OPPOSITES always attract, right? Not quite. A new experiment has shown that a drop of water with positive electrical charge can be made to 鈥渂ounce off鈥� a negatively charged object.
of the University of California at Davis accidentally applied a strong electric field to a beaker filled with oil and water. At first the mixture erupted into a turbulent mess, but as he turned down the voltage Ristenpart saw droplets of water suspended in the oil bouncing between the electrode at the top of the beaker and the oil-water boundary below. The droplets were positively charged, so why didn鈥檛 they merge with the negatively charged body of water?
Ristenpart set out to reproduce the happy accident, now filming with an ultra-high-speed camera. The video shows that when a droplet nears the water-oil boundary it elongates slightly, forming a tiny bridge. Ristenpart thinks that positive ions drain out of the droplet and negative electrons come in through the bridge, so the droplet, now negatively charged, is drawn up to the positive electrode, where it regains its original positive charge, and so on. The discovery could lead to new microfluidic devices and better methods for separating salt water from crude oil.
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