Superslippery Surfaces

Repulsive van der Waals forces are one of the keys to sliding without friction, according to researchers at the Swedish Royal Institute of Technology, in Stockholm (Langmuir, DOI: 10.1021/la7036907). For most materials in sliding contact, van der Waals forces are attractive and cause the interacting surfaces to be drawn together. That attraction leads to friction, which limits the lifetime of most moving parts. In contrast, repulsive van der Waals interactions are uncommon. Adam A. Feiler and coworkers, however, have observed such repulsive forces by using an atomic force microscopy setup in which a gold sphere attached to a cantilever interacts with a Teflon surface coated with cyclohexane. For that system, the team measures a friction coefficient of zero, within the limits of the instrument's resolution. Materials that exhibit these repulsive interactions are uncommon, the researchers say, but include technologically relevant systems such as metal bearings in contact with a Teflon housing (and an organic lubricant) and some combinations of ceramics.