Date of Award

Spring 5-25-2024

Document Type

Dissertation

Degree Name

Doctor of Philosophy (PhD)

Department

Micro and Nanoscale Systems

First Advisor

Arden Moore

Abstract

Despite the significant importance and widespread use of phase-change cooling techniques, there are still fundamental questions about the microscopic processes that govern the heat transfer mechanisms. In order to gain a better understanding of the underlying physics involved, it is essential to have information at the microscale regarding the surface temperature distribution with time as well as the location and speed of the moving contact line (MCL). A comprehensive understanding of heat transfer mechanisms and phase-interface behavior during phase-change cooling is crucial for improving heat transfer models, optimizing surface engineering, and maximizing overall effectiveness. Firstly, this dissertation presents a capacitance-based microdevice capable of tracking a moving phase interface at the microscale for unconstrained liquid droplets. This microdevice is comprised of an array of planar interdigitated electrodes beneath a thin insulating polymer layer. During the experiments, monitoring changes in capacitance with time facilitated sensing the MCL location and speed as it passes over each capacitance sensor. This capacitive sensing scheme is noninvasive to the system under study, allowing its implementation into many types of existing hardware and devices and not requiring optical access to the phase change area of the device. Implementing multiple capacitance sensors in adjacent proximity for a semiconducting based demonstrated a few limitations, including coupling effects, but it did not prevent the effective detection of MCL. Utilizing a dielectric substrate demonstrated notable improvements, including but not limited to increased capacitance signal outputs and reduced coupling effects for multiple sensors in adjacent proximity. Moreover, this sensing scheme demonstrated the efficient tracking of MCL during droplet evaporation across different surface temperatures, establishing its functionality at elevated temperatures and during phase-change heat transfer processes. Next, multifunctional sensing in an evaporation phase change process is demonstrated by combining the capacitance-sensing microsensors with a series of resistance temperature detectors (RTDs) to form a multifaceted MEMS device. This composite MEMS device also includes a resistance heater, making it an independent experimental setup and ensuring its implementation in investigating phase-change cooling processes. The composite MEMS device has been utilized to measure the local heat transfer characteristics and MCL behavior simultaneously for the evaporation of individual sessile water droplets on the heated surface of the device. The microdevice's resistance- and capacitance-based operating principles mean that it can detect temperature changes and track MCL at the microscale in real time, even for applications with limited or no visibility, such as within thermal management hardware or processing equipment. Results of this study showed that the MCL passage precedes the change in local surface temperature, and the duration of the time difference between these events depends on the MCL's speed. In addition, the passage of the MCL accounts for more than 70% of the overall temperature change during the evaporation process. This work also presents a series of studies in which this composite MEMS device was modified to investigate heat transfer mechanisms and simultaneous tracking of the MCL for subcooled impinging droplets across a range of surface temperatures at multiple impact velocities. Experimental results of this study showed that when a droplet impacts a heated surface and evaporates, the process can be divided into two segments based on the effective heat transfer rate: an initial conduction-dominated segment followed by another segment dominated by surface evaporation. Results also showed that heat flux at the solid-liquid interface of an impinging droplet increases with the rise of either impact velocity or surface temperature. Additionally, this study demonstrated that convection within evaporating droplets contributes negligibly to overall heat transfer; instead, heat conduction into the droplet and surface evaporation dominates the process. In the final study presented here, the composite MEMS device was further modified and implemented to measure surface temperature variation and track the movement of the MCL for isolated bubbling events during nucleate boiling of water. Experimental results showed that the rewetting process of the superheated sensing region acted as a fast-quenching event that caused a sharp and sudden temperature drop, and the duration of this event shortened with increasing surface temperature. The bubble nucleation followed the rewetting process and caused a gradual increase in surface temperature as the bubble started to grow with the advancing movement of the MCL. When the growing bubble diameter reached its maximum, the MCL began to recede, indicating the beginning of the rewetting process, which led to the next bubbling cycle. Collectively, this work represents the first known examples of independent microscale sensing of MCL behavior and surface temperatures and does so for multiple instances of phase change processes. The data produced are analyzed and discussed within the content of fundamental heat transfer processes and prior works. Major findings are presented and guidance on next steps and future studies are provided.

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