Controlling chemical chaos.

Pale yellow, then colorless. Pale yellow, then colorless. First one color, then none.

Instead of mixing to produce a uniformly colored solution, certain combinations of reacting chemicals display regularly repeating patterns of alternating colors. By carefully adjusting the concentrations of the ingredients required for these chemical oscillators, researchers can also transform this periodic behavior into the erratic, unpredictable changes characteristic of a chaotic system. Now Kenneth Showalter and his co-workers at West Virginia University in Morgantown have demonstrated that they can keep such a chemically unstable system oscillating regularly by applying a sequence of small adjustments to the concentrations of some of the chemicals involved in the reaction.

"This is the first example of controlling chaos in a chemical system," Showalter says. Researchers had previously used similar techniques for controlling the chaotic behavior of heart tissue (SN: 9/5/92, p. 156), solid-state lasers (SN: 2/22/92, p. 119), and magnetoelastic ribbons (SN: 1/26/91, p. 60). Showalter and his group report their findings in the Jan. 21 NATURE.

The researchers carried out their experiment in a tank continuously fed separate solutions of malonic acid, cerium sulfate, and sodium bromate. One pump delivered malonic acid at a fixed flow rate, and another pump delivered the cerium and bromate solutions at a rate regulated by a computer. The reaction was monitored by tracking fluctuations in the voltage of an electrode sensitive to bromide ions (a reaction product).

Showalter and his co-workers used these voltage measurements to plot a three-dimensional portrait of the system's dynamical behavior (see diagram). One coordinate represents the voltage at a given moment, the second coordinate represents the voltage 38 seconds earlier, and the third represents the voltage another 38 seconds earlier. If the voltage were to remain constant throughout the experiment, the plot would consist of a single point. However, because the voltage fluctuates erratically, the plotted points trace out a complicated shape (dotted lines). Using data obtained from such a plot, the researchers can compute how much and when to increase or decrease the cerium and bromate flow rate to stabilize the voltage into a repeating pattern (solid line).

Using this technique, Showalter and his group could not only stabilize oscillatory behavior with a given period but also switch that behavior readily from one per iod to another-all by making modest adjustments to the flow rates. "I think the real challenge in this business will be controlling chaos in spatialtemporal systems:' which display moving waves of color often in the shape of spirals or concentric rings-in thin liquid films, Showalter says. "There's also a tremendous opportunity there for biological applications:'