New Way To Drive Chemical Reactions: Collision Of Liquids At High Speed

October 03, 1997

CHAMPAIGN, Ill. -- When a liquid moves fast enough, gas bubbles will form and collapse. This process -- called cavitation -- is responsible for the pleasant babbling sound of streams and rivers, and for the stealth-defying sound of propellers on submarines. Chemists at the University of Illinois have discovered that in addition to making noise, high-velocity liquids also can drive chemical reactions.

"By colliding two streams of liquids together at a combined speed of 450 mph, we can break some of the strongest chemical bonds," said Kenneth Suslick, a U. of I. professor of chemical sciences. "With water, for example, the oxygen-hydrogen bond ruptures. The fragments can recombine to form hydrogen peroxide and other highly reactive intermediates that can destroy contaminants in the water."

Some contaminants can be destroyed directly by the implosive collapse of the bubbles. Other less volatile contaminants can be destroyed through secondary reactions with some of the fragments, such as free hydrogens and hydroxyl radicals -- both of which are extremely reactive. "This raises the possibility of using turbulent liquid jets as a simple way of purifying water contaminated with low levels of chemical waste," Suslick said.

The jets are made by pumping liquids at very high pressures through very small holes drilled in gemstones. "Only gems are hard enough to take the pressure without cracking or eroding," Suslick said. Currently, liquid jets are used industrially for making emulsions (such as cosmetic lotions) 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 bubbles in the moving liquid," Suslick said. "This is very similar to the effects of high-intensity ultrasound in a liquid, where the collapse of sound-driven bubbles generates intense local heating, forming a hot spot in 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 billionth of a second."

Any turbulent flow can cause cavitation in liquids, Suslick said. "But generating bubbles doesn't necessarily generate chemistry. The bubbles have to collapse pretty intensively to create the required heat and pressure. By colliding two liquid jets, we can concentrate the collisional energy in 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's not very often that we find a new way to drive chemistry, especially one as simple as fast-moving liquids. Although we can create very-high-energy chemistry using these liquid jets, the reaction rates are pretty slow so far."

Suslick and graduate students Millan Mdleleni and Jeff Ries reported their findings in the Oct. 1 issue of the Journal of the American Chemical Society.

University of Illinois at Urbana-Champaign

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