Labs-on-a-chip gain micro mixer

By Kimberly Patch, Technology Research News

Very small things act differently than their larger counterparts. Water flowing in a channel as small as the width of a human hair, for instance, acts more like honey. Even under pressure it travels less than a centimeter per second.

The physics of the very small makes it more difficult for scientists to create devices like microchips that blend tiny amounts of chemicals. These labs-on-a-chip are being designed to do things like sense very small amounts of chemicals, or detect the order of the four bases that make up a segment of DNA. Because mixing solutions is a basic step in many chemical processes, it is important to be able to blend tiny amounts of chemicals on these chips.

A group of researchers from Harvard University, the University of California at Santa Barbara, and the School of Industrial Physics and Chemistry in Paris have developed a method for mixing liquids in small channels. The researchers' device looks more like the bottom of a waxless cross-country ski than a conventional mixer, however.

Mixing in microchannels is intrinsically difficult because on such a small scale, turbulence disappears, said Abraham Stroock, a doctoral student and researcher at Harvard University. "In the absence of turbulence, mixing is more like kneading dough then stirring coffee. In order to mix a volume of dough, you must explicitly fold it into itself. To mix the cup of coffee, you can casually stir with a spoon and turbulence will do the rest," he said.

This means it takes a long time for fluids to mix in microchannels. "If streams of two solutions were injected into the same microchannel, the streams would flow side-by-side with only diffusive mixing between them. For typical proteins, full mixing of the streams would take many minutes even in a channel that's just 100 microns wide," said Stroock.

At the same time, it has not proven practical to shrink the standard, macroscopic mixing devices to a 100-micron scale, said Stroock. "There are no micro-blenders available," he said.

The researchers' device speeds mixing by patterning part of the channel with a series of herringbone-like ridges that encourages the fluids to interact. "We realized that we could create twisting flows with a simple pattern of grooves on one wall of the channel. Once we knew that we could make twisting flows, it was clear that we can design a chaotic mixer," said Stroock.

The twisting motion of the fluid is generated by diagonal grooves. "The grooves act like the helical rifling structure on the inside of a gun barrel. The interaction of the fluid with the grooves transfers some of the [linear] motion of the fluid along the channel into a rotational motion," said Stroock.

This type of mixing is appropriate for microdevices because, unlike turbulence, its effect doesn't become weaker as the channel grows smaller.

By varying the location of the grooves along the channel, the researchers were able to make the liquid fold into itself like kneading bread dough. "The best way to mix, or knead, is to periodically change the orientation of the folding process relative to the volume that is being mixed," said Stroock. "If the folding is done correctly, then the number of folds grows exponentially with the number of steps. A flow that does this is called chaotic," he said.

To confirm the kneading action of the flow, the researchers had to look at cross sections of the fluid. They did this using a type of microscopic imaging system that uses fluorescence. "We achieve the contrast by flowing streams of fluorescent solutions alongside streams of non-fluorescent solutions. As the streams mix we see the bright layers of fluorescent fluid amid darks layers of non-fluorescent fluid," said Stroock.

The mixer could be incorporated into existing lab-on-a-chip devices now, said Stroock. "We already use this mixer in our lab in unrelated microfluidic projects that require mixing," he said.

The researchers have done some nice work, said David Beebe, an assistant professor of biomedical engineering at the University of Wisconsin-Madison. The work "extends chaotic mixing principles across a broader range of flow regimes. And it [expands] the mixing options available to microsystem designers," Beebe said.

Passive mixing schemes have many advantages, but they also have a significant disadvantage, Beebe added. "They require quite a bit of real estate -- channel length -- which may be a drawback in some applications."

The researchers are currently looking into ways to use the mixer in new types of chemical separations, said Stroock.

Stroock's research colleagues were Stephen K. W. Dertinger, Howard A. Stone and George M. Whitesides of Harvard University, Armand Ajdari of The School of Industrial Physics and Chemistry of the City of Paris, and Igor Mezic of the University of California at Santa Barbara. They published the research in the January 25, 2002 issue of Science. The research was funded by Defense Advanced Research Projects Agency (DARPA), the National Institutes of Health (NIH) and the National Science Foundation (NSF).

Timeline:   Now
Funding:   Government
TRN Categories:  Microfluidics and BioMEMS
Story Type:   News
Related Elements:  Technical paper, "Chaotic Mixer for Microchannels," Science, January 25, 2002.


February 6, 2002

Page One

Tiny chain revs microdevices

Labs-on-a-chip gain micro mixer

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Nudged nested nanotubes may oscillate

Portfolios boost quantum computing


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