Direct observation of shear-induced orientational phase coexistence in a lyotropic system using a modified X-ray surface forces apparatus

The second generation x-ray surface forces apparatus (XSFA-II) allows for t he first time simultaneous in situ small-angle x-ray scattering and surface force measurements. We have used the XSFA-II to monitor shear-induced orie ntational transitions in a lyotropic model lubricant system. Upon applying small shear amplitudes (similar to 20 mum) to a relatively thick (similar t o 800 mum) film, we observed evidence for the formation of an orientational boundary layer at the shearing surface. Time-resolved x-ray diffraction re vealed the gradual transition to shear-favored orientation by growth of the boundary layer.