MIXING two liquids together might sound simple, but for engineers designing a new generation of microchip-sized laboratories this apparently straightforward task has become a major headache. However a new technique offers a simple, cheap and elegant solution.
Fluids behave differently on a micro scale: the absence of convection in minuscule pipes dramatically slows down the mixing process, and diffusion is far too slow to be useful. In the past, engineers have used complex, expensive, micrometre-sized mechanical mixers to do the job. Tiny motors on the chips drive mixing vanes, but they are costly, unreliable and can easily gum up.
But now, Ian Glasgow and Nadine Aubry of the New Jersey Institute of Technology in Newark have solved the problem by simply alternating the flow of fluid in two microchip pipes that feed into a third mixing pipe. They reveal their method in the latest issue of the journal Lab on a Chip.
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Normally when two 鈥減ipes鈥� in a microscale chemical reactor meet, one blocks the flow of the other. As a result, the two liquids fail to mix. But by switching the flow several times per second, Glasgow and Aubry created a turbulent, stripy fluid (see Diagram) composed of the two liquids. Each stripe was only about 50 micrometres long. This is short enough for turbulence and diffusion to completely blend most liquids within a second, the researchers have found.
鈥淓verybody looks at creating turbulence in three dimensions to mix liquids. We traded spatial complexity for time complexity 鈥� which is much simpler to handle,鈥� says Aubry.
Andreas Manz, a microfluidics expert at Imperial College in London, described the development as a 鈥渧ery neat and elegant鈥� solution to a problem on which lab-on-a-chip engineers have been expending so much effort. 鈥淭he absence of convection, which depends on temperature differences in the liquid, means that chemical reactions will occur at exactly the same rate throughout the whole liquid,鈥� said Manz. 鈥淭his will be a huge advantage to chemists.鈥�
The New Jersey idea is expected to have many applications. Chip-sized chemical plants will be able to mix minuscule quantities of reagents and churn out drips of designer chemicals to order, predicts Aubry. This will be useful in the preliminary phases of drug discovery, where reagents need to be very well mixed in order to produce purer test drugs with fewer unwanted by-products.
The technique will also let engineers make smaller, more sensitive detectors for nerve gases and pollutants. Closer to home, cheap lab-on-a-chip devices could be used to make simple sensors that detect rotting food in your fridge.