A particle suspended in a liquid typically jitters haphazardly about as it’s struck randomly by nearby liquid molecules. But when a temperature gradient is applied to the liquid, that random motion instead becomes directional. In a recent study, researchers directly mapped the motions underlying this thermophoresis.
In their experiment, the team placed a 7-micron sphere in water laced with 500-nanometer fluorescent tracers. Using a laser, they optically trapped the sphere, pinning it in place. Then, with a second laser, they heated the water on one side of the sphere and observed, under a microscope, what happened. After a few seconds, the tracers began moving toward the hot region, creating a slip flow along the surface of the sphere. Had the sphere been able to move freely, they found, the flow would have been strong enough to move it. (Image and research credit: T. Tsuji et al.; via APS Physics)