Nanoscale friction could lead to surface improvement

Scientists from the Technische Universitaet Muenchen (TUM) have observed a previously unknown type of friction that occurs at the nanoscale.

In search of low-friction components for nanosystems, the physicists led by professors Thorsten Hugel and Alexander Holleitner examined how and why single polymer molecules in various solvents slide over or stick to certain surfaces.

Their goal was to understand the basic laws of physics at the molecular scale in order to develop targeted anti-friction surfaces and suitable lubricants.

For their studies the scientists attached the end of a polymer molecule to the nanometre-fine tip of an atomic force microscope (AFM). While they pulled the polymer molecule over test surfaces, the AFM measured the resulting forces, from which the researchers could directly deduce the behaviour of the polymer coil.

Besides the two expected friction mechanisms such as sticking and sliding the researchers detected a third one for certain combinations of polymer, solvent and surface. The researchers dubbed this ‘desorption stick’.

‘Although the polymer sticks to the surface, the polymer strand can be pulled from its coiled conformation into the surrounding solution without significant force to be exerted,’ said Prof Hugel. ‘The cause is probably a very low internal friction within the polymer coil.’

According to a statement, desorption stick doesn’t depend on the speed of movement or the support surface or adhesive strength of the polymer. Instead, the chemical nature of the surface and the quality of the solvent are decisive; hydrophobic polystyrene exhibits pure sliding behaviour when dissolved in chloroform. In water, however, it shows desorption stick.

‘The understanding gained by our measurement of single-molecule friction opens up new ways to minimize friction,’ said Prof Holleitner. ‘In the future, with targeted preparation of polymers, new surfaces could be developed specifically for the nano- and micrometer range.’

The work was supported by the German Research Foundation (DFG) and the Cluster of Excellence Nanosystems Initiative Munich (NIM).

Their paper, Nanoscale Friction Mechanisms at Solid–Liquid Interfaces, can be found here.