SMALL sensors powered only by sunlight could float in the sky indefinitely and make weather-related measurements. Flat objects with two sides can be levitated by what is called photophoresis. This occurs when one of the sides absorbs lots of light and the other very little, creating a difference in temperature. Just like how temperature differences in the atmosphere cause winds, this temperature gradient makes molecules move in such a way as to create a lifting force on the object. Benjamin Schafer at Harvard University and his colleagues designed a device that could use photophoresis to levitate small atmospheric sensors that wouldn't need motors or batteries to stay aloft. While photophoretic devices have been tested before, the researchers designed a larger version optimised for carrying sensors.
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