Anomalous ordering in wormlike micelles under electric field

The rotational dynamics of cetyltrimethylammonium tosylate (CTAT) micellar solutions using transient electric birefringence (TEB) are reported. The birefringence signal is resolved into three components-a normal one, related to the alignment of free or partially free cylindrical micelles with the field, an anomalous one of opposite sign, observed for concentrations around the overlap concentration, c*, and a third one of same sign as the normal one observed for c > c*. The effects of surfactant concentration and temperature on these components are studied. The origin of the anomalous signal is analyzed by combining recently proposed pictures by Hoffmann and by Cates. In the latter case, disklike clusters are suggested to exist around c*. The dimensions of these clusters are estimated in two different ways: using the rotational diffusion of disklike clusters and considering the polarization of a charge cloud of the counterions. They are found to agree reasonably. The anomalous birefringence exhibits a maximum when plotted against the square of the electric field. It obeys a master curve, independent of concentration, when data are normalized by the maximum values.