Dillavou Sam, Rubinstein Shmuel M
Department of Physics, Harvard University, Cambridge, Massachusetts 02138, USA.
John A. Paulson School of Engineering and Applied Sciences, Harvard University, Cambridge, Massachusetts 02138, USA.
Phys Rev Lett. 2020 Feb 28;124(8):085502. doi: 10.1103/PhysRevLett.124.085502.
We simultaneously measure the static friction and the real area of contact between two solid bodies. These quantities are traditionally considered equivalent, and under static conditions both increase logarithmically in time, a phenomenon coined aging. Here we show that the frictional aging rate is determined by the combination of the aging rate of the real area of contact and two memory-erasure effects that occur when shear is changed (e.g., to measure static friction.) The application of a static shear load accelerates frictional aging while the aging rate of the real area of contact is unaffected. Moreover, a negative static shear-pulling instead of pushing-slows frictional aging, but similarly does not affect the aging of contacts. The origin of this shear effect on aging is geometrical. When shear load is increased, minute relative tilts between the two blocks prematurely erase interfacial memory prior to sliding, negating the effect of aging. Modifying the loading point of the interface eliminates these tilts and as a result frictional aging rate becomes insensitive to shear. We also identify a secondary memory-erasure effect that remains even when all tilts are eliminated and show that this effect can be leveraged to accelerate aging by cycling between two static shear loads.
我们同时测量了两个固体之间的静摩擦力和实际接触面积。传统上认为这些量是等效的，并且在静态条件下，它们都会随时间呈对数增长，这种现象被称为老化。在此我们表明，摩擦老化速率由实际接触面积的老化速率与剪切力改变（例如测量静摩擦力时）所发生的两种记忆擦除效应共同决定。施加静态剪切载荷会加速摩擦老化，而实际接触面积的老化速率不受影响。此外，负向的静态剪切拉力（而非推力）会减缓摩擦老化，但同样不会影响接触的老化。这种剪切对老化的影响源于几何因素。当剪切载荷增加时，两个物体之间微小的相对倾斜会在滑动之前过早地擦除界面记忆，从而抵消老化的影响。改变界面的加载点可消除这些倾斜，结果摩擦老化速率对剪切力变得不敏感。我们还识别出一种即使在所有倾斜都消除后仍然存在的次要记忆擦除效应，并表明通过在两个静态剪切载荷之间循环，可以利用这种效应来加速老化。
