A paper by three physicists from Université Bordeaux I in France describes a new and highly effective method of sorting and routing nanolitre droplets flowing through microchannels. The technique will be useful in chemical microreactors and biological assays.


These pictures superpose images captured at 100 fps to show the path of 59-μm-radius droplets that are moving at 2.2 mm/s. With no laser light, they head naturally toward the lower outlet. In the bottom picture, the laser switches the water/dye drops to the other outlet. Reprinted with permission from Applied Physics Letters.

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In the technique, water with 0.1 per cent by weight of fluorescein absorbs visible light from a Coherent argon-ion laser at 514.5 nm. The local heating induces a thermocapillary force that changes the path of the droplet. An Olympus inverted microscope and Omega dichroic mirror were used to direct the laser beam into the microchannel, and an Olympus 2.5× objective focused the beam to a waist of 10.3 μm. A high-speed CMOS camera captured images of the droplet traffic.

The scientists experimented with various beam powers and flow rates and showed that they could route droplets at 13 mm/s into the appropriate branch of a Y-junction, with a kilohertz sorting capability. The researchers intend to try infrared excitation to avoid the need for fluorescein.

(Applied Physics Letters, 17 April 2008, Vol. 92, 154105)