Friction attacked at an atomic level

Stick slip, a term for friction between two sliding surfaces in machines, is an expensive problem for industry, causing wear, overheating and wasted energy. To solve the problem, a research team lead by Dr Uzi Landman, director of the Georgia Institute of Technology’s centre for computational materials science, is using new concepts in the study […]

Stick slip, a term for friction between two sliding surfaces in machines, is an expensive problem for industry, causing wear, overheating and wasted energy.

To solve the problem, a research team lead by Dr Uzi Landman, director of the Georgia Institute of Technology’s centre for computational materials science, is using new concepts in the study of lubricants.

Atomic-level studies of thin-film lubricants showed that in a confined space of, typically, 20 Angstroms, a lubricant like hexadecane forms into long molecular chains in four or five layers parallel to the bearing surfaces. In this state, it becomes semi-solid, resisting the shearing forces needed to make the surfaces slide and increasing the energy needed to get them moving again.

By constantly varying the gap between the two surfaces, Landman has found that the lubricant stays highly fluid. An oscillation of just one Angstrom, equal to one- twentieth of the normal film thickness, causes molecular disorder and frustrates the lubricant’s effort to become an orderly semi-solid.

Landman says this makes it possible to achieve super-kinetic sliding a situation where velocity is high enough to overcome frictional forces and overcome stick slip at much lower velocities than normal. This could make stopping and starting machines a much smoother process.

The lubricant’s viscosity determines the frequency at which the gap must oscillate. Thicker liquids need less frequent oscillations to maintain molecular disorder, as they are slower to adjust to the gap size. Landman believes small oscillations in the gap would not cause bearings to heat up or the cavitation effect, where energy is released by collapsing bubbles.

The research, sponsored by the US Air Force Office of Scientific Research and the US Department of Energy, is likely to benefit machines such as magnetic disc drives, for which energy consumption is a critical factor.