75th Annual Meeting of The Society of Rheology edition:75 location:Pittsburgh, PE, USA date:12-16 October 2003
We describe the design and construction of two new microrheometers designed to facilitate the study of complex fluids using very small sample volumes (1-10 µl). The shear-rate-dependent viscosity is measured using a sliding plate microrheometer with optical flats (polished flat to within 30nm) as the shearing surfaces. White light interferometry and a three-point nanopositioning stage employing 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 between a drive spring and an independent sensor spring. Alignment fidelity and device orthogonality are minimized by machining the entire instrument frame from a single monolithic aluminum block using water-jet and EDM technology. 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. The lower plate is driven by an ‘inchworm’ motor with a resolution of 0.1 nm and a maximum displacement of 6 mm, thus allowing large strains to be obtained. The transient extensional rheology is also measured using a 1 µl fluid droplet in a microscale capillary break-up extensional rheometer. The extensional flow is driven by capillarity and resisted by the viscous and elastic stresses in the elongating fluid thread.
These devices are used to quantify the rheological properties of the spinning dope extracted ex vivo from the major ampullate gland of a Nephila clavipes spider. The present study shows that the shear viscosity of this 30wt% protein solution decreases ten-fold as it is pushed through the long narrow converging S-duct in the abdominal cavity of the spider, whereas the extensional viscosity increases more than one hundred-fold during the spinning process. Quantifying the properties of native spinning solutions provides new guidance for adjusting the spinning processes of synthetic or genetically-engineered silks to match those of the spider.