Shear-Induced Gelation and Fracture in Micellar Solutions.

A wide variety of surfactant molecules self-assemble in solution to form very long flexible micelles. While great progress has been made in understanding the flow behavior of such systems [1], there exists a large class of wormlike micellar solutions which exhibit anomalous behavior, namely, a dramatic increase in viscosity above a critical shear rate and a concomitant decrease in the turbulent drag. It has been suggested that these phenomena are associated with aggregation of micelles induced by the shear flow [2–4]. However, there is only indirect evidence for the existence of any shear-induced structures and very little is known about them or how they might contribute to shear thickening or drag reduction. In this Letter, we report the first direct observation of shear-induced structures (SIS) using a novel light scattering microscopy technique. These microscopy measurements are augmented by light scattering measurements of changes in the micelle structure during shear flow, by velocity profile measurements, and by rheological data. These data show a fascinating series of shear-induced structural changes which give rise to the observed shear thickening. Moreover, our measurements indicate a correspondence between the shear-induced structural changes we measure and a number of well-documented but puzzling observations in the rheological response of the system. In particular, we have observed shear-induced gelation, fracture, and the formation of intricate cyclic patterns caused by an elastic instability. For our studies, we used aqueous solutions of hexadecyltrimethylammoniumbromide (CTAB) and sodium salicylate (NaSal). Extreme care was taken to ensure reproducibility and allow samples to reach thermal equilibrium before each measurement. The concentrations of CTAB ranged from 30 to 2500 ppm by weight; the molar ratio of salt to surfactant was 1:1. The range of concentrations and salt-to-surfactant ratio were chosen so as to produce the strongest shear-thickening effect. Some of the unusual flow properties of these dilute wormlike micellar solutions are illustrated in Fig. 1, where we show viscosity vs shear rate for three different