Micro- and macro-shear viscosity in dispersed lamellar phases

Surfactant phases, such as dispersed lamellar gels, are extremely useful in commercial products because they are very weight-effective at building viscosity. An enduring challenge is to determine the microstructural features responsible for the bulk rheology so that we can design products with improved performance. The samples described here have very different theological profiles as exemplified by an order-of-magnitude difference in their zero-shear-rate viscosity, and infinite-shear-rate viscosities which differ by half an order of magnitude. As a first approximation we consider the dispersed lamellar system to be analogous to a high-intemal-phase-volume emulsion which is described by the well-known Kreiger-Dougherty equation. This requires us to establish the value of a number of parameters of which the continuous phase viscosity is the one that defines the baseline viscosity. We measured this in situ by a micro-viscosity technique involving Fluorescence Correlation Spectroscopy using microscopic probes: viz. a fluorescent dye molecule (rhodamine) of size 0.85 nm; a lyzozyme protein of 2 nm size and a quantum dot of 12.5 nm size. We show that the continuous phase has a viscosity about twice that of water. Moreover, this viscosity is the same for the all three probes indicating that the system is quite uniform at the microscopic level investigated. Interestingly, this micro-viscosity was practically the same for all the samples and thus could not be correlated with zero-shear-rate viscosity or other theological characteristics. We conclude that the macro-viscosity arises from structures much larger than 25 nm (twice the hydrodynamic diameter of the quantum dot). Our future intention is to use larger probes to establish the length-scale at which the microstructure begins to be apparent in the bulk rheology characteristics. (c) 2007 Elsevier B.V. All rights reserved.