Laser Unveils a 3-D Model of Chaos

Daniel C. McCarthy

Does coffee taste different if you pour the cream into the cup first? Does the temperature of the cream influence the texture? These were the questions one recent technical conference attendee was exploring aloud during a midmorning coffee break. Some people never switch off from their work, and if the coffee-mixing attendee works with chemicals he should be excused. Problems related to mixing chemicals have resulted in poor or unusable products near 0.5 to 3 percent of total output, which could represent up to $20 billion in losses each year for the US chemical industry.
A problem of that magnitude elicits research. And one arm of that research has enlisted the use of photonics to help study a three-dimensional visualization of chaos at Northwestern University's Robert R. McCormick School of Engineering and Applied Science. This study by professor Julio M. Ottino and his staff is created in a clear plastic cylindrical tank filled with glycerin and equipped with an impeller and needles injected with colored dyes. The impeller consists of a not-quite-vertical rod spinning a not-quite-horizontal disc. The dyes selectively fluoresce at certain wavelengths of light.
As the mix gets under way, a HeCd laser from Liconix aimed through a cylindrical lens creates a vertical sheet of light through the tank, illuminating specific dyes. According to Gerald O. Fountain, a graduate student working with Ottino, the laser operates in two modes, 325 and 442 nm. Other than the laser and the illuminated dyes in the tank, there is no light source in the room.
"We're looking for regular regions in the flow that are fixed and steady as well as the areas that are in chaos," said Fountain.
What Ottino's team learned was that the tilted impeller induces some dye streams to circle the tank in a spiral, nearly missing itself for several passes. Over time, the dye path forms a circular region revolving around the impeller, which in the laser's cross section appears as side-by-side ring shapes, referred to as "regular islands."
"What we've seen is that some of these regular areas can be completely engulfed in chaotic regions and not be penetrated by the dyes around them, even after 17 hours," Fountain said.
Other dye streams may circle the tank and never return to their original path. Those divergent paths indicate where the most efficient mixing occurs, providing valuable information for companies that blend chemicals that react with each other faster than they mix.