Microfluidics of Complex Fluids: Extensional Rheology from Optimisation to Experiment

Microfluidics finds application in technologies ranging across energy, medicine, biotechnology, chemistry and engineering. The current market for microfluidic devices is some US$1.5Bn per year, and is set to rise over coming decades. Examples include lab-on-a-chip devices for the production of emulsions, chemical reactors, medical diagnostics, the delivery of drugs and chemicals, isolation and tagging of biomaterials, and analytical chemistry. Many of these applications require handling complex fluids (e.g. polymeric solutions or biofluids) that have non-Newtonian rheological behaviour, such as shear thinning and viscoelasticity, the effects of which are enhanced at the microscale. For viscoelastic complex fluids the effect of extension on the fluid behaviour often leads to much larger flow resistances than with Newtonian Fluids due to strong extensionally-thickening behaviour. This makes thorough experimental characterisation of the extensional properties of viscoelastic fluids crucial in an industrial context:
- to accurately describe their behaviour,
- to effectively control their flow,
- for designing efficient and safe devices/components,
- to detect subtle dissimilarities in their composition (e.g. for product quality control),
- for quality-assurance of the final product (e.g. in polymer or food processing industries).
Moreover, properties of viscoelastic physiological fluids (eg. synovial fluid, saliva and blood) are closely linked to their functionality, and changes to the extensional viscosity provide indications of fluid degradation and an inability to achieve the desired in vivo functionality. Rheological information about healthy and diseased biofluids is bound to shed new light upon the onset and progression of diseases (e.g., diabetes and arthritis sufferers), leading in turn to improved therapeutics and formulation of analogue or prosthetic fluids.
The project aim is to develop a microfluidic-based extensional rheometer design; this requires sophisticated experiments guided by shape-optimisation computational tools. Microfluidic characterisation of the extensional properties of weakly viscoelastic fluids will provide new insight into important fluid mechanics that is practically intractable using conventional instruments. The project's technological outcome will be a proof-of-concept optimal extensional rheometer design; the engineering science outcome will be new insight into viscoelastic flows at the microscale. The long-term impact will be a much higher degree of control over processes using viscoelastic fluids (e.g. inkjet printing and coating processes), and this is likely to open the door to new, currently unforeseen, applications of these materials.

The project is anticipated to make an impact in several fronts:

Fundamental science: This research will produce new and fundamental insight into complex fluid flow dynamics at the microscale, particularly in understanding the effects of enhanced elasticity under extensional flows. We will also make advances in developing computational tools needed for the design of microfluidic devices for the future. Potential beneficiaries include researchers in academia and R&D teams in industry dealing with complex fluids, such as inks, surfactants, food stuffs and polymer solutions.

Technological development and Industry: The enhanced rheometer design we will develop can be integrated into lab-on-a-chip platforms, which will be a key technological advance in instrumentation for extensional rheometry of low viscosity and elasticity fluids covering a broad range of strain rates - currently a major technological challenge. For example the inkjet printing industry (the top commercial application in microfluidics, with sales in excess of US$2 billion) has been struggling for years to measure the extensional viscosity at high extension rates for low viscosity fluids. Additionally, in the long term, these platforms can serve as an inline rheological sensor in industrial processes to monitor the product properties and for quality control throughout manufacturing processes (eg. food, oil and chemical industries). Our computer-based optimization approach aims to overturn the current empirical design methodology (which is typically by trial-and-error), leading to better and more efficient microfluidic devices, for applications in the pharmaceutical, chemical and dispensing industries.

Training and education: The project will train a Strathclyde-funded PhD student in UK priority themes of 'microsystems' and 'complex fluids and rheology' (as defined by EPSRC), using cutting-edge experimental and numerical techniques in a multidisciplinary and international research environment. Moreover, the outcomes and multimedia materials from this project will be incorporated into the university's final-year MEng course ME514 "Advanced Topics in Fluid Systems Engineering".

Society: Many physiological fluids (eg. synovial fluid, saliva and blood) are viscoelastic. This project therefore is likely to have a long-term impact for human health by increasing our understanding of the fundamental properties of these physiological fluids, which are closely linked to their functionality. For example, it is well known that diseases like diabetes and malaria affect the deformability of red blood cells and the rheological properties of blood, compromising, as a consequence, its ability to deliver oxygen around the body. Rheological information about healthy and diseased blood and other biofluids will produce new insight into the causes and development of diseases, leading in turn to improved therapeutics and formulation of analogue or prosthetic fluids. Because the project will enhance our capabilities in the design of microfluidic components, it will also contribute towards the current trend of device miniaturization in healthcare, which can have profound implications on point-of-care diagnostics as well as drug delivery and synthesis.

Science awareness: The very small size of microfluidic devices, the strong visual component of this project and the vast range of applications of lab-on-a-chip make this research very attractive to secondary school students and the general public. As a consequence, we plan to motivate their interest in this scientific area, and in mechanical and chemical engineering more generally, by producing multimedia materials, including photos, short movies, and presentations that will be placed on the Project website and on file sharing sites (eg. YouTube). These will be promoted to the general public and school children via initiatives such as the university's Schools Programme and Departmental Open Days.