Characterizing the rheology of lamellar gel networks with optical coherence tomography velocimetry

Lamellar gel networks based on mixtures of cetostearyl alcohol and a cationic surfactant, cetyl-trimethylammonium chloride, were studied using a combination of rheometry and optical coherence tomography (OCT) velocimetry. Experiments were conducted in a stress-controlled rheometer with a parallel plate geometry. Each formulation was found to exhibit a yield stress and thixotropy. The shear start-up behavior in response to a constant stress was directly observed using OCT velocimetry. Close to the yield stress, the velocity had a power law behavior with time after an initial period of transience. At larger stresses, the velocity undergoes two successive increases in power law scaling with time. When sheared at low, constant, shear rates 1-5 s(-1), the fluids exhibit plug flow with strong wall slip at both rheometer plates. At rates of 10-150 s(-1), the fluids separate into a distinctive three shear band morphology while the wall slip reduces. These rheological properties can be explained by a multilamellar vesicle to planar lamellae transition.