Extensional flows that involve stretching of fluid elements arise in many natural and industrial processes, including inkjet printing, filament spinning, porous media flow, extrusion, moulding, coating, particle suspension/sedimentation, turbulent drag reduction, blood circulation, mucus flows (in e.g. lungs, eyes, mouth, etc) and the flow of synovial fluid between joints under compression. The fluids used in most of these applications are termed complex fluids or viscoelastic materials as their response to applied stress (or strain) involves a combination of solid-like elasticity and liquid-like ability to flow. For a fluid confined between two plates, the response to an applied stress (or strain) is measured typically using a torsional rheometer, where the shear viscosity describes the resistance offered by the fluid to a parallel displacement of the plates. In this case the velocity gradient is always perpendicular to the flow direction. If, on the other hand, the two plates are displaced so as to increase their separation, both flow and the velocity gradient are in the same direction resulting in a stretching or extensional flow field. Macromolecular stretching caused by the exposure of complex fluids containing long polymer molecules to extensional flow fields can lead to a much larger resistance to flow than expected on the basis of the shear viscosity measured in conventional torsional rheometers.

One of my main areas of inerest is the development of microfluidic extensional rheometry using the cross-slot geometry as a platform. Cross-slots consist of perpendicular bisecting channels with flow through opposing inlets and outlets. This generates a point of zero flow velocity (a stagnation point) at the centre of the cross, where macromolecules become subjected to a strong extensional flow. Macromolecular stretching can be observed by looking for flow-induced birefringence and near the stagnation point and results in an enhance pressure drop across the cross-slot, which can be used to assess the apparent extensional viscosity.

Flow in the cross-slot can be driven continuously, or in an oscillatory manner using piezo-electric micropumps on each arm of the cross. The oscillatory flow is particularly useful for the study of biofluids or in cases where the available volume of fluid is low. The system is called the Extensional Flow Oscillatory Rheometer (EFOR) and the movie below provides a schematic demonstration of how it works.