TECH SPACELubricating Nanoscale Machines Challenges Conventional Expectations
The unique and often unexpected properties of fluids confined to very small spaces will force designers of future nanometer scale devices to reexamine conventional expectations regarding lubrication and fluid flow.
At these small size scales, considerations pertaining to molecular architecture, structural conformations and packing -- along with the increased importance of surface roughness, surface tension, frictional losses and fluctuations -- produce dramatic changes in the behavior of lubricants and other fluids. These considerations come into play as devices approach the size of lubricant molecules that interact with them.
"We are accumulating more and more evidence that such confined fluids behave in ways that are very different from bulk ones, and there is no way to extrapolate the behavior from the large scale to the very small," said Uzi Landman, director of the Center for Computational Materials Science in the School of Physics at the Georgia Institute of Technology. "We must find clever ways to harness and control these new behaviors in order to realize the opportunities in nanotechnology."
Landman described nanoscale lubrication and fluid flow February 15 at a seminar on nanotechnology held at the 167th annual meeting of the American Association for the Advancement of Science (AAAS) in San Francisco.
Using supercomputer-based molecular dynamics simulations to model the behavior of these fluids at the atomic and molecular level, Landman's research center has developed a series of predictions that will help guide future device designers. Some of the theoretical predictions have already been borne out by experimental results.
Among the predictions:
Landman's research group has reported on the tendency of lubricant molecules such as hexadecane and other molecular fluids to form highly ordered layers in planes parallel to the motion of the confining surfaces.
On size scales that approximate multiples of the molecular width, these layered lubricants appear to increase their viscosity, "becoming, at equilibrium and at various stages of the sliding motion, liquid-like in the plane parallel to the sliding surfaces and solid-like in the direction perpendicular to the surfaces," Landman said.
This phenomenon manifests itself in several ways, including an increasing amount of pressure required to squeeze the lubricant out of the confining spaces. The pressure required shows distinct steps that correspond to the molecular diameter, suggesting the lubricant is squeezed out layer by layer.
"Viscosity and other concepts that we commonly use are taken from bulk behavior, and one of the questions we must answer is whether it is appropriate to adopt the same concepts on the molecular levels," Landman said.
Increased friction caused by nanoconfinement-induced layering poses a significant concern for future devices, but Landman and his colleagues propose several countering techniques:
In a second phase of the nanojet simulations, the researchers reformulated the traditional hydrodynamics equations to include fluctuations whose influence becomes dominant at small sizes. The newly derived equations extend hydrodynamics to the nanoscale, and they were shown by Moseler and Landman to yield results that agree with their atomistic simulations.
Molecular dynamics simulations allow researchers to study the behavior of each atom and molecule in a system with very fine resolution in space and time by integrating the equations of motion with interatomic interactions derived from quantum mechanical calculations and/or experimental data from larger systems.
The classical and quantum mechanical simulation methodologies developed by Landman and his coworkers were the basis for his 2000 Feynman Prize in theoretical nanotechnology. These "computational microscopies and spectroscopies" allow scientists to make predictions and draw molecular-based designs that could guide the fabrication of devices this small.
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Georgia Institute of Technology
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