  Using a novel bi-axial confocal rheoscope, we investigate the structure and
rheology of sheared colloidal suspensions under confinement. Consistent with
previous work [X. Cheng \textit{et al., Proc. Natl. Acad. Sci. U. S. A.}, 2011,
109, 63], we observe a vorticity-aligned string phase in moderate concentrated
colloidal suspensions under uniaxial shear. Using bi-axial shear protocols, we
directly manipulate the orientation and morphology of the string structures.
Simultaneously, we measure the suspension rheology along both the flow and
vorticity directions with a bi-axial force measurement device. Our results
demonstrate that despite the highly anisotropic microstructure, the suspension
viscosity remains isotropic and constant over the shear rates explored. These
results suggest that hydrodynamic contributions dominate the suspension
response. In addition they highlight the capabilities of bi-axial confocal
rheoscopes for elucidating the relationship between microstructure and rheology
in complex fluids.
