A MICROSCOPIC pump powered by a swirling beam of light could be just the thing to move fluids around tiny chemical labs built on silicon chips.
Researchers have known for about a decade that an ordinary laser beam can grab a tiny object, less than a few micrometres across, and hold it in an 鈥渙ptical trap鈥�. The object, such as a red blood cell, is ensnared by the beam鈥檚 electric field, which is greatest at the centre.
Biologists routinely use optical traps to grab and isolate particular cells from mixtures. And a few years ago they found that if the laser energy is set to spiral through the air rather than travel along the beam鈥檚 axis, the object trapped in the beam spins (快猫短视频, 14 February 1998, p 34).
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Now David Grier and Jennifer Curtis at the University of Chicago in Illinois have found a way of controlling tiny spheres so they race round in circles rather than simply spinning on the spot. This behaviour has been harnessed to make a laser-powered microscopic pump that doesn鈥檛 require a mechanical drive.
Their device consists of two parallel rows of polystyrene spheres just a micrometre across, separated by a narrow channel 100 micrometres long and about 50 micrometres wide (see Diagram). To make the laser beams spiral, they are passed through a filter made from a sheet of liquid crystals divided up into a square array of over 200,000 individually programmable pixels.
An electric field is used to adjust the alignment of the crystals in each pixel, which introduces a tiny shift in the phase of the photons passing through. If you give each photon a slightly different phase to its neighbour the beam will move like a corkscrew.
鈥淭he planar wave comes out looking like a spiral ramp in a car park,鈥� says Grier. Because the photons transfer some of their momentum to the spheres, the spheres are driven round in a small circle, each sphere spinning in its own beam of light.
Grier and Curtis were also able to systematically vary the twist in the helical light, giving them precise control over an individual sphere鈥檚 speed and circular path (Physical Review Letters, vol 90, p 133901). And by controlling the direction in which the laser beams spiral, they can make the spheres on one side of the channel turn anticlockwise while those on the other side turn clockwise. This movement forces fluid to flow through the channel, as seen by the team when they watched molecules of dye move through the pump under a microscope.
Grier and Curtis have also built a microscopic switch by arranging four spheres at the corners of a square glass slide 20 micrometres across. Depending on which way the spheres are driven, the switch can pump fluid in or out of the four channels leading to the centre of the switch. The result is a junction that can redirect flow in up to 12 different ways.
The tiny light pump and switch could be perfect for moving fluids around the tiny lab-on-a-chip devices that are being developed for high-speed chemical and biochemical analysis.
鈥淲e are taking the motors out of microfluidics and replacing them with beams of light,鈥� says Grier. However, the pump can鈥檛 yet move fluids very fast. As the spheres rotate, small variations in their paths make them bump against each other and at high speeds they 鈥渟tall鈥�.
This limits the fluid flow to about 10 micrometres a second. Grier is confident that he can reduce bumping by swapping to a higher-resolution liquid-crystal filter. Until then it will be difficult to tell whether optically driven pumps will prove practical for microfluidic devices, says Michael Gaitan, head of microelectromechanical systems at the US National Institute of Standards and Technology.