Proceedings of the 6th European Conference on Rheology pages:473-474
6th European Conference on Rheology edition:6 location:Erlangern, Germany date:1-6 September 2002
We describe the design and construction of a new microrheometer designed to facilitate the viscometric study of complex fluids with very small sample volumes (1-10 l) in gaps of micrometer dimensions. The device is configured as a sliding plate rheometer with optical flats (polished flat to within /20, or 30nm) as the shearing surfaces. White light interferometry and a three-point nanopositioning stage using piezo-stepping motors are used to control the parallelism of the upper and lower surfaces. A compound flexure system is used to hold the fluid sample testing unit between a drive spring and an independent sensor spring. Alignment fidelity, device orthogonality and total error stack-up are all optimized by machining the entire instrument frame from a single monolithic aluminum block using water-jet and EDM technology. The dynamic force range of the instrument can be varied greatly by changing the length and/or thickness of the compound flexures. Displacements in the sensing flexure are detected using an inductive proximity sensor with a resolution of ± 3 nm allowing the detection of loads up to 6 N with an accuracy of 3 mN. Calibrations give a standard deviation from linearity of the sensor spring of less than 2%. The lower plate is attached to a drive flexure which is moved by an ‘inchworm’ motor with a maximum displacement of 6 mm and a resolution of 0.1 nm. The rheological properties of fluids such as microgels, emulsions, or self-assembling peptide solutions can be probed as a function of time and plate separation using small amplitude sinusoidal oscillations. Sawtooth (periodic triangular) waveforms also allow large straining motions (up to = 100 at a gap of 10 µm) to be probed. Because there is convenient optical access to the sample circular dichroism, birefringence and optical microscopy can all be performed during the imposed shearing deformation.