Rheology of Complex Fluids in Microscopic Flows: Quantitative Characterisation from Molecular Dynamics to Fluid Flows

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Recently experimental evidence on the intriguing flow phenomena of complex fluids in microfluidic systems has attracted enormous attention. The small scale of microfluidics makes flow of large deformation rate easily accessible. Hence even a low-viscosity polymer solution with a short relaxation time can reach the high Weissenberg (We) number flow regime, in which elastic forces dominate over viscous forces, and so exhibit strong viscoelastic effects. Flow-induced phase separation of polymer solutions can be much more pronounced in this regime. There are also examples of turbulence-like instabilities in the flows of polymer solutions at low Reynolds (Re) number but high We number regimes. Such elastic turbulence could be harnessed in microfluidic devices to act as powerful and efficient mixers and as dynamic valves, and even to construct functional memory and control devices which are insensitive to electromagnetic noise. Thus progress in microfluidics technology gives rise to new opportunities in understanding the fundamental physics of complex fluid flows, while innovation and optimisation of the technology itself can also greatly benefit from the new knowledge generated from a fundamental study.There are few quantitative results available concerning complex fluid flow in the characteristic flow dimension less than 200 micron. The industrial sectors involving rheology of complex fluids in microscopic flow, such as ink-jet printing/direct-writing and enhanced oil recovery in porous media, encounter a major difficulty as experimental data produced in macroscopic flows (characteristic dimension larger than 500 micron) by conventional rheometric techniques are of little relevance to behaviour in microscopic flow environments at typical deformation rates of 10^6 s-1 or higher. In such a fast flow regime, the addition of even small amounts of polymer to a formulation results in a profound perturbation of complex fluid flows, for example in the formation of long-lived ligaments connecting the ejected droplet with the nozzle of the printer. The length and lifetime of the ligaments is strongly dependent upon the molecular weight, functionality and concentration of the polymer. Above certain concentrations of polymer the capillary force of the ejected droplet is not able to break the ligament and the elastic ligament retracts the ejected droplet back into the nozzle. This can be related to the timescale of the coil-stretch transition and the subsequent relaxation compared to the timescale of the inkjet drop ejection event. A better understanding of the relationship between the molecular physics of complex fluids and their flow behaviour in microfluidics through quantitative characterisation is a prerequisite for a novel technology breakthrough of this area, especially for establishing design principles for ink formulation and printheads.The essential physics of complex fluids in microfluidics lies in the constitutive relationship which forms a bridge between flow behaviour and microstructure evolution in flow. We propose to study quantitatively aqueous solutions of poly(ethylene oxide) (PEO) and self-assembling PEO-based block copolymers under benchmark and rheometric microscopic flows. A state-of-the-art flow characterisation platform will be developed for measurement of velocity, stress fields and concentration fluctuations across the flow geometry. The experimental data for various flow configurations will be used to validate the constitutive model and its parameters by comparison with calculated results. The effects of shrinking the flow geometry, from a characteristic length scale of 600 micron to one of 3 micron, on complex fluid flows will be carefully investigated. Such a quantitative approach promises to extract total constitutive information for any given complex fluid in microscopic flow. This systematic and integrated approach will be the first of its kind to be applied to microfluidic technology.