  A sheared microscopic graphite mesa retracts spontaneously to minimize
interfacial energy. Using an optical knife-edge technique, we report
comparative measurements of the speeds of such self-retracting motion (SRM) in
air and low vacuum (about 1 Pa). We found that the retracting speed Vm of SRM
is almost the same in the two environments at both room temperature and high
temperature (above 150 celsius degree). We also extract the acceleration of the
SRM, and found some typical behaviors. In the high speed range (above 5 m/s),
the acceleration decreases linearly with the logarithm of the SRM speed. We
attribute this phenomenon to the thermally activated sliding process in SRM. In
the low speed region (between 0.1 m/s and 1 m/s), the acceleration increases
linearly with the logarithm of the SRM speed.
