Cavity optofluidics: a μdroplet's whispering-gallery mode makes a μvortex.

We experimentally demonstrate light-flow interaction, in which the angular momentum of circulating light excites micro-vortices. In contrast with the solid-phase of matter, where one has to overcome static friction in order to start motion, liquids have no "static drag." Relevant to almost all optofluidic micro-systems hence, μWatt optical power is sufficient to start flows, even in liquids 50 times more viscous than water. We map the flows to be three-dimensional (3D) by using a technique based on fluorescent nano-emitters; to reveal, as expected, flow speeds proportional to power divided by viscosity.