Head-On Collisions Break Water Bonds

Violently colliding water jets can create tiny implosions capable of driving chemical reactions, according to a paper in this week's Journal of the American Chemical Society. The process is a close cousin to sonoluminescence--the sound-driven creation of gas bubbles that emit light as they collapse--and may even have practical applications.

When a fluid flows rapidly into a region of low pressure, the turbulence can generate pockets of even lower pressure, momentarily lowering the boiling point and generating tiny pockets of vapor. This so-called cavitation corrodes boat propellers and gives brooks their babble as they flow around rocks. Scientists have known for decades that carefully orchestrated sound waves can produce similar bubbles, and they have recently learned that these bubbles collapse so violently that they reach temperatures of 5000 kelvins and pressures of 1000 atmospheres--high enough to drive chemical reactions. But the power of the bubbles produced by fluid flow was unknown.

Now three researchers at the University of Illinois have created bubbles without sound waves, instead forcing two streams of water through tiny holes drilled in gemstones. The streams collided with a combined speed of 725 kilometers per hour, and the resulting bubble implosions were energetic enough to sever the oxygen-hydrogen bonds of water molecules. The researchers measured the rate of this reaction by dissolving potassium iodide in the water. A photospectrometer revealed the formation of triiodide, which is encouraged by OH radicals and hydrogen peroxide--two products of the breakdown of water molecules. The group says that means the temperatures inside the bubbles are probably in the 1000-degree range.

The technique may one day be used to chemically purify water, because the high-temperature bubbles could break down toxic solvents like carbon tetrachloride, says team leader Kenneth Suslick, although he adds that the reaction rates so far are slow. William Moser, a chemical engineer at the Worcester Polytechnic Institute in Massachusetts, says the new paper at least proves the power of the tiny implosions: "It's extremely clean and well done."