in earlier attempts to describe the phenomenon. Read more.