The Society of Rheology 79th Annual Meeting edition:79 location:Salt Lake City, UT (USA) date:7-11 October 2007
The rheology of many materials relevant to consumer products and biomedical applications often depends strongly on the characteristic length scale of the device or flow of interest. For these highly structured fluids, the flows occurring during the actual application of the product are often confined to geometries with dimensions of a few micrometers. Examples include the spreading of skin lotions, or the organoleptic perception of food and pharmaceutical products. In this study, we measure the nonlinear rheology of microgel particle dispersions in narrow gaps from 1-100 micrometers. We use an interferometry-based sliding plate microrheometer to impose large shearing deformations on microliter fluid samples confined between parallel optical flats. The macrorheological response is compared with the flow in the micro-confined geometry. Whereas the bulk rheology is typical of the broad class of soft solid materials with a pronounced yield stress followed by strong shear-thinning, the flow in the confined geometry is dominated by wall slip and the apparent viscosity depends on the degree of confinement, the mass concentration and the volumetric swelling ratio of the microgel particles. The evolution in the slip velocity and extrapolation length of the confined microgel can be systematically determined in this sliding plate rheometer as a function of degree of confinement and shear stress. Thixotropy in the confined samples can also be determined by repeated shearing using periodic large amplitude oscillations.