  The sliding motion of objects is typically governed by their friction with
the underlying surface. Compared to translational friction, however, rotational
friction has received much less attention. Here, we experimentally and
theoretically study the rotational depinning and orientational dynamics of
two-dimensional colloidal crystalline clusters on periodically corrugated
surfaces in the presence of magnetically exerted torques. We demonstrate that
the traversing of locally commensurate areas of the moir\'e pattern through the
edges of clusters, which is hindered by potential barriers during cluster
rotation, controls its rotational depinning. The experimentally measured
depinning thresholds as a function of cluster size strikingly collapse onto a
universal theoretical curve which predicts the possibility of a
superlow-static-torque state for large clusters. We further reveal a
cluster-size-independent rotation-translation depinning transition when
lattice-matched clusters are driven jointly by a torque and a force. Our work
provides guidelines to the design of nanomechanical devices that involve
rotational motions on atomic surfaces.
