Characterization of Microscopic Thermofluidic Transport for Development of Porous Media Used in Phase-Change Devices

Author(s): Suh, Youngjoon | Advisor(s): Won, Yoonjin | Abstract: As the heat generation at the device footprint continuously increases in modern high-tech electronics, there is an urgent need to develop cooling devices that can dissipate highly concentrated heat loads. Phase-change cooling strategies leverage the high latent heat of vaporization of the working fluid to effectively dissipate large heat fluxes. Typical passive two-phase cooling devices utilize the thermal and fluidic transport properties of porous microstructures called the wick that facilitate evaporative heat transfer. Despite the increasing attention that the wick receives for phase-change devices, developing the optimal wick has been a daunting challenge due to the limited resolution of extant thermofluidic characterization techniques. From the designing perspective, it has been difficult to precisely predict how microstructures affect thermofluidic transport through traditional low-resolution experiments. From the manufacturing perspective, large-scale (g 100 m) fabrication of self-assembled wick templates are restricted due to solvent evaporation-induced structural defects called grain boundaries. Motivated by this multistep task, this thesis reports experimental characterization of multiscale thermal and fluidic transport phenomena occurring in porous media using a novel micro laser induced fluorescence (μLIF) technique. Primarily, we corroborate the use of unsealed temperature-sensitive dyes by systematically investigating their effects on temperature, concentration, and liquid thickness on the fluorescence intensity. Considering these factors, we analyze the evaporative performances of microstructures using two approaches. The first approach characterizes local and overall evaporation rates by measuring the solution drying time. The second method employs an intensity-to-temperature calibration curve to convert temperature-sensitive fluorescence signals to surface temperatures. Then, submicron-level evaporation rates are calculated by employing a species transport equation for vapor at the liquid-vapor interface. Using these methods, we decompose key thermofluidic parameters contributing to the wick’s evaporative performance.We then extend this work to study in situ interparticle interactions in colloidal assemblies that decide grain boundaries in colloidal wick templates. The fluorescence microscopy measures the saturation levels with high fidelity to identify distinct colloidal structuring regimes during self-assembly as well as cracking mechanics.The presented studies help decompose the key thermofluidic parameters contributing to the evaporative performance of microscale structures and offer fundamental groundworks for developing large-scale crack-free colloidal wick templates using colloidal self-assembly fabrication.