CHAMPAIGN, Ill. -- When a liquid moves fast enough, gas bubbles willform and collapse. This process -- called cavitation -- is responsiblefor the pleasant babbling sound of streams and rivers, and for the stealth-defyingsound of propellers on submarines. Chemists at the University of Illinoishave discovered that in addition to making noise, high-velocity liquidsalso can drive chemical reactions.
"By colliding two streams of liquids together at a combined speedof 450 mph, we can break some of the strongest chemical bonds," saidKenneth Suslick, a U. of I. professor of chemical sciences. "Withwater, for example, the oxygen-hydrogen bond ruptures. The fragments canrecombine to form hydrogen peroxide and other highly reactive intermediatesthat can destroy contaminants in the water."
Some contaminants can be destroyed directly by the implosive collapseof the bubbles. Other less volatile contaminants can be destroyed throughsecondary reactions with some of the fragments, such as free hydrogens andhydroxyl radicals -- both of which are extremely reactive. "This raisesthe possibility of using turbulent liquid jets as a simple way of purifyingwater contaminated with low levels of chemical waste," Suslick said.
The jets are made by pumping liquids at very high pressures through verysmall holes drilled in gemstones. "Only gems are hard enough to takethe pressure without cracking or eroding," Suslick said. Currently,liquid jets are used industrially for making emulsions (such as cosmeticlotions) and for cutting extremely hard materials.
"The chemistry of turbulent liquids comes from 'hydrodynamic cavitation,'which causes the formation, growth and implosive collapse of small gas bubblesin the moving liquid," Suslick said. "This is very similar tothe effects of high-intensity ultrasound in a liquid, where the collapseof sound-driven bubbles generates intense local heating, forming a hot spotin the cold liquid with a transient temperature of about 9,000 degrees Fahrenheit,the pressure of about 1,000 atmospheres and the duration of about a billionthof a second."
Any turbulent flow can cause cavitation in liquids, Suslick said. "Butgenerating bubbles doesn't necessarily generate chemistry. The bubbleshave to collapse pretty intensively to create the required heat and pressure. By colliding two liquid jets, we can concentrate the collisional energyin the bubbles."
There are only a few ways to force chemical reactions: heat, light,radiation and ultrasound are the common ones, Suslick said. "So, it'snot very often that we find a new way to drive chemistry, especially oneas simple as fast-moving liquids. Although we can create very-high-energychemistry using these liquid jets, the reaction rates are pretty slow sofar."
Suslick and graduate students Millan Mdleleni and Jeff Ries reportedtheir findings in the Oct. 1 issue of the Journal of the American ChemicalSociety.
The above story is based on materials provided by University of Illinois at Urbana-Champaign. Note: Materials may be edited for content and length.
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