Unified Modeling Framework for Thin-Film Evaporation from Micropillar Arrays Capturing Local Interfacial Effects

Biomimetic Micropillar Wick for Enhanced Thin-Film Evaporation

Sustainable liquid cooling solutions are recognized as the future of thermal management in the chip industry. Among them, phase change heat transfer devices such as heat pipes and vapor chambers have shown tremendous potential. These devices rely on the physics of capillary-driven thin-film evaporation, which is inherently coupled with the design and optimization of the evaporator wicks used in these devices. Here, we introduce a biomimetic evaporator wick design inspired by the peristome of the Nepenthes alata that can achieve significantly enhanced evaporative cooling. It consists of an array of micropillars with multiple wedges along the sidewall of each micropillar. The efficacy of the wedged micropillar is evaluated based on a validated numerical model on the metrics of dryout heat flux and effective heat transfer coefficient. The wedge angle is chosen such that wedged micropillars cause liquid filaments to rise along the micropillar vertical walls. This results in a significant increase in thin-film area for evaporation. Additionally, the large mean curvature of the liquid meniscus produces strong capillary pumping pressure and simultaneously, the wedges increase the overall permeability of the wick. Consequently, our model predicts that the wedged micropillar wick can attain ∼234% enhancement of dryout heat flux compared to a conventional cylindrical micropillar wick of similar geometrical dimensions. Moreover, the wedged micropillars can also attain a higher effective heat transfer coefficient under dryout conditions, thus outperforming the cylindrical micropillar in terms of heat transfer efficiency. Our study provides insight into the design and capability of the biomimetic wedged micropillars as an efficient evaporator wick for various thin-film evaporation applications.

Electro-osmosis Aided Thin-Film Evaporation from a Micropillar Wick Structure

The heat-dissipating capacity of a surface having micropillar wick structures, which resembles the evaporator section of a vapor chamber, is mainly limited by the liquid flow rate through the porous structure (permeability) and the capillary pressure gradient. The efficacy of a regular vapor chamber is determined from two parameters, namely, the dry-out heat flux and temperature of the evaporator surface. These two parameters possess a counter relation to each other. The work described herein introduces and evaluates the performance of a novel idea of electro-osmosis-aided thin-film evaporation from a micropillar array structure. This study is conducted using a discretized approach that is validated against the thin-film evaporation model and additionally the electro-osmotic flow model with pre-existing pressure gradient conditions. The unique feature of this approach is that it results in an increment in the magnitude of dry-out heat flux without significantly changing the surface temperature, wherein the increase in permeability is due to the addition of electro-osmotic flow. This comprehensive model considers various geometries, zeta potentials, and extremal electric fields and establishes the beneficial effects of the application of an external electric field. The results are used to predict the sensitivity and the dependence of the dry-out heat flux and the evaporator surface temperature on these parameters. For a host of electro-osmotic parameters considered herein, a maximum increment of up to 320% in the dry-out heat flux is observed for an external electric field of 10⁵ V/m. The study, therefore, conclusively demonstrates the beneficial impact of electro-osmosis in enhancing the dry-out heat flux without any significant Joule heating.

Multiobjective Optimization of Graded, Hybrid Micropillar Wicks for Capillary-Fed Evaporation

As electronic device power densities continue to increase, vapor chambers and heat pipes have emerged as effective thermal management solutions for hotspot mitigation. A crucial aspect of vapor chamber functionality depends on the properties of the microporous wick that drives heat and mass transport within the device. While many prior studies have focused on the optimization of these porous structures to increase the maximum capillary-limited dryout heat flux, an equally important aspect of porous wick design is the minimization of the thermal resistance above heated areas. Segmented wicks with geometries that vary along the length of the wick are attractive candidates that can potentially be used to fulfill these simultaneous design goals. Previous studies on bisegmented wicks with only two distinct adiabatic and heated region geometries, however, have shown mixed results regarding the degree of performance benefit over homogeneous wicks. In this work, we present a systematic modeling approach to investigate the optimal composition of segmented micropillar wicks comprising multiple, discrete regions of graded geometry. Using a genetic algorithm, we generate Pareto fronts of optimal segmented wick distributions that maximize the dryout heat flux and minimize the thermal resistance for a given heating configuration. We find that optimal, graded segmented wicks are capable of dissipating dryout heat fluxes more than 200% higher than baseline homogeneous wicks with significantly lower thermal resistance. The sensitivity of the wick performance to the total number of geometry segments is found to vary depending on the desired heat flux and thermal resistance operating regimes.

Elucidating the Mechanism of Condensation-Mediated Degradation of Organofunctional Silane Self-Assembled Monolayer Coatings

Dropwise condensation is favorable for numerous industrial and heat/mass transfer applications due to the enhanced heat transfer performance that results from efficient condensate removal. Organofunctional silane self-assembled monolayer (SAM) coatings are one of the most common ultrathin low surface energy materials used to promote dropwise condensation of water vapors because of their minimal thermal resistance and scalable synthesis process. These SAM coatings typically degrade (i.e., condensation transitions from the efficient dropwise mode to the inefficient filmwise mode) rapidly during water vapor condensation. More importantly, the condensation-mediated coating degradation/failure mechanism(s) remain unknown and/or unproven. In this work, we develop a mechanistic understanding of water vapor condensation-mediated organofunctional silane SAM coating degradation and validate our hypothesis through controlled coating synthesis procedures on silicon/silicon dioxide substrates. We further demonstrate that a pristine organofunctional silane SAM coating resulting from a water/moisture-free coating environment exhibits superior long-term robustness during water vapor condensation. Our molecular/nanoscale surface characterizations, pre- and post-condensation heat transfer testing, indicate that the presence of moisture in the coating environment leads to uncoated regions of the substrate that act as nucleation sites for coating degradation. By elucidating the reasons for formation of these degradation nuclei and demonstrating a method to suppress such defects, this study provides new insight into why low surface energy silane SAM coatings degrade during water vapor condensation. The proposed approach addresses a key bottleneck (i.e., coating failure) preventing the adoption of efficient dropwise condensation methods in industry, and it will facilitate enhanced phase-change heat transfer technologies in industrial applications.

Stable and Efficient Nanofilm Pure Evaporation on Nanopillar Surfaces

Molecular dynamics simulations were conducted to systematically investigate how to maintain and enhance nanofilm pure evaporation on nanopillar surfaces. First, the dynamics of the evaporation meniscus and the onset and evolution of nanobubbles on nanopillar surfaces were characterized. The meniscus can be pinned at the top surface of the nanopillars during evaporation for perfectly wetting fluid. The curvature of the meniscus close to nanopillars varies dramatically. Nanobubbles do not originate from the solid surface, where there is an ultrathin nonevaporation film due to strong solid-fluid interaction, but originate and evolve from the corner of nanopillars, where there is a quick increase in potential energy of the fluid. Second, according to a parametric study, the smaller pitch between nanopillars (P) and larger diameter of nanopillars (D) are found to enhance evaporation but also raise the possibility of boiling, whereas the smaller height of nanopillars (H) is found to enhance evaporation and suppress boiling. Finally, it is revealed that the nanofilm thickness should be maintained beyond a threshold, which is 20 Å in this work, to avoid the suppression effect of disjoining pressure on evaporation. Moreover, it is revealed that whether the evaporative heat transfer is enhanced on the nanopillar surface compared with the smooth surface is also affected by the nanofilm thickness. The value of nanofilm thickness should be determined by the competition between the suppression effect on evaporation due to the decrease in the volume of supplied fluid and the existence of capillary pressure and the enhancement effect on evaporation due to the increase in the heating area. Our work serves as the guidelines to achieve stable and efficient nanofilm pure evaporative heat transfer on nanopillar surfaces.

Local Meniscus Curvature During Steady-State Evaporation from Micropillar Arrays

Micropillar arrays are an ideal model system for capillary-aided thin film evaporation that can be fabricated with precise geometric control using microfabrication methods. The capillary limit leading to dryout is a critical performance metric for capillary-aided thin film evaporation and is proportional to the product of the permeability and capillary pressure. Capillary flow models for steady-state thin-film evaporation typically employ capillary pressure and permeability as separate parameters; however, it is difficult to separate the two from experimental hemiwicking characterizations or dryout observations. Furthermore, for micropillar arrays, local permeability depends on meniscus curvature varying spatially and with the evaporation rate. In this work, we use thin-film interference microscopy to profile local meniscus curvature during steady-state evaporation of water in a pure vapor environment. Local capillary pressure is calculated from curvature without requiring knowledge of contact angle or permeability. Results are compared against a Darcian semianalytical model for flow through micropillar wicks incorporating local permeability due to meniscus curvature. Although traditionally a slip boundary condition has been assumed at the liquid-vapor interface, we find much better agreement using a no-slip condition. The consequence of no-slip behavior is larger pressure gradients for a given evaporation flux and a lower dryout heat flux relative to a full slip condition. Heat transfer coefficient data are also presented and discussed in terms of curvature and sample geometry.