Sound Turns Liquid to Jelly

A burst of high-frequency sound waves is enough to turn a range of oily liquid mixtures to jelly. Because the reaction is reversible, it could be used to remotely control the viscosity of liquid shock absorbers in cars or of lubricants in robotic joints, or to temporarily solidify fuels and paints so they don’t leak during transport. Engineers may one day even use the technology to make building dampers that absorb energy from external forces, prolonging a structure’s life and preventing a catastrophic event such as an earthquake from destroying it.

Gels are semi-solid mixtures that consist of a liquid trapped within the pores of a continuous network of chain-like molecules. They are usually created by adding an acid to a liquid with a solid suspended in it, known as a sol, or illuminating a sol with a flash of UV light.

But both processes create strong chemical bonds, making it difficult to turn the gel back into a liquid. Shaking is enough to turn some sols into a gel, but the gel tends to “melt” when the shaking stops. “The sol-to gel transition itself is a very common phenomenon,” says chemist Takeshi Naota from Osaka University in Japan. “But there is no method to achieve instant, remote and reversible control between stable liquid and stable gel phases at the same ambient temperature.” Now he and his colleague Hiroshi Koori have devised a way to reversibly gelatinise a range of liquids. They dissolved a new type of compound, made of small organic molecules containing palladium, in acetone to produce a transparent, oily solution that they then blasted with ultrasound waves at a frequency of 40 kilohertz. After just 3 seconds, it formed an opaque white gel (Journal of the American Chemical Society, DOI: 10.1021/ja050809h). A blast of heat changed the gel back to a liquid, although Naota has already partially succeeded in disrupting the gel with ultrasound as well. The compound had the same effect on three other organic solvents, including dioxane.

No one has worked out exactly what causes this effect, but chemists find it intriguing. “This looks new and very innovative,” says Eric Bescher, a chemist who specialises in gels at the University of California, Los Angeles. “I think it is fascinating because it’s unexpected,” says Mitchell Winnik, a polymer chemist at the University of Toronto in Canada.

Sonochemists, who regularly use ultrasound to induce chemical reactions, are no less fascinated. “Exciting and interesting, but unexplained,” says Kenneth Suslick, a sonochemist at the University of Illinois in Urbana-Champaign. But Naota has a theory about what is going on. He says the key is the shape of the small organic molecules. They consist of two hydrocarbon chains, each comprising a flat arrangement of carbon, nitrogen and oxygen atoms with a palladium atom at the centre that acts as a hinge (see Graphic). Naota has shown that in the liquid phase, before the ultrasound is applied, the chains of different molecules overlap, but only temporarily.

He suspects that the ultrasound blast causes the surrounding solvent molecules to vibrate. That exerts pressure on the overlapping hydrocarbon chains of the organic molecules, forcing them to rotate about the palladium atoms and come together. The chains then become locked together by strong attractive forces known as stacking interactions.

Once these molecules start sticking together, they form long chains throughout the solution. The liquid solvent gets trapped in the gaps, producing a gel. “No one found such a phenomenon before because it requires the gelator to have a very special molecular structure,” says Naota.

Bescher says the next step is to see whether the phenomenon is observed when different molecules are dissolved in these organic liquids.

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