Lamellar phases under shear: variation of the layer orientation across the couette gap
The shear induced orientation of a lyotropic lamellar phase in the system sodium dodecylsulfate (SDS)/decanol/water was studied. The distribution of the layer orientation across the gap of a couette type shear cell was investigated by means of small-angle X-ray scattering. As a function of shear rate, a transition from parallel to perpendicular layer orientation was observed. The measurements showed that the lamellar orientation depends on the position within the gap, with a parallel orientation preferred near the rotating outer wall of the shear cell. The inhomogeneous orientation distribution across the gap explains both the bimodal alignment reported earlier as well as the transition from parallel to perpendicular orientation over a broad range of shear rates in a shear controlled experiment. Therefore, quantitative studies on layer orientation need to consider the exact position within the gap where the beam has probed the sample.