Recent Developments in Shear and Extensional Microrheometry of Complex Fluids

We review the design and construction of several new microrheometers designed to facilitate the study of non-Newtonian fluids using very small sample volumes (1-10 μl) and on length scales of micrometer dimensions. These devices enable both the steady shear and transient extensional rheological properties of complex fluids to be measured. The shear-rate-dependent viscosity and gap-dependent yield stress of a fluid sample are measured using a sliding plate microrheometer with optical flats as the shearing surfaces. Compound flexure arrangements serve to generate a steady or oscillatory plane Couette flow and as the ‘pick-up’ or force transducer. 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 transient extensional rheology of very small samples is also measured using 1–100 μL fluid droplets in a microscale capillary break-up extensional rheometer. In this device, the extensional flow is driven by capillarity and resisted by the viscous and elastic stresses in the elongating fluid thread. These two devices are used to quantify the rheological properties of a wide range of complex fluids from consumer products to biopolymer solutions. Examples considered include mayonnaise and yogurt, biopolymer solutions such as the spinning dope extracted ex vivo from the silk worm, Bombyx mori and from the major ampullate gland of the Nephila clavipes spider, plus the physically-crosslinked mucin gel of slugs and snails.