Slip-stick and the evolution of frictional strength

The minutiae of friction The behaviour of systems as diverse as earthquakes and hard drives is influenced by frictional motion and its strength. What at first glance appears to be a continuous sliding process between touching surfaces is in fact a product of a series of 'slip' and 'stick' events on the microscopic scale. The mechanism of evolution of frictional strength at this level, though, is still unclear. Ben-David et al . have studied the evolution of the local contact area between two sliding bodies (PMMA plastic blocks) and the motion of their interface, and find that it involves four distinct phases. Within microseconds, all the contact area reduction has occurred. This is followed by a rapid slip phase, then a sharp transition to much slower slippage culminating in a 'stick' phase when motion is arrested. After several hundred microseconds the contact area begins to increase again. These results provide a basis for a better understanding of this kind of motion in many technologically important contexts. From earthquakes to hard drives, frictional motion and its strength are involved in a wide range of phenomena. The strength of an interface that divides two sliding bodies is determined by both the real contact area and the contacts' shear strength. By continuous measurements of the concurrent local evolution of the real contact area and the corresponding interface motion from the first microseconds when contact detachment occurs, frictional strength is now characterized from short to long timescales. The evolution of frictional strength has great fundamental and practical importance. Applications range from earthquake dynamics 1 , 2 , 3 , 4 to hard-drive read/write cycles 5 . Frictional strength is governed by the resistance to shear of the large ensemble of discrete contacts that forms the interface that separates two sliding bodies. An interface’s overall strength is determined by both the real contact area and the contacts’ shear strength 6 , 7 . Whereas the average motion of large, slowly sliding bodies is well-described by empirical friction laws 3 , 8 , 9 , 10 , interface strength is a dynamic entity that is inherently related to both fast processes such as detachment/re-attachment 11 , 12 , 13 , 14 and the slow process of contact area rejuvenation 6 , 7 , 13 , 15 , 16 . Here we show how frictional strength evolves from extremely short to long timescales, by continuous measurements of the concurrent local evolution of the real contact area