Gap-Dependent Microrheometry of Complex Liquids

We review recent developments in the field of microrheometry and describe a new flexure-based microrheometer (FMR) that can make measurements of the steady shear viscosity of transparent or optically-opaque complex fluids using 1-10 μl samples. We document the complex microrheology that may arise in a typical consumer product such as a skin crème, when the microstructure of the multiphase material (in this case a oil-water emulsion containing microcrystalline wax particles) and the characteristic length scale of the flow device become comparable. The material is a highly filled emulsion and conventional cone-and-plate rheometry shows that the material exhibits an apparent yield stress. Analogous tests in the microrheometer show that a sequence of flow transitions occur which lead to an apparent viscosity that is both gap– and shear-stress– (or shear-rate)–dependent. The microrheological measurements are combined with a slip analysis and optical microscopy to formulate a ‘phase diagram’ identifying the critical stress and corresponding slip velocity for each flow transition as a function of the gap. In the case of the skin crème investigated in the present study, the flow behavior in small gaps below 100 μm is dominated by the presence of micro-crystalline wax particles with mean diameters in the range of 50μm. The sudden decrease in the apparent viscosity, or equivalently the sudden increase in the slip velocity between two surfaces with decreasing gap and increasing applied stress is of importance in end-user perceptions of textural quantities such as ‘slipperiness’ or ‘lubricity’ which are typically used to characterize creams and other complex consumer products. The ability to progressively vary the gap between the two shearing surfaces means that the flexure-based microrheometer developed in the present work may find future applications in helping to bridge the gap between the traditionally-distinct fields of bulk rheological characterization and tribological testing. Gap-Dependent Microrheology