Axisymmetric flow of surfactant solutions past a sphere moving at constant velocity is experimentally investigated. Test fluids used are entangled wormlike micelle solutions of 30mM cetyletrimethylammonium bromide and sodium salicylate at various molar ratios. To obtain full-field flow birefringence image of transient flows behind an impulsively started sphere, a division of focal plane type polarization image sensor is utilized. Left circularly polarized light is used as incident light and the degree of linear polarization (DOLP) and the angle of polarization (AOP) of transmitted light are measured, from which distributions of spatially averaged values of the retardation and the angle of fast axis of the fluid are obtained. Distributions of the retardation along the centerline of the sphere are investigated at various Deborah numbers. At a critical Deborah number, the retardation distributions become propagate at a constant speed away from the sphere. The measurements also show the abrupt change in the wake accompanied by the necking of the cross section when Deborah number exceeds a critical value.
