Understanding and Leveraging Flow Instabilities: From Fundamentals to Applications 

Abstract

Flow instabilities refer to the sudden or irregular changes in a fluid's flow state caused by external forces or variations in internal properties during its motion. They are prevalent in natural phenomena, everyday life, and engineering applications, where they play a significant role. Flow instabilities often manifest as unwelcome side effects, such as viscous fingering (VF) instability, which can lead to uneven fluid dispersion, resulting in problems such as resource waste and environmental pollution. As a result, the focal point has focused on suppressing and eliminating flow instabilities in the past decades. However, flow instabilities can sometimes yield desired results, such as Plateau-Rayleigh instability (PRI), which can break up a liquid column into countless droplets with characteristic distances to form monodisperse droplets as templates to manufacture materials. Therefore, it is essential to comprehend and harness the unique advantages of flow instability while mitigating its detrimental effects to facilitate satisfactory applications, which is the research objective of this thesis.

To enrich and enhance the scope and comprehensiveness of the research, we studied flow instability types that occurred on both macro- and micro-fluid interfaces. The first part focuses on VF to explore the feasibility of obtaining adequate tunability of finger size in a single Hele-Shaw cell to develop hierarchical structures. VF is characterized by finger-like patterns formed when a low-viscosity fluid displaces a high-viscosity fluid sensitive to boundary conditions. Utilizing a one-end-lifted Hele-Shaw cell (HSC), we systematically examine the characteristics of VF, which induces dynamic changes in gap thickness as the upper substrate is lifted. The resulting structure features a hierarchical arrangement of dense finger-like structures with gradually decreasing sizes. Notably, the finger width is inversely proportional to the range between −0.53 to −1.22 power of the capillary number, deviating from the expected -1/2, highlighting VF's significant dependence on varying boundary conditions.

Subsequently, based on the first work, we thoroughly explore the ability of VF to prepare hierarchical structures in one-end-lifted HSCs. To put the above-mentioned liquid structures into practical use, solid-state hierarchical structures with long lifespans are successfully created by employing rapidly solidifying materials as high-viscosity fluids. In addition, by leveraging engineered substrates, various hierarchical structures are prepared to explore their applications. This strategy is mold-free, user-friendly, scalable, and cost-effective compared to existing manufacturing technologies. Most importantly, the fabricated hierarchical structures are physically unclonable, offering promising applications in the anti-counterfeiting domain.

The resultant results on VF are generally based on relatively larger fluid interfaces. Afterward, we directed our focus towards microscale flow instability—PRI in droplet-based microfluidics. PRI is extensively used for droplet generation in microfluidic systems, enabling efficient and controllable liquid sample processing and fostering the development and innovation of microfluidic technology. Based on this, our research focuses on the preparation of calibrated droplets as templates to fabricate microparticles and microcapsules with different properties and functions using droplet-based microfluidic technology and explores its applications. We prepared a microparticle with dual anisotropy in both shape and composition and investigated its properties as a micromotor driven by the photothermal Marangoni effect. Under full-area near-infrared irradiation, the micromotor exhibits multiple motion modes, including translation and revolution, while the micromotor assembly shows additional rotational motion. The self-assembly of these micromotors is highly controllable and programmable, and custom patterns can be created to achieve specific motion by adjusting the direction, angle, and relative position of the micromotors on the water surface.

The preparation of the above microparticles mainly involves a single jet. Ultimately, we use the composite jet to prepare more complex microcapsules with thermochromic property for smart windows. The core consists of thermochromic Poly-N-isopropylacrylamide (PNIPAM) hydrogel, while the shell is made of polydimethylsiloxane (PDMS), providing protection for the core. When the external temperature exceeds the lower critical solution temperature, the PNIPAM core undergoes a reversible phase transition from transparent to turbid, effectively shielding infrared radiation and adjusting transmittance. This process enhances the thermal regulation capabilities of the windows, ultimately reducing building energy consumption.

In summary, by leveraging the advantages of viscous fingering instability and Plateau-Rayleigh instabilities, this study has developed a variety of materials and explored their applications, providing new insights into the utilization of fluid instabilities.

Date of Award	29 Jul 2025
Original language	English
Awarding Institution	City University of Hong Kong
Supervisor	Pingan ZHU (Supervisor)