UC Irvine

Through the extreme integration and compaction of high-power and high-frequency electronics, nano- and micromaterials have played a critical role in enabling next-generation thermal management technologies. Enormous efforts aim to leverage nanomaterials through advanced microfabrication techniques to reduce the thermal resistance between the device junction and the heat sink at different length scales. While the traditional convective air-cooling approach is limited in its effectiveness at dissipating high heat fluxes, the use of latent heat in liquid-vapor phase change offers an order of magnitude enhancement in thermal dissipation efficacy. Owing to their high reliability, passive operation, and effective thermal transport, heat pipes and vapor chambers are extensively used as thermal management devices, which rely on internal porous wicks to passively capillary-feed the working liquid to phase-change surfaces while dissipating heat from localized hot spots. The design, characterization, modeling, and fabrication of these microporous wicks enable the development of heat pipes and vapor chambers as well as the determination of their operational performance limits. In addressing application-driven demands to understand the fundamental hydraulic and heat transfer mechanisms of wick materials, a relatively new class of well-ordered microporous media known as inverse opal is evaluated due to its periodic and three-dimensionally interconnected, permeable pore network. The regularity in the pore packing arrangement provides the unique opportunity to extract structure-property relations with high reproducibility.

While the attractive characteristics of inverse opals are attributed to their deterministic and well-ordered structures, their long-range periodicity is often naturally disturbed with defects, such that these materials are categorized as polycrystalline porous media and are defined by morphologies with discrete crystalline porous domains that intersect at grain boundaries. Much like solid-state transport in an atomic polycrystal, hydraulic transport in a polycrystalline microporous medium is determined by the summative effects of transport within the crystalline domains and transport across the grain boundaries. While hydraulic transport in well-defined geometries can be predicted with relatively high accuracy using numerical simulations, grain boundaries introduce flow field complexities that are significantly more challenging to model while potentially having a significant impact on the collective transport characteristics of the porous medium. To elucidate the hydraulic resistance induced by grain boundaries, a novel combination of fluorescent microscopy and electron microscopy is employed to visualize capillary-assisted liquid propagation in polycrystalline inverse opals with high spatial, contrast, and temporal resolutions.

Under extremely high heat fluxes from concentrated hot spots, the operational performance of two-phase thermal management devices is limited by their boiling heat transfer efficacy. Utilizing the uniform and controllable features of inverse opals, structure-dependent properties and optimal heat transfer performance can be determined based on the effective balance between competing liquid-vapor interactions within the confinement of the porous media. The surface wettability of inverse opal is investigated in details for its dominant role in bubble nucleation and departure that influence the heat removal rate and surface dryout.