chemistry : Waxy races

PHILIP BALL

Snail-racing aficionados can now switch allegiance to an animal-free micro-sport: droplet racing. Place the liquid drop of your choice on the track, and watch it race against its competitors at the awesome speed of about half a centimetre per second. Two chemists from the Massachusetts Institute of Technology now reveal how best to make this ideal, pocket-sized travelling game in the Journal of the American Chemical Society 1.

Seok-Won Lee and Paul Laibinis demonstrate how liquid droplets can be made to propel themselves along predetermined tracks -- in this case two millimetres wide, but in principle they could be much narrower. Devices like this that transport liquids in controlled ways are being explored for 'microfluidics', the production of microchip-sized chemical laboratories for the analysis of tiny samples for medical or forensic science and environmental monitoring.

Liquids in microfluidics systems can be contained by capillary forces alone: surface tension is strong enough to prevent a tiny stream of liquid from spreading out of a channel, whereas an industrial-scale flow needs to be confined in a pipe. One way of making flow channels is to coat a surface with patterned films that attract and repel the liquids. For example, a narrow channel of a water-wettable film edged on either side with water-repelling films will become a conduit down which water will flow in an elongated, bead-like droplet.

But how do you drive the flow? Some researchers are busy building miniaturized pumps for microfluidics, such as microscopic water-wheels carved out of wafers of silicon. Lee and Laibinis have a different idea: they make the droplets self-propelled.

In 1992, US scientists George Whitesides and Manoj Chaudhury made it advantageous for a water droplet to move in a predefined direction across a surface by giving the surface a gradually varying 'wettability'. The researchers coated a silicon wafer with a film of organic molecules that had sticky heads and waxy, water-repelling tails. The molecules stuck head down, and their exposed tails made the surface waterproof.

But these researchers let the film molecules find their way onto the surface by evaporating them from a bath of the stuff placed next to one edge. So the part of the surface close to the liquid had a higher concentration of water-repelling molecules than the more distant regions. A water droplet placed on a more highly coated part of the silicon wafer would therefore start to move towards the less repelling region, like a bug trying to escape from a hot plate by following the gradient of decreasing temperature. The droplet would even travel uphill if the 'wettability' gradient was strong enough to counteract gravity.

Then British chemist Colin Bain and co-workers showed in 1994 that if the film-forming molecules were dissolved in the droplet itself, they would stick to the surface tail-up at the trailing edge of a moving droplet, leaving a water-repelling trail behind them like the slime of a racing snail. This trail would then push the droplet forward, as its rear edge recoiled from the film.

Lee and Laibinis have used this same principle to propel droplets along wettable tracks on a surface covered elsewhere with a waxy film. The droplets are of an organic, but water-soluble, liquid called DNH; dissolved within them are organic molecules called amines. The amines have a 'head' group that sticks to the DNH-friendly molecules of the track, leaving a DNH-repelling tail poking up. Different kinds of amine create differing degrees of non-wettability, driving the droplets at different speeds. On a parallel track, a droplet containing a long-tailed amine overtakes a droplet with a short-tailed amine by moving at about twice the speed.

An attractive feature of this system is that the 'slime trail' of the droplets, created from the dissolved amines, is not fixed firmly to the surface, but can be easily washed off to expose the fresh tracks, ready for a new race.

1. Lee, S.-W. & Laibinis, P. E. Directed movement of liquids on patterned surfaces using noncovalent molecular adsorption. Journal of the American Chemical Society 122, 5395-5396 (2000).