Author:

Abstract:

The field of microrheometry encompasses experimental techniques that extend classical rheological techniques down to the determination of the viscoelastic fluid properties on the micro- and nanometer scale. Such techniques enable the study of flow transitions, wall slip, yield and frictional processes of complex fluids on small scales. We report on new applications of three different microrheometers that have been designed to facilitate the study of complex fluids using very small sample volumes (1-10 µl) and very small gaps (200nm-100m): - A sliding plate microrheometer (FMR) with optical flats as the shearing surfaces, held in a compound flexure frame machined from a single monolithic aluminum block. The device employs white light interferometry and nanopositioning motors to control the parallelism of the upper and lower surfaces. Inductive proximity sensors and ‘inchworm’ motors allow for the detection loads up to 6 N with an accuracy of 3 mN in a rate controlled strain with a resolution of 0.5 nm and a maximum displacement of 6 mm. Steady shearing motions and large amplitude oscillatory shear (LAOS) flows can then be imposed on structured materials such as food stuffs, consumer products and biopolymer gels. - A normal force controlled triborheometer that allows a precise determination of the coefficient of sliding friction for complex biofluids such as hyaluronic acid solutions in the hydrodynamic, mixed and boundary lubrication regimes. - A microcapillary break-up extensional rheometer that allows the measurement of transient extensional stresses of an elongating fluid thread using stretched 1 µl fluid droplets. These devices are utilized to investigate the nonlinear rheological properties of µl-samples of biopolymers, wall-slip and gap-dependent changes in the yield stresses, and the load- and gap-dependent ‘lubricity’ of complex fluids.