Bistability in non-Newtonian flow: Rheology of lyotropic liquid crystals

We study the rheological properties of self-assembled surfactant structures: lyotropic liquid crystals exhibiting plane lamellar and ``onion'' (concentric bilayer) textures. Both systems behave similarly: they are gel-like and exhibit extreme shear-thinning behavior. We demonstrate that this results in a hysteresis in the stress-strain curve when the stress is imposed. Controlled shear-rate experiments reveal the origin of the hysteresis by demonstrating the existence of two extrema in the stress-strain curve. The bistability leads to the formation of shear bands in the system and is illustrated by demonstrating the existence of sudden jumps in the viscosity of the fluid, following the viscosity of the fluid in time.