Nonlinear microrheology of wormlike micelle solutions using ferromagnetic nanowire probes

Ferromagnetic nanowires were employed to investigate the microrheology of wormlike micelle solutions composed of equimolar cetylpyridinium chloride-sodium salicylate. For a wire rotated about a short axis, the drag at low rotation rate omega(R) is independent of omega(R) and strongly temperature dependent, consistent with the macroscopic shear viscosity. Above a critical rotation rate omega(c), the drag is independent of temperature and decreases as a power law with increasing rate. The onset of nonlinear drag is characterized by a peak associated with contributions from extensional flow. Above omega(c), the fluid generates an additional torque that tilts the wire out of regions of high shear flow and that is interpreted as a consequence of a shear-induced transition to nematic order among the micelles. Rotation of the wire in response to this torque reveals directly the anisotropy of the drag in the nonlinear state.