  Understanding the correlation between structure and rheology in colloidal
suspensions is important as these suspensions are crucial in industrial
applications. Moreover, colloids exhibit a wide range of structures under
confinement that could considerably alter the viscosity. Here, we use a
combination of experiments and simulations to elucidate how confinement induced
structures alter the relative contributions of hydrodynamic and repulsive
forces to produce up to a ten fold change in the viscosity. We use a custom
built confocal rheoscope to image the particle configurations of a colloidal
suspension while simultaneously measuring the viscosity. We find a
non-monotonic trend to the viscosity under confinement that is strongly
correlated with the microstructure. As the gap decreases below 15 particle
diameters, the viscosity first decreases from its bulk value, shows
fluctuations with the gap, and then sharply increases for gaps below three
particle diameters. Further, we compare our experimental results to two
simulations techniques that enables us to determine the relative contributions
of hydrodynamic and short range repulsive stresses. The first method uses the
lubrication approximation to find the hydrodynamic stress and includes a short
range repulsive force between the particles and the second is a Stokesian
dynamics simulation that calculates the full hydrodynamic stress in the
suspension. We find that the decrease in the viscosity at moderate confinements
has a significant contribution from both the hydrodynamic and repulsive forces
whereas the increase in viscosity at gaps less than three particle diameters
arises primarily from short range repulsive forces. These results provide new
insights to the unique rheological behavior of confined suspensions and further
enable us to tune the viscosity by changing properties such as the gap,
polydispersity, and the volume fraction.
