MOLECULAR “brushes” that reduce the friction between two surfaces a thousand-fold could provide a new means of lubrication, say materials scientists in Israel. Jacob Klein and colleagues at the Weizmann Institute of Science in Rehovot have demonstrated the technique for the first time.
Klein’s brushes are made of chains of the common polymer polystyrene, fixed to mica. “Each chain is attached by one end to the surface of the mica, while the other end dangles down,” says Klein. “The chains look like the hairs on a brush.”
When two surfaces covered with the brushes are doused in a toluene solvent and rubbed together, the polymer chains run past each other, instead of interlocking. Thermodynamics holds the key to how the system works: the polymer molecules find it energetically favourable to crumple as the pressure increases, rather than lock “bristles” with the brush opposite. This phenomenon keeps them apart under loads as high as a megapascal – the pressure typically experienced in a human knee joint, for example.
Klein says that the brushes reduced the friction to a thousandth of what it was when the surfaces were lubricated by toluene alone. “For this model system, the polymer brushes provide quite remarkable lubrication,” he says.
Klein believes that similar molecular interactions occur in nature, lubricating joints such as those in the knee. He speculates that if similar systems can be developed with water as the solvent instead of toluene, some of the first uses could be in orthopaedics, perhaps to ease the movement of artificial joints.