  We investigate the dynamics and rheological properties of a circular
colloidal cluster that is continuously sheared by magnetic and optical torques
in a two-dimensional (2D) Taylor-Couette geometry. By varying the two driving
fields, we obtain the system flow diagram and report the velocity profiles
along the colloidal structure. We then use the inner magnetic trimer as a
microrheometer, and observe continuous thinning of all particle layers followed
by thickening of the third one above a threshold field. Experimental data are
supported by Brownian dynamics simulations. Our approach gives a unique
microscopic view on how the structure of strongly confined colloidal matter
weakens or strengthens upon shear, envisioning the engineering of rheological
devices at the microscales.
