Water droplets that form on strongly repellent surfaces often coalesce and leap off. When they do, they carry an electric charge that can be used to control them. Evelyn Wang at the Massachusetts Institute of Technology in Cambridge and her colleagues observed that jumping droplets sometimes repel each other in the air (pictured). To learn why, they studied droplets coalescing on a superhydrophobic copper oxide nanostructure, and found that the droplets sometimes gain a small positive charge as they merge and leap into the air.
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