If you apply an electric voltage across two water-filled beakers and separate them, something strange happens: The water stretches from beaker to beaker, creating a bridge that defies gravity. Water bridges were discovered 120 years ago, but no one has ever been sure why they do not collapse. One theory is that the voltage makes the water molecules line up, creating a “dielectric” tension that stops the bridge from falling. Another argues that surface tension—the tendency of a water’s surface to shrink inwards—keeps the bridge aloft. Now, researchers believe that water bridges rely on both strategies. Reza Namin at the Sharif University of Technology in Tehran and colleagues measured various parameters across the length of a water bridge, including voltage, current, and bridge diameter. Then they plugged the data into a computer simulation to calculate the forces involved. The results, to be published next month in Physical Review E, reveal that dielectric tension and surface tension each carry about half a water bridge’s weight. The results, the researchers believe, could help engineers develop electrowetting, a method of using electricity to adjust the adhesion of fluids to a screen that is expected to be used in the next generation of e-book readers.
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