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 has 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 3D map the flows by using a technique based on fluorescent nano-emitters; to reveal, as expected, flow speeds proportional to power divided by viscosity.