Soft Drink Secrets

When a clear, fruity soft drink called Orbitz appeared on supermarket shelves 2 years ago, consumers marveled at the dozens of tiny gelatinous spheres suspended in the liquid that spin around when the bottle is shaken. While Orbitz's manufacturer, Clearly Canadian Beverage Corp., attributes the trick to an equal density between the spheres and the liquid, a team of chemical engineers now has a more sophisticated answer. At last month's annual meeting of the Society of Rheology, scientists said the Orbitz liquid has the unusual property of being much more viscous at rest than in motion.

When he first encountered Orbitz, Skip Rochefort, a chemical engineering professor at Oregon State University, Corvallis, tested the equal density explanation by adding enough alcohol to the drink to lower the liquid's density by 20% to 30%. The beads, however, remained suspended. Puzzled, Rochefort teamed up with local high school student Jason Hower to carefully extract the spheres and test the fluid's viscosity. The two then discovered the secret to Orbitz: When the fluid is in motion, its viscosity is roughly the same as water and the balls can move freely. But at rest, the liquid's viscosity is 100,000 times higher, keeping the spheres suspended and immobile.

Rochefort explains this phenomenon by pointing to the behavior of molecules of xanthan gum and gallan gum, two of the drink's ingredients. These long molecules, he explains, bind weakly to each other to form a spiderweblike network in liquid that holds the balls in place--a phenomenon few beyond Orbitz's creators understood. But once shaken the bonds are broken and the web disintegrates, he says.

Rochefort's description is essentially correct, says Ross Clark, a rheologist from Kelco, the company that makes the Orbitz ingredients. But Orbitz really shouldn't be thought of as a futuristic fluid, he adds: "There's been a lot of fuss over what is a relatively simple material."

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