Microchannel folds fluids

Mixing a pair of liquids may seem like a quick easy task, but at very small scales turbulence disappears, making mixing more like kneading dough than stirring cream into coffee.

Researchers from the University of Michigan have devised a passive mixing scheme that causes the flow of liquids on a microfluidic chip to split, rotate and recombine so that the fluid repeatedly folds in on itself and so mixes relatively quickly despite the lack of turbulence.

The folding process is akin to making a puff pastry, according to the researchers. The flow is split into two identical streams, then rotated in opposite directions and recombined, which cuts the size of any non-mixed portions in half. After a few folds, the non-mixed portions become small enough that diffusion is enough to mix the liquids.

The method can be easily integrated into existing microfluidic platforms, or labs-on-a-chip, according to the researchers.

Key to the method's utility is it does not take a lot of space on a chip. Other passive fluid mixing schemes involve flowing the liquids through relatively long channels, according to the researchers.

The method could be used in practical commercial applications in five years, according to the researchers. The work appeared in the March 22, 2004 issue of Applied Physics Letters.