Picking Up Good Vibrations
Machines from car engines to computer chips emit heat as they run, and cooling is essential to keep the heat from destroying them. But the tiny machines of tomorrow could cool in strange and unforeseen ways, according to a study by LAS chemists. The results open a new window on chemical reactions and could help nanotechnologists design effective molecule-sized machines.
As a car engine heats up, trillions of molecules of metal in it jiggle randomly. Cooling systems like radiators decrease the average motion of those molecules, and no one worries about what each molecule is doing. But it's different for nanostructures—assemblages of molecules less than one-thousandth the diameter of a human hair, says LAS physical chemist Dana Dlott. Atoms in molecules are attached by a variety of spring-like bonds. To understand how nanostructures cool, it's essential to understand which springs vibrate and when.
A decade ago, chemists had no good way to measure how vibrations in one part of a molecule altered vibrations of nearby bonds. So Dlott designed, built, and fine-tuned a million-dollar, ultrafast laser spectrometer with energy as powerful as three nuclear reactors. One laser in the spectrometer zaps a single type of molecular bond. That vibrating bond causes other bonds to vibrate. The rest of the machine measures the resulting vibrations, all within a trillionth of a second. "It's a very special thermometer that measures how much energy is in each vibration," Dlott says.
In the study, Dlott, along with chemist John Deak, a former LAS postdoctoral fellow who's now at the University of Scranton, and their colleagues measured vibrational energy in a tiny detergent-coated water droplet sitting in an oil-like solvent. In a typical machine, heat flows slower when it has farther to go—a principle that underlies the insulating abilities of down coats and fiberglass attic insulation. But everything worked differently in the tiny droplets. Zapping the water inside the droplets caused vibrations to move twice as fast into the solvent as zapping the detergent coating itself.
The ultrafast laser spectroscopy technique enables chemists to map the detailed motion of molecular bonds during chemical reactions, which will help them understand and better predict how chemicals react, Dlott says. And understanding how vibrations move among molecules, he predicts, "will assist in the design of future nanomachines."