We demonstrate that friction is not always an exact number. In a simple lubricated system (atomically smooth mica separated by molecularly thin films of squalane), sinusoidal shear forces were applied for millions of cycles. The kinetic friction increased steadily over repetitive cycles and collapsed intermittently to the average value. Individual slip cycles also consisted of a cascade of smaller slip events. The distribution of fluctuation size and duration followed a power law.

• Received 4 January 1996

DOI:https://doi.org/10.1103/PhysRevLett.77.4330