PORTLAND, Ore. A newly discovered natural way to form nanoscale wires from bulk droplets could simultaneously cut their cost and increase yields. The discovery could also lead to advances in composite materials, electronics and pharmaceuticals, researchers said.
"Today, if you want a tiny wire, you force material through a tiny nozzle," said the University of Chicago's Wendy Zhang, who helped explain the mechanism behind the extrusion method. "At the nanoscale, however, we think it will be better to use naturally occurring processes like ours, rather than brute force. We also believe our method will be more economical than growing high-aspect-ratio shapes like wires."
Some researchers developing nanowires see them as fundamental structures that could possibly replace silicon transistors in future-generation circuitry.
Working with Zhang were researchers Peter Howell at Oxford University and Michael Siegel at the New Jersey Institute of Technology. Purdue University professor Osman Basaran formulated a computer simulation of the laboratory results with his student Pankaj Doshi.
"We are not materials engineers," said Zhang, an assistant professor at the university's James Franck Institute, "but we think there will be many useful applications of our technique." For instance, she said, Chicago professor Sidney Nagel is extruding monomers-the precursors to polymers-into networks "and making cross-links optically."
Itai Cohen, a former student of Nagel's and now a postdoctoral scientist at Harvard, made the seminal discovery while observing a drop of water drip into a viscous oil. It began as a smooth parabolic shape, but when it appeared ready to break off, material from inside the droplet spewed out the tapered end in a controllable, consistent stream that eventually transformed the contents of the droplet into a long, narrow wire 8 microns in diameter and 2 millimeters long-that is, one with a 1:4,000 aspect ratio.
"From our theoretical calculations, we think that we can control this process to produce even higher aspect ratios and we also think we can get the size down to 10 or 20 molecules," or 2 to 4 nanometers, Zhang said.
The novelty of the process, Zhang said, is that the droplet maintains its shape-a phenomenon called memory-while its contents are smoothly flowing out the end.
The new theory, invented to explain the memory effect, maintains that an equilibrium in flow velocity is achieved just at the point when the extrusion begins (see photos, page 1).
Before that the inside of the droplet is relatively static, but at the point when the outside oil's flow slows to match the stasis inside, the hitherto undiscovered extrusion dynamic is exhibited.
"This dynamic is unique, because it preserves a memory of the drop shape right down to very small length-scales," Zhang said.
Zhang also made it clear that the theoretical side is not ironclad yet. It appears to be holding up to new experimental results as they accumulate, but some aspects, such as the speed at which the effect arises, remain mysterious, she said.
The research was supported by the Energy Department and the National Science Foundation.