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Posted: Feb 28, 2008
Slip sliding away in the nanoworld
(Nanowerk Spotlight) When two surfaces approach each other in air, they attract. A phenomenon that is explained by van der Waals forces that affect molecules' interaction. Without these - very weak - intermolecular forces, life as we know it would be impossible. They are responsible for a number of properties of molecular compounds, including crystal structures, condensing, melting and boiling points, surface tension, and densities. Intermolecular forces form molecules like enzymes, proteins, and DNA into the shapes required for biological activity.
Van der Waals forces can also be repulsive, for instance when two surfaces approach each other in liquid: the same force which causes attraction in air (and which is responsible for so called stiction and adhesion) can be made repulsive by choosing the right combination of surface materials and intervening liquid. This force has the characteristic that it increases very rapidly with very small changes in separation when the surfaces are close to each other.
Researchers now have shown that if repulsive van der Waals forces exist between two surfaces prior to their contact then friction is essentially precluded and supersliding is achieved. This opens the possibility that, in certain material systems, the controlled use of repulsive van der Waals forces could be a way to reduce, if not eliminate, friction.
Friction forces act wherever two solids touch. In simple terms: friction is a force that slows things down (this is one reason why you can't have a perpetuum mobile). In scientific terms: friction between two surfaces is caused by energy loss as atoms from the opposing surfaces smash against each other (and in extreme cases of course, surface damage). In the presence of an attractive/adhesive force the atoms are in intimate contact and much energy is lost in forcing the atoms to slide past each other. Friction is not just a physical phenomenon, it also causes huge economic costs: friction wear causes progressive damage between working machine parts; friction also causes heat, which represents wasted energy. Lubrication reduces the effects of friction but, of course, lubricants are a cost factor, too.
"Our work clearly shows that two surfaces experiencing a repulsive surface force, which diverges at small separations, can slide essentially without friction" Dr. Mark Rutland explains to Nanowerk. "The number of systems in which repulsive van der Waals forces could occur is limited but includes metal bearings in a PTFE housing with an organic lubricant and certain combinations of technically interesting ceramic materials. Recent proposals concerning the control of van der Waals forces and the fact that van der Waals forces can be enhanced by electromagnetic radiation in the microwave and visible ranges also open up the possibility of achieving friction-free sliding in a much wider range of systems."
PFTE is a synthetic fluoropolymer which finds numerous applications. Its most well known trademark in the industry is the DuPont brand name Teflon™.
An artist's depiction of superlubricity using repulsive van der Waals forces (Image: Fredrik Dahlström, i3D)
The findings by Rutland, a professor of surface chemistry in the School of Chemical Science and Engineering at the Royal Institute of Technology (KTH) in Stockholm, Sweden, and his collaborators are new in two regards: no one has ever used such a force to control friction or demonstrated such astonishingly low friction on a metal. In fact, says Rutland, only a handful of groups have ever observed repulsive van der Waals forces.
The KTH scientists report their results in the February 16, 2008 issue of Langmuir ("Superlubricity Using Repulsive van der Waals Forces"). Rutland's co-authors were Dr. Lennart Bergström, now a professor at Stockholm University and head of the Department of Physical, Inorganic and Structural Chemistry, and Dr. Adam Feiler, the paper's first author, who was a postdoc with Rutland at KTH's surface chemistry group and has now moved to the Institute for Surface Chemistry.
The advance reported in this research is the ability to reduce the friction experienced by a metal to hitherto impossibly small levels (such a reduction is also possible for ceramic
materials and Rutland's team is working on this). The friction measurements presented here are of the same order as the lowest ever recorded friction coefficients in liquid, though achieved by a completely different approach.
"The steeply repulsive van der Waals force acts to prevent the surfaces achieving contact and they tend to slip past one other, rather like two magnets when you try to push them together" says Rutland.
In their experiments, the researchers attached a gold sphere to an AFM cantilever and forced it to interact with a smooth Teflon surface. They found that the normal surface forces are repulsive when cyclohexane is chosen as the intervening liquid between the gold sphere the Teflon surface. When the refractive index of the liquid is changed, for instance by adding water, the force can be tuned from repulsive to attractive and adhesive.
If these findings could be turned into commercial reality, numerous high-friction systems could benefit: bearings could be made using metal/organic-liquid/ teflon to essentially preclude friction; tools for cutting and drilling could be modified to radically improve performance; in the nanoworld, MEMS and hard discs are obvious areas where one would want to reduce friction (in a previous Spotlight we have reported about an alternative approach to reducing friction at the nanoscale). However, the friction reduction achieved in these lab experiments can only be achieved with an organic film between the surfaces, which will render its application challenging.
Rutland and his collaborators are now working on extending the range of systems where such forces can be induced, and to find new, environmentally friendly, non-toxic organic liquids with suitable dielectric properties.