Lubricating Nanoscale Machines: Unusual Behavior Of Highly Confined Fluids Challenges Conventional Expectations

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 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 will discuss nanoscale lubrication and fluid flow Thursday, February 15 at a seminar on nanotechnology to be 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:

• When confined to tight spaces, long-chain lubricant molecules act more like "soft solids," forming, for energetic and entropic reasons, ordered layers that significantly influence the movement of sliding surfaces. This poses significant challenges in systems such as ultra-high-density computer disk drives.

• Confined fluids composed of molecular mixtures segregate themselves by size, with the longer chain molecules adsorbing near the surfaces and the smaller ones remaining in the middle region of the confining gap.

• Liquid jets just a few nanometers in diameter propagate over shorter distances than predicted by conventional fluid flow equations. Generation of such nanojets requires special treatment to prevent clogging of the nozzle.

• Molecular fluids confined between slightly roughened surfaces exhibit a more liquid-like behavior than when confined by smooth surfaces, resulting in significant modification of the resistance to sliding.

In addition to pointing out potential issues involved in the nanotribology of future nanodevices, the simulations -- involving as many as hundreds of thousands molecules -- also allow researchers to explore and test potential solutions.

Writing in journals such as Science, Nature, Physical Review Letters, Langmuir, and the Journal of Physical Chemistry, 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.

"The confinement of these liquids brings about sluggishness to their response," Landman explained. "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."

Increased friction caused by nanoconfinement-induced layering poses a significant concern for future devices, but Landman and his colleagues propose several techniques for countering it:

• Chemically altering the long-chain molecules to include branched structures that inhibit the formation of layers. The researchers have shown that a nanoconfined liquid made of branched alkane molecules has a lower viscocity then a confined liquid of the same molecular weight but made of straight chain molecules. This behavior is opposite to that found in much larger environments.

• Roughening the surfaces of the confining plates to disrupt the molecular ordering. Instead of forming ordered layers, the molecules closest to the rough surfaces adhere to them, leaving free-flowing molecules in between. Consequently, patterning of the surface morphology could be used to control friction and lubrication processes.

• Varying the distance between the two confining surfaces in an oscillatory manner, just enough to keep the lubricant molecules in a "frustated" state of disorder. Varying the distance by one Angstrom in a 20-Angstrom gap should be enough to prevent the layering. The frequency of the applied oscillations depends on the characteristic molecular relaxation times and the viscosity of the lubricant, which in turn are governed by the nature and structure of the fluid molecules.

Recent work published by Landman and post-doctoral fellow Michael Moseler in Science predicts the feasibility of generating nanojets just a few nanometers in diameter. Such jets could one day be used for printing circuitry patterns, injecting genes into cells, producing droplets of uniform size and serving as fuel injectors for nanoengines.

The nanojets, however, would differ significantly from their larger cousins. For example, nanojets would have to overcome the effects of surface tension and wetting that are of much less importance at larger scales. In a nanojet, liquid molecules wet the outer surface of the nozzle, eventually creating an adsorbed film that clogs the nozzle. To prevent that, the researchers suggest heating the outer surface of the nozzle to evaporate the condensing films, or coating the outer surfaces with an anti-stick non-wetting compound.

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.

"In the nanorealm there is a whole new world that is full of surprises and opportunities," added Landman.