The way in which an object starts to slip under an applied shear force, the transition from static to dynamic friction, is critical for many domains ranging from tectonics and earthquake dynamics to stability and wear of mechanical systems. The fundamental questions like what is the origin of the larger friction of the onset of slip and how it drops to a smaller value of dynamic or steady-state friction have remained largely unanswered, while having very important practical implications.
Our recent precise measurements of the real contact area, using a novel fluorescence microscopy, show that, unexpectedly, slipping can cause the area of real contact to grow instead of shrink [1] Therefore, since the dynamic friction is smaller than the static friction, the average interfacial shear strength decreases in the slip process, disparaging the idea of a constant friction per unit contact area.
A related observation about frictional slip is that stationary interfaces demonstrate aging, meaning that the onset friction depends on the time that the surfaces have spent in contact. This behavior puts the frictional interfaces in one group with glassy and jammed matter.
We combine surface profilometry, contact area imaging, and friction force measurement techniques to study the elastoplastic changes of asperity contacts at frictional interfaces, hoping to understand the above aspects and their relations.
Contact:
Kasra Farain (k.farain@uva.nl)
Reference:
[1] Weber et al. , Frictional weakening of slip interfaces, Science Advances, 2019, 10.1126/sciadv.aav7603