Date of Award
Doctor of Philosophy (PhD)
Dr. Xiangchun Xuan
Dr. Joshua Bostwick
Dr. Phanindra Tallapragada
Dr. Chenning Tong
Non-Newtonian fluids such as polymer solutions often flow under microscale extensional conditions in many natural and engineering flow fields such as in microfluidic chips, porous rocks, biological membranes and filters, printheads in additive manufacturing, etc. The changing cross sectional areas of the internal flow passages therein exert additional extension on the flow along with the shearing. Numerous studies have been dedicated to understanding the extensional flows of polymer solutions over the years. However, most of these studies only focused on flexible polymers exhibiting elasticity in their macroscopic rheology, whereas rigid polymers that portray shear-thinning but often elude elasticity in the shear and elastocapillary tests are mostly ignored. This dissertation aims to broaden the current understanding on extensional flows by systematically considering both rigid and flexible polymers and connecting their responses with their macroscopic identities. Planar microfluidic abrupt contraction-expansion channels have been employed which are known as the benchmarks for such tests due to the high extension rates and least dimensional stretching of the fluids thus easier validations of constitutive laws. For a complete fundamental understanding in the planar abrupt extensional channel, two configurations must be considered, the constriction (contraction-expansion) and cavity (expansion-contraction) channel, latter of which allows direct interactions of the flow features in the contractions and expansions, while the former doesn’t. Other channel attributes that can dictate flow features, such as the vertical confinement and the constriction length have also been emphasized for a comprehensive understanding.
In the first part, we focus on the constriction configuration. To begin with, polymer chain conformation was investigated by varying the molecular weight or concentration for a flexible linear polymer, which in turn manifested as distinct macroscopic rheological properties. However, if the specific macro-rheology attained by altering either of the polymer properties would generate the same continuum extensional flow responses in the microchannels was an interesting question seldom explored previously. Next, polymers having discrete shear-thinning and elasticity were emphasized using different rigid and flexible chained solutions. Elastic solutions produced asymmetric disturbances whereas shear-thinning facilitated steadiness and symmetry. This understanding is then extended to the effect of changing the constriction length (i.e., the contraction and expansion wall separation), relevant to actual porous media and lab-on-a-chip devices. Overall, the elastic effects can get stabilized or destabilized based on the constriction length. Shear-thinning can suppress such events. Moving on, the focus was shifted to the depth of the planar channels (i.e., the top and bottom wall separation) which can dictate important flow features such as the velocity profile and the wall depletion of the polymers. The results indicated that the destabilization of the contraction and expansion flows features of both the elastic and shear-thinning solutions may occur as the depth is increased except the contraction flows of the elastic fluids where it gets stabilized.
In the second part, we focus on the cavity configuration similar to the real pores in porous media. The contraction and expansion wall flow features in this structure being allowed to directly interact, strong deviations of flow events may be expected from the constriction structure. But surprisingly unlike in the constriction structure, rheological understanding even based only on the flexible elastic solutions was lacking herein. Thus, a full-scale rheological investigation was conducted probing diverse combinations of the shear-thinning and elastic strengths based on a number of flexible, semiflexible, and rigid biopolymers. Strong elasticity induced contraction flow instabilities dominated the expansion flow inertial events. Weak shear-thinning and/elasticity is dominated by Newtonian like inertial vortices. And interaction of high shear-thinning and inertia produced strong turbulent mixing. Finally, the understandings from here were applied to demonstrate a particle size-based sorting method in shear thinning fluids flowing through the cavity channel. Moreover, the influence of the shear-thinning index, inertia, and particle sizes on the trapping efficiency were provided as guidelines.
Raihan, Mahmud Kamal, "Extensional Flows of Polymer Solutions in Planar Microchannels" (2023). All Dissertations. 3423.
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