Self-stirring liquids

Chemistry, not force, mixes it up

In a tail wagging the dog reversal, researchers have found that simple chemical reactions can mix a solution. Usually, chemicals are stirred to enhance a reaction, but a new study finds that the reverse is also true: Simple chemical reactions can trigger fluid flows, reports a paper in the January 29 Physical Review Letters.

CHEMICAL FLOW Hydrochloric acid (top) and sodium hydroxide (bottom) create an asymmetrical pattern as they react, mixing a solution. The diameter of the view shown here is three centimeters and the entire sequence lasts about a minute. C. Almarcha

The research has implications for many chemical reactions, including those inside stars or when carbon dioxide stored deep in the earth encounters water, says study coauthor Anne De Wit of the Université Libre de Bruxelles in Belgium.

De Wit and her colleagues wondered what would happen to fluid flows if the reacting liquids were left alone and not stirred. The researchers watched a very simple reaction — the neutralization that occurs between hydrochloric acid and sodium hydroxide, a common chemical base — in the absence of stirring.

The researchers carefully injected the denser sodium hydroxide into a container and then added the hydrochloric acid. The sodium hydroxide stayed on the bottom and the hydrochloric acid sat on top. Where the two reactive chemicals met, the reaction’s products — table salt and water — began to form. As the salty solution formed, it crept upward and hit the lower-density acid, creating tendrils that started to mix the solution. But the same didn’t happen below the reaction line. This difference in how the reaction product interacted with each of its chemical parents drove the mixing the team observed.

These asymmetrical patterns, the researchers say, distinguish mixing during a chemical reaction from what happens when two nonreactive liquids meet, which may look more like diffusion or other kinds of mixing.

“These kinds of beautiful patterns can be observed with very well-known reactions,” says study coauthor Christophe Almarcha, also of the Université Libre de Bruxelles. “This is quite fascinating for someone who’s done this reaction hundreds of times.”

The researchers also describe reaction-driven mixing mathematically by creating a model that predicted a pattern that looked like the real thing. The model can be tweaked to predict patterns for other chemical reactions, which would vary widely, Almarcha says.

“Our little model system says ‘pay attention,’” De Wit says. “If there are reactions, then new things will happen.” For instance, if stored carbon leaches into an aquifer and starts reacting with water, “those reactions will trigger flows, which will enhance the mixture,” she says.

Chemical Reactions Make Moves from Science News on Vimeo.

The neutralization reaction of hydrochloric acid (top) and sodium hydroxide (bottom) creates flows in an otherwise still system.

Video credit: Christophe Almarcha

Laura Sanders is the neuroscience writer. She holds a Ph.D. in molecular biology from the University of Southern California.

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