Using NMR microscopy we measure the velocity distribution for a wormli
ke surfactant solution in the gap of a small angle cone-and-plate rheo
meter, This system, cetylpyridinium chloride/sodium salicylate 100 mM/
60 mM, exhibits biphasic shear band structure when the applied shear r
ate exceeds the critical rate of strain beyond which a plateau is obse
rved in the shear stress. The structure is characterized by two low/hi
gh shear interfaces and the region of high shear evolves by increasing
width as the average gap shear is increased.