Molecules shed light on the secrets of friction

29 January 2015

Researchers at the University of Amsterdam (UvA) and Foundation for Fundamental Research on Matter (FOM) have developed a new microscopy method that enables the visualization of nanoscale contacts between two surfaces. This allows one to see precisely where two objects touch and thus where forces are transferred between the objects. Their findings were recently published in the leading scientific journal 'Angewandte Chemie'.

Mechanical friction is responsible for 30% of worldwide energy consumption, but it is poorly understood scientifically and therefore hard to predict. Surfaces always have a certain roughness on the microscopic scale. Even large objects ultimately touch one another at the scale of atoms, but how such contacts look like in detail is unknown. The researchers conceived a unique method to visualize such details. 

A chemical look on physical contact

By combining their expertise physicists and chemists of the UvA and FOM have succeeded in creating molecules that light up under pressure. The molecules are attached to one of the surfaces using chemical bonds. When the two objects are brought into contact, the molecules show exactly where the force is exerted on the surface to which they are attached. The researchers demonstrated their technique by bringing a plastic sphere into contact with a flat glass surface and observing the fluorescence of the molecules in the touching region with a microscope. The famous physicist Heinrich Hertz already gave a theoretical description in 1881 of the increase of the contact area when two objects are pressed together with increasing force. In their experiments, the researchers showed that the theory works perfectly on the scale of micrometers, but they could also see fine details within the overall contact region. Those are a result of the roughness of the surface of the plastic sphere.

Smaller objects suffer more friction

With the new technique, it is possible for the first time to visualize contacts at the molecular scale. Such detailed observations can provide deeper insight into the ubiquitous phenomenon of friction. For example, small machines, or machines in which small parts are moving, suffer increasingly from friction as they are further miniaturized. A gearwheel with teeth of only a few micrometres thick can wear much faster than its larger analogue. Friction, therefore, currently limits the further miniaturization of applications with moving parts.

Details

Tomislav Suhina, Bart Weber, Chantal E. Carpentier, Kinga Lorincz, Peter Schall, Daniel Bonn & Albert M. Brouwer: ‘Fluorescence Microscopy Visualization of Contacts between Objects’, in: Angewandte Chemie International Edition (28 January 2015/DOI:10.1002/anie.201410240).

Published by  University of Amsterdam