A FINE beam of light could push tiny liquid samples through 鈥渕icrofluidic鈥� devices, instead of the mechanical pumps or electric fields now used in these labs-on-a-chip. A laser would be a more versatile tool to pump, divide and sort single cells or viruses for analysis.
鈥淎 laser would be a more versatile tool to sort single cells for analysis in microfluidic devices鈥�
Jean-Pierre Delville from the University of Bordeaux 1 in France and colleagues passed a beam of light perpendicularly through the interface between two different liquids to create a jet of liquid that flowed along the beam, but in the opposite direction to the flow of photons. This happens because the liquids have different refractive indexes, causing the speed of the photons to change as they flow across the interface, altering their momentum. As momentum has to be conserved, the interface experiences a force in the opposite direction, creating a tiny bulge that slowly grows.
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At some point, the interface can no longer support the pressure in the ever-increasing bulge, and a jet of fluid bursts through. As it does so, the light scatters, changing the momentum in such a way that it causes the fluid to flow towards the source of the beam, travelling up to 300 micrometres.
The team then used a second beam of light to manipulate the droplets of the fluid jet once they were moving. For example, the heat from the second beam could be used to increase the surface tension on only one side of the droplets, causing them to move horizontally away from the beam. This can also be used to stop and start a flow, or split and merge droplets, a technique that they say could be helpful when trying to control liquid flow in the tiny channels of a lab-on-a-chip (Journal of Optics A: Pure and Applied Optics, ).
As laser beams can be easily focused, switched on and off rapidly, and reflected around the chip, the team believe that researchers would be able to alter the flow of liquid very precisely.
David Erickson, an expert in microfluidics at Cornell University in Ithaca, New York, thinks the technique is more precise and flexible than current methods. However, he warns that the heat generated by the laser beam could damage some biological samples.
