|
Sliding friction is often simulated by moving sliders at some externally imposed relative velocity v, and extracting the resulting frictional force f. That procedure is at variance with most real life cases, where a force f is applied, and the sliding velocity v results. If the velocity-force curve is monotonic the two protocols are equivalent; but if it is nonmonotonic, it is not. In the realistic case of an externally applied force one may expect characteristic velocity instabilities, due to regions of "negative differential friction," where dv/df <0. We exemplify this phenomenon for the case of two single walled nanotubes sliding coaxially inside one another. Here the v-f characteristics is S-shaped in correspondence to frictional peaks due to the selective sliding-induced excitation of outer tube vibrations. Due to negative differential friction the sliding velocity is predicted to exhibit striking
plateaus and jumps as the pulling force increases.
|