Detailed insight into friction: how objects start to slide

A collaboration of chemists and physicists have shed light on a crucial aspect of friction: how things begin to slide. Using fluorescence microscopy and dedicated fluorescent molecules, they were able to pinpoint how and when friction at the contact between two objects is overcome and when sliding starts to occur. The researchers from the University of Amsterdam, research institute ARCNL and the University of Twente report on the details of this important transition from static to dynamic friction in The Journal of Physical Chemistry Letters.

Friction is responsible for an estimated 25% of the world energy consumption. One of the key questions for the stability of many systems is how and when objects start to slide with respect to each other; think of earthquakes or your feet on the ground. When two objects touch, the contact area is formed by the many microscopic protrusions of the two interfaces that touch and interlock. Application of a shear force makes the objects slide along each other, breaking these initial contacts.

Investigation of friction phenomena using fluorescence microscopy from Universiteit van Amsterdam on Vimeo. This video captures the transition from static to dynamic friction which marks the onset of movement. Dark colors represent regions of contact where the shearing force is too small to overcome the friction force. In the light regions, the movement starts at the micro-scale until the entire object begins to slide.

Dragging a sphere over a glass surface
In the paper just published the researchers report on experiments in which a sphere is dragged over a glass surface. The glass surface has been decorated with a special type of molecule (fluorogenic mechanophores) that start to emit light (fluorescence) when under the stress of the shearing force. The moment this force disappears, the molecules return to their stable, non-fluorescent form. This allows to directly visualize and quantify the microscopic shear force down to the microscopic roughness level, and establish how it evolves during the transition from the static to the moving state. The researchers find, among other things, that just before sliding occurs, a slip wave propagates from the edge toward the center of the macroscopic contact area. This allows for a quantitative and microscopic local understanding of how surfaces start to slide.

Reference
Chao-Chun Hsu, Feng-Chun Hsia, Bart Weber, Matthijn B. de Rooij, Daniel Bonn, and Albert M. Brouwer, Local Shearing Force Measurement during Frictional Sliding Using Fluorogenic Mechanophores, J. Phys. Chem. Lett, 13, 8840−8844 (2022).
DOI: 10.1021/acs.jpclett.2c02010