On the optical characterisation of low-viscosity viscoelastic fluid flows within microchannel geometries

Contemporary communications infrastructure is undergoing major enhancements via the integration of various network components in order to support growing worldwide data requirements. Microfluidic cooling is required to mitigate induced localised hot spots within Photonic Integrated Circuits (PICs) in the packaging technology, as microscale lasers must be maintained within a tolerance of ± 0.1 K of their operating temperature. Convective heat transfer is limited due to the laminar conditions of Newtonian fluids imposed by pumping limitations. Previous works determined that enhanced heat transfer is possible in such microchannels via non-Newtonian fluids including viscoelastic solutions, through a phenomenon termed elastic turbulence, at low Reynolds numbers. These fluids are typically quite viscous (η > 200 cP), resulting in an increased pumping penalty. The primary objective of this thesis is to study the dynamics of Newtonian and low viscosity (and thus easier to pump) non-Newtonian fluids in such channels, in order to determine how the flow of such fluids behaves over a range of pressure drops within microchannel geometries. If sufficient mixing at low pressure drops is present in such flows, they could be employed to enhance the heat transfer rate of devices such as PICs. The pressure-flow measurements of microchannel configurations (e.g. micro gaps of planar area 1 mm x 200 µm) were initially conducted to determine the optimum channel design for laminar Newtonian flow. A channel containing micro posts (diameter = 50 µm) was further characterised using micro-Particle Image Velocimetry (µPIV) for both a Newtonian (DI water) and a non-Newtonian 100 parts per million (ppm) polyacrylamide solution. Enhanced mixing was observed for polyacrylamide flow (fluctuation magnitude, I ≈ 38% of the mean velocity) compared to a negligible 4% for the Newtonian counterpart at an applied pressure of 150 kPa. Due to the fouling of the geometry and possibility of solution degradation, wormlike micellar solutions (500 and 1,000 ppm CTAC/NaSal fluids), which exhibit a birefringent response under flow induced stresses, were chosen for further investigation. The dynamics of both solutions and a Newtonian counterpart were optically characterised in a serpentine channel of two different aspect ratios (w = 250 µm, h = 250 µm and 500 µm respectively) using µPIV over a pressure drop range of 2.5 – 100 kPa. Birefringence measurements were also recorded to qualitatively evaluate the flow-induced stress patterns within the CTAC solutions. The behaviour of the velocity fields and colour changes observed for the fluids were analysed to determine if fluid mixing occurred. It was concluded that larger I occurred within the 500 µm deep singular serpentine bend channels for the 500 ppm and 1,000 ppm CTAC/NaSal solutions (≈ 51% and 42% respectively) at 2.5 kPa in comparison to negligible I for a Newtonian flow. The fluctuation magnitude for both viscoelastic solutions was also higher (≈18%) in comparison to Newtonian flow in 250 µm deep channels. Such enhanced mixing could help to increase convective heat transfer. However, there is a pressure drop penalty (≈ 42%) associated with the 1,000 ppm solution compared to DI water at higher flow rates. The mixing occurs from elastic turbulence of the fluid, and from rotation of the flow approaching the second serpentine bend. A corkscrew effect is apparent from the flow field analysis. Steady out-of-plane flow fluctuation is present at higher pressures, as noted from divergence patterns, indicating that Dean flow is occurring with higher flow rates. The observed colour changes from the birefringence measurements indicate that the maximum shear stress occurred at the location of turbulent-like mixing. A significant decay of I at higher pressures is postulated to be due to the scission of the micelles due to increasing shear rates. The findings of this thesis enhance the understanding of 2D and 3D mixing behaviour of viscoelastic flow, which could be exploited for future microchannel cooling and mixing applications.