The 234th American Chemical Society (ACS) National Meeting edition:234 location:Boston, MA (USA) date:19-23 August 2007
The rheology of complex fluids in consumer products 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 contribution, we discuss the rheology of microgel particle dispersions in narrow gaps from 1-100μm. 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, the latter is dominated by apparent wall slip and the apparent viscosity depends on the degree of confinement, the mass concentration and the volumetric swelling ratio of the microgel particles.