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 the first time simultaneous in situ small-angle x-ray scattering and surface force measurements. We have used the XSFA-II to monitor shear-induced orientational transitions in a lyotropic model lubricant system. Upon applying small shear amplitudes ( ~20 μm) to a relatively thick ( ~800 μm) film, we observed evidence for the formation of an orientational boundary layer at the shearing surface. Time-resolved x-ray diffraction revealed the gradual transition to shear-favored orientation by growth of the boundary layer.