  Surface and interfacial energies play important roles in a number of
instability phenomena in liquids and soft matters, but are rare to play a
similar role in solids. Here we report a new type of mechanical instabilities
that are controlled by surface and interfacial energies and are valid for a
large class of materials, in particular two-dimensional layered materials. When
sliding a top flake cleaved from a square microscale graphite mesa by using a
probe acted on the flake through a point contact, we observed that the flake
moved unrotationally for a certain distance before it suddenly transferred to a
rotating-moving state. The theoretical analysis that agrees well with the
experimental observation reveals that this mechanical instability is an
interesting effect of the structural superlubricity (a state of nearly zero
friction). Our further analysis shows that this type of instability holds
generally for various sliding joints on different scales, as long as the
friction is ultralow. Thus, the uncovered mechanism provides useful knowledge
for manipulating and controlling all these sliding joints, and can guide design
of future structural superlubricity based devices.
