Dynamic transitions in molecularly thin liquid films under frictional sliding

The friction properties of the molecularly thin films of an asymmetric ether, 1,3-dimethylbutyl hexadecyl ether (DBHE), confined between mica surfaces were investigated using the surface forces apparatus. Kinetic friction was measured as a function of normal load and sliding velocity, and the static friction (stiction) was measured as a function of normal load and surface stopping time. Kinetic friction measurements exhibited unstable sliding dynamics: the friction force exhibited cyclic bumps and valleys in the sliding velocity range from about 10(-2) to 1 microm/s, but above and below the velocity range, smooth sliding was observed. Stop-start experiments showed a stiction spike when surface stopping time exceeded a characteristic nucleation time, indicative of the static friction state at very low sliding velocity. These results imply that the friction of the confined DBHE film has at least three responsible friction states--static friction and two different kinetic friction states--depending on the sliding velocity. The unstable sliding (bumps and valleys of the friction force) reflects the dynamic transition between two different kinetic states. The different friction states and their transitions are discussed on the basis of the recent experiments and theories of "inverted" stick-slip friction.