Carbopol: From a simple to a thixotropic yield stress fluid

There is an ongoing discussion in the literature about the flow behavior of the widely used model yield-stress fluid Carbopol. Some papers show that it is indeed the simple model yield-stress fluid that many people believe it to be. However, other authors report rheological hysteresis in the flow curve, transient shear banding that persists for a very long time, and the breaking of fore-aft symmetry in a falling ball experiment. Such behaviors have in the past been associated with thixotropic yield stress fluids, which are very different from simple ones. We present experiments that suggest that both types of behavior may be found in the same type of Carbopol, depending on the preparation: After strong stirring for a long time, the system becomes slightly thixotropic. Flow visualization experiments with fluorescently labeled Carbopol elucidate the difference between the simple and thixotropic behavior of the systems: The stirring breaks the polymers into smaller fragments, some of which are so small that they exhibit Brownian motion. We propose that this generates a depletion interaction that leads to gel formation, which in turn leads to the thixotropy.