Uzi Landman, director of Georgia Tech's Center for Computational Materials Science, will receive recognition from the MRS, the world's largest materials professional society. The medal award is given to recognize "a specific outstanding recent discovery or advancement that is expected to have a major impact on the progress of any materials-related field." Charles M. Lieber of Harvard University will also be honored with a medal at the ceremony.
Landman's award is for the development and implementation of research methodologies that use molecular dynamics simulations to predict the often-surprising behavior that occurs at the nanoscale when surfaces of solid and liquid materials meet. Landman's research team has examined the effects of friction and lubrication in these small-scale systems, predicting how such systems would behave long before they could be fabricated. Over time, most of their key predictions in this new science of nanotribology have been confirmed experimentally.
Performed on large parallel-processing computers, the simulations use known laws of physics - including quantum mechanics - to predict how hundreds of thousands of molecules or atoms interact and respond to external influences such as the exertion of a load or the application of shear relative motion between bodies in contact. The resulting calculations will help engineers design smaller and smaller disk drives, nanometer-scale machines and even biomechanical implants used in the body.
A faculty member in Georgia Tech's School of Physics since 1977 and currently a Regents' and Institute Professor and Fuller E. Callaway chair, Landman began working on molecular dynamics simulations in the late 1970s. A 1990 article he published in the journal Science brought particular attention to the field by "demonstrating the capacity of realistic molecular dynamics simulations to make specific predictions that could be compared to quantitative measurements in the field of tribology," the MRS citation says.
"In many respects, Landman helped create the field of nanotribology as he contributed to both classical and quantum mechanical molecular dynamics simulation methodologies, leading to an understanding of the atomic origins underlying nanoscale tribological processes," the MRS added.
The 1990 Science paper used large-scale molecular dynamics simulations done on large supercomputers to show that when a nickel tip was brought into close proximity to a sheet of gold, gold atoms would jump from the sheet to the probe.
"To our amazement, we found the gold atoms jumping to contact the nickel probe at short distances," Landman recalled. "Then we did simulations in which we withdrew the tip after contact and found that a nanometer-sized wire made of gold was created. That gold would deform in this manner amazed us, because gold is not supposed to do this."
The simulations were done several years before scientists began to make and measure the properties of wires on that size scale, and prior to the rise of the nanoscience and nanotechnology field.
These early simulations and subsequent ones showed that nanometer-scale wires, called interfacial junctions, would be formed whenever bare metal surfaces, as well as other material surfaces, were brought into close proximity. Transforming earlier notions in the microscopic domain, the simulation results suggested that breaking these junctions is the cause of friction - resistance to motion - in small-scale mechanical devices.
"It is the need to shear these junctions when you move two bodies parallel to one another that dissipates energy," Landman noted. "Work is dissipated and irreversibly lost in the process of formation and breakup of these junctions. That's what causes the resistance to motion."
Landman also showed that lubricant molecules may no longer behave as lubricants when trapped in very small gaps between surfaces. Instead, they organize themselves into "soft crystals" that increase rather than decrease resistance to motion.
"It turns out that in this kind of a nanotribological situation, molecules do not behave like they do in bulk," he said. "When you confine fluids of this type to such small dimensions, they behave very differently. The viscosity is different, the degree of order is different and they have a tendency to self-order themselves parallel to the sliding surfaces. Under such circumstances, the lubricant can become a source of problems because it is no longer a liquid."
Landman and his group have formulated and simulated techniques that could be used to maintain the lubricating properties by "frustrating" the molecules' attempts to align themselves. One strategy would be to use branch-chain molecules that cannot align themselves into soft crystals. Another technique would be to rapidly vary the distance between the moving surfaces, keeping the lubricant molecules in motion - thereby preventing them from forming soft-solid crystals.
Other simulations showed that nanometer-scale wires would behave like ideal materials because they are made up of too few atoms to have single and extended defects known as dislocations. Without defects to impede the flow of electrons, these wires allow "ballistic electronic transport," leading to reduced resistance and quantized conductance. This will be important to the design of future small-scale electronic devices, Landman notes.
In August 2000, Landman and collaborator Michael Moseler published a cover article in Science in which they reported on a molecular dynamics simulation that predicted the behavior of nanometer-scale jets of liquid.
The underlying theme behind all of this work can be summed up as "small is different," the title Landman will use for his MRS medal presentation.
"New behavior emerges on the nanoscale," he explained. "This new behavior creates interesting physical phenomena, and that is where the technological opportunities may lie. To take advantage of them, we must understand how these small systems behave."
For more information, please contact Uzi Landman (404-894-3368); E-mail: (uzi.landman@physics.gatech.edu).
Georgia Tech Research News and Research Horizons magazine, along with high-resolution JPEG images, can be found on the Web at (www.gtresearchnews.gatech.edu).