A novel far-field nanoscopic velocimetry for nanofluidics.

For the first time we have been able to measure the flow velocity profile for nanofluidics with a spatial resolution better than 70 nm. Due to the diffraction resolution barrier, traditional optical methods have so far failed in measuring the velocity profile in a nanocapillary or a closed nanochannel without an opened sidewall. A novel optical point measurement method is presented which applies stimulated emission depletion (STED) microscopy to laser induced fluorescence photobleaching anemometer (LIFPA) techniques to measure flow velocity. Herein we demonstrate this far-field nanoscopic velocimetry method by measuring the velocity profile in a nanocapillary with an inner diameter of 360 nm. The closest measuring point to the wall is about 35 nm. This method opens up a new class of functional measuring techniques for nanofluidics and for nanoscale flows from the wall.