On the nanoscale, heat can flow in unexpected ways. This uncertainty poses challenges and opportunities for researchers aiming to understand and exploit the behavior of tiny structures.
Anomalous heat flow recently became apparent in the surprising behavior of ultrasmall clusters of water molecules enclosed in molecule-thick skins of surfactant molecules, the key ingredient of detergents. When scientists laser-zapped the watery centers of the blobs, which boosted the water clusters to the equivalent of about 3,000°C, that energy flowed into the solvent much faster than when the laser was tuned to excite the surfactant skins, say Dana D. Dlott of the University of Illinois at Urbana-Champaign and his colleagues there and at the University of Scranton (Pa.)
A laser jolt induces bending, stretching, and other specific movements of molecules that make up these nanostructures, Dlott explains. When the laser is tuned to zap the water within the nanostructures, a cascade of motions of water and surfactant molecules quickly incites random vibrations, or heating, of the solvent. However, when the laser is tuned to excite the surfactant molecules, a different cascade of motions leads more slowly to solvent heating, he says.
According to the study report, which appears in the Oct. 15 Science, simple heat conduction, the diffusion of random molecular vibrations, can’t explain how energy propagates through those nanoscale structures. In addition to providing a new way to look at energy flows through biomolecules, the work also may guide researchers in designing nanodevices that exploit or shed heat effectively.