Thin-film flows and soft hydraulics with complex fluids for environmental and microfluidic-based applications

A wide range of practical fluids, such as polymer solutions, colloidal suspensions, foams, and biologically relevant fluids, show non-Newtonian behaviors in engineering applications. Unlike their Newtonian counterparts, these fluids exhibit complex nonlinear responses to deformation and shear stress. In particular, non-Newtonian fluid flows in confined geometries are ubiquitous across natural and engineered systems and across scientific disciplines, ranging from enhanced oil recovery to geophysical problems, to polymer and materials processing, to biomicrofluidics and organs-on-chips, to flexible and wearable electronics. However, the interplay between fluid rheology and geometrical confinement presents intricate challenges to understand and manipulate the fluid behaviors. Here, to address this research frontier, my PhD dissertation is dedicated to advancing the fundamental understanding of non-Newtonian fluid flows in confined geometry considering two canonical configurations, with a combination of experimental and theoretical methodologies. The first configuration involves thin-film flows, where a confined bubble propagates through non-Newtonian fluids, influencing the deposition of liquid films and bubble morphologies. The second configuration focuses on fluid-structure interactions in soft hydraulic systems, where the interplay between complex fluid rheology and structural compliance governs the flow behavior. By bridging experimental findings with theoretical frameworks, this dissertation advances the fundamental understanding of non Newtonian fluid mechanics in both thin film flows and soft hydraulics. These insights may provide valuable engineering guidance for controlling flows in porous media, optimizing coating processing, enhancing design of biomedical devices, microfluidic systems as well as soft robotics. In the first part of the dissertation, the motion of an elongated bubble within a confined geometry filled with non Newtonian fluids is explored through both experimental and theoretical approaches. While the dynamics of film deposition in Newtonian fluids is well understood, the behavior of complex non-Newtonian fluids remains elusive, especially in the development of predictive scaling laws for film thickness. In this dissertation, I systematically investigate the influence of shear-thinning viscosity and viscoelasticity on thin film deposition, bubble velocity, and bubble shape formation. Building upon the recent advancements in hydrodynamic theory, scaling laws are derived to capture the intricate interplay between complex fluid rheology and interfacial dynamics, incorporating dimensionless parameters such as the capillary number, Carreau number, and Weissenberg number. The theoretical scaling laws for the film thicknss are in good agreement with experimental observations across a wide range of experimental parameters. In addition, a high degree of undulations on the bubble surface results in an intricate rear viscosity distribution for the rear meniscus and the deviation between the experiments and theory may require a further investigation to resolve the axial velocity field. In the second part of the dissertation regarding soft hydraulics, while low Reynolds number flows of Newtonian fluids through rigid and compliant conduits are well-characterized, it is not the case for non-Newtonian fluids, where complex rheology interacts with wall compliance in a nontrivial manner. Even for steady, low-Reynolds-number flows, a complete understanding of how viscoelasticity and shear-dependent viscosity influence the flow rate–pressure drop relation in deformable configurations remains an open challenge. To systematically investigate the interplay between fluid rheology and wall compliance, I establish an experimental framework to obtain high precision experimental data on the flow rate–pressure drop relationship in two canonical deformable systems: a rectangular channel with a top deformable wall and an axisymmetric tube. By introducing key dimensionless parameters. i.e. the Carreau, Deborah, and compliance numbers, to describe fluid rheology and wall compliance, I demonstrate good agreement between theoretical predictions and experimental measurements for the flow rate–pressure drop relations in steady shear-thinning and purely viscoelastic fluids flows.

Name thesis:degree_name

Ph.D.

Level thesis:degree_level

Dissertation

Discipline thesis:degree_discipline

Mechanical Engineering

Grantor

University of Illinois Urbana-Champaign

Year dc:date

2025

Author dc:creator

• Chun, Sunggyu

Contributors dc:contributor

• Feng, Jie
• Ewoldt, Randy H.
• Chamorro, Leonardo P.
• Christov, Ivan C.

• Thin-film flows
• soft hydraulics

Statement dc:rights

• Copyright 2025 SungGyu Chun

Language dc:language

en, eng

Handle dc:identifier

https://hdl.handle.net/2142/129708

OAI record oai:identifier

https://www.ideals.illinois.edu/oai-pmh?verb=GetRecord&metadataPrefix=etdms&identifier=oai%3Awww.ideals.illinois.edu%3A2142%2F129708