Friction-reduction recipe: Add two atoms and lots of heat
Richard Stratt pictures the process this way: "It's like pushing through a crowded airport terminal. Everyone is jostling each other. And you just can't seem to get to your gate. That's the kind of motion – and friction – at play in chemical reactions."
But Stratt, a Brown University professor of chemistry, and colleagues at the University of Southern California have hit upon a surprise. They found an example of molecular motion in a chemical reaction that actually destroys friction. This superfast molecule, which makes a whopping 270 trillion rotations per minute, literally pushes away molecules that surround it in solvent, wiping out most of the resulting frictional force.
"We found a case where this molecule is spinning so fast that it manipulates the arrangement of molecules around it in just the right way to annihilate friction," Stratt said. "To go back to the airport analogy, we found the obnoxious guy who muscles his way through the crowd and makes his plane."
Results of the experiments, published in the current issue of Science, add a new wrinkle to old beliefs about molecular motion and energy transfer during chemical reactions.
"Most things just don't work this way in liquids," Stratt said. "Basic laws of physics suggest that the faster you move, the easier it is to slow you down. And when you push hard against something, it pushes back. But in this one case, these rules don't apply."
Stephen Bradforth, a University of Southern California chemist, is an expert in ultrafast dynamics in chemical reactions. Bradforth's team, which included Amy Moskun and Askat Jailaubekov, devised a clever way to study rapidly spinning molecules in liquids.
The team made a solution of iodine cyanide and water, sent drops of it streaming down a wire, then hit the drops with pulses from an ultraviolet laser. The laser shattered some molecules in the solution. One resulting bit was a two-atom cyanide radical. It was intensely hot – 4,700 degrees Kelvin, about three-quarters of the temperature at sun's surface – and started a superfast spin.
Bradforth's group measured the tilt of the spin, expecting it to quickly lean to one side, in the same way a spinning top lists as it slows. But the molecule didn't appear to alter its tilt for 10 picoseconds – an eon in chemical reactions. What was going on?
So Bradforth, an experimentalist, and Stratt, a theorist, started collaborating. Guohua Tao, a graduate student in Stratt's lab, devised a computer simulation that may explain the molecule's confounding behavior.
What Tao and Stratt found was a "wiggle." According to the simulation, the tilt of the molecule started changing almost instantly due to friction in the liquid. But in about the time it took to make a quarter turn in rotation, the molecule broke free from its neighbors and continued to spin nearly friction-free for 10 picoseconds. How? The simulation shows the two-atom wonder repelling surrounding molecules, creating a protective bubble allowing unrestrained rotation.
"It seemed to be saying, 'Wham! Get out of my face,'" Stratt said, "which was surprising. You can see molecules behave this way in gases, but not in liquids."
The Brown-USC team plans further tests to see if the friction-free phenomenon applies to different molecules in different solutions. If it does, Stratt said chemists might need to rethink the pathways molecules use to move energy.
The National Science Foundation and the David and Lucile Packard Foundation funded the work.
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