Rheo-SAXS on colloidal platelets probed in the vorticity direction

In previous experiments we have shown, using time-resolved SAXS in combination with the cone-plate shear cell, that dispersions of colloidal platelets in the nematic phase display a complex responses to oscillatory shear flow. These experiments were performed in a so-called plate-plate geometry, where the flow-vorticity plane of the structure is probed by X-rays. Though the experiments were really successful [1], the final goal was to do experiments in a Couette geometry where structure in the more relevant flow-gradient plane is probed, whilst it is also possible to scan the gap in order to have special resolved information. In our last beam-time we succeeded to achieve this goal, using a couette geometry that was produced in Julich. Figure 1 shows the structure of nematic platelets submitted to oscillatory shear flow at 5 different positions throughout the gap of the shear cell. Care was taken that the scatter patterns were taken at the same time within an oscillation. Clearly in the middle of the gap there is a significant tilt of the director, while close to the wall the anchoring of the nematic is so strong that the tilt is suppressed. The fact that a tilt is observed is interesting in itself since in the standard scattering geometry there is tilt cannot be observed, though it has important implications for the flow behaviour. The fact that the tilt is only observed in the middle hints to a competition between wall anchoring and the stress-reducing flow response of the nematic phase. Thus, these images give a unique insight into the complex flow behaviour of this model colloidal system