Influence of a velocity gradient on small angle neutron scattering of a sheared solution with rodlike micelles

A solution consisting of 20 mM cetylpyridiniumsalicylate (CPS) and 20 mM sodiumsalicylate (NaSal) in which rodlike micelles are present, was sheared and studied in a small angle neutron scattering (SANS) experiment. Without a velocity gradient the scattering curves of constant intensity were circles which reflect an isotropic distribution of the orientation of the scatterers. If a velocity gradient is applied to the solution the curves of constant scattering intensity show a characteristic deviation from the circles due to the partially ordering of the rods. From that deviation the rotational diffusion coefficient can be obtained. A theoretical treatment of this problem is given and applied to the experimental data.