  Based on (overdamped) Stokesian dynamics simulations and video microscopy
experiments, we study the non equilibrium dynamics of a sheared colloidal
cluster, which is confined to a two-dimensional disk. The experimental system
is composed of a mixture of paramagnetic and non magnetic polystyrene
particles, which are held in the disk by time shared optical tweezers. The
paramagnetic particles are located at the center of the disk and are actuated
by an external, rotating magnetic field that induces a magnetic torque. We
identify two different steady states by monitoring the mean angular velocities
per ring. The first one is characterized by rare slip events, where the inner
rings momentarily depin from the outer ring, which is kept static by the set of
optical traps. For the second state, we find a bistability of the mean angular
velocities, which can be understood from the analysis of the slip events in the
particle trajectories. We calculate the particle waiting- and jumping time
distributions and estimate a time scale between slips, which is also reflected
by a plateau in the mean squared azimuthal displacement. The dynamical
transition is further reflected by the components of the stress tensor,
revealing a shear-thinning behavior as well as shear stress overshoots.
Finally, we briefly discuss the observed transition in the context of
stochastic thermodynamics and how it may open future directions in this field.
