Rheology of complex fluids and kinematics of complex flows



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Complex fluids are key to commercial and biological applications, exhibiting diverse flow kinematics such as elongational flows, polymer-induced purely elastic instability, slip and shear banding. Accurate measurement of such complex flow kinematics can provide insights into the nonlinear rheology and the intrinsic meso structure of the complex fluid. Ideally, such measurements should be done in three dimensions (3D) with high temporal resolution to adequately capture the strong nonlinearities inherent in complex flows. In past, 3D imaging techniques like confocal microscopy and tomography have provided valuable insights into the investigation of 3D structures, however, they are limited by temporal resolution, small depth of field (DOF) within the sample and expensive setups. Alternately, the advent of digital holography microscopy (DHM) has resulted in an inexpensive, user-friendly, 3D visualization paradigm with large DOF and rapid analysis. Consequently, DHM with particle tracking velocimetry (PTV) has been applied to micromixer flows, active matter flows and polymer-induced purely elastic instability. However, its application to complex fluid flows and their rheology, so far, has been limited.

My dissertation primarily focusses on holographic characterizations of flow kinematics and rheology by investigating Newtonian and viscoelastic flows in linear, extensional and curved microfluidic channels, which provide for shear, extensional and curvilinear flow behaviors, respectively. In linear microchannels, DHM PTV was used to capture the 3D flow profiles of Newtonian and viscoelastic fluids, which was then used to calculate the shear rheology without requiring a-priori assumptions of constitutive equations or material properties. Additionally, surface slip observation in viscoelastic flows enabled slip length characterization which remain challenging with conventional 2D approaches. In the case of elongational flows in a hyperbolic contraction expansion (CE) channel, DHM PTV led flow field visualizations were used to interrogate the shear and extensional fields permeating the flow volume. Experiments with Newtonian, viscoelastic shear thinning and a Boger fluid indicated marked variation in kinematic features of strongly elastic flows vs. viscous shear flows. The DHM PTV interrogations were complemented with investigations using the eCapillary technique to quantify extensional viscosity and extensional thickening of the flow. The high-fidelity 3D flow field information enabled assessment of extensional effects in mixed flows where measurements are often corrupted by the presence of background shear. Next, the case of curvilinear viscoelastic flows, undergoing polymer-induced purely elastic instability is considered. The hydrodynamic resistance due to polymer-induced purely elastic instability is mapped with the iCapillary technique and flow visualization is done with DHM PTV. The hydrodynamic resistance rapidly diverges from the base case of inelastic shear thinning flows beyond a critical Weissenberg number value indicating the onset of polymer-induced purely elastic instability. Additionally, characterization of the 3D flow kinematics provides direct visualization of polymer-induced purely elastic instability led flow fluctuations.

Finally, we embark on the modulation of flow profiles in microfluidic channels using the concept of geometrically encodable shear rates to achieve high throughput rheology. Our approach involves using a tapered microchannel with co-flowing laminar streams. The tapered geometry allows the shear rate to be modulated continuously in the streamwise direction of flow with a single snapshot of the co-flow interface providing viscosity vs. shear rate curve(s) over an order of magnitude variation from a single input flow state. The methodology is tested experimentally and numerically based on a lubrication type analysis to investigate the viscosity and shear rate range for both Newtonian and Power Law fluids. In sum, this dissertation aims to contribute to microfluidic rheology using holography and microfluidic design for high throughput rheology.



Holography, Rheology