Three-Tier Hierarchical Structures for Extreme Pool Boiling Heat Transfer Performance

Synergistic Boiling Enhancement on Hierarchical Micro-Pit/Carbon Nanotube Surfaces

Pool boiling offers exceptional heat transfer performance, making it crucial for advanced thermal management. However, simultaneously optimizing both critical heat flux (CHF) and heat transfer coefficient (HTC) is challenging due to the inherent trade-off between promoting bubble nucleation and mitigating detrimental bubble coalescence. This study presents a micro/nano-hierarchical surface architecture designed to overcome this limitation. Fabricated via laser machining and chemical vapor deposition, the architecture comprises an array of micro pits (MPs) decorated with Co-catalyzed carbon nanotubes (CoCNTs). Computational fluid dynamics (CFD) simulations demonstrate that the MP array enhances HTC by increasing the density of nucleation sites and reducing the bubble departure diameter. Simultaneously, the CoCNTs within the MPs enhance interfacial heat transfer and promote capillary-driven liquid replenishment to the heating surface, effectively mitigating dry-out and significantly improving CHF. The synergistic effects of these micro/nanofeatures yield remarkable performance enhancements on Cu substrates, with the HTC and CHF increasing by 211.5% and 125.2%, respectively, compared to a bare Cu surface. This hierarchical surface design offers a promising strategy for developing high-performance boiling heat transfer surfaces for next-generation thermal management applications.

Revealing Microstructured Surface Critical Heat Flux Degradation Mechanisms and Synergistic Pool Boiling Enhancement in Fluorinated Fluids

Fluorinated dielectric fluids are widely utilized as heat transfer fluids for two-phase cooling of electronics, capitalizing on the fluids' large latent heat release during boiling for efficient heat flux removal. Recent studies have optimized surface micro/nanostructures on aluminum alloy through chemical etching, achieving more than 2× enhancements in boiling heat transfer coefficients (HTCs) of these fluids compared to plain surfaces. However, these microengineered surfaces suffer from critical heat flux (CHF) reduction of nearly 40%, with the mechanisms driving this CHF reduction remaining unclear. Here, we investigate the mechanism resulting in the poor CHF of microstructured surfaces and develop a guideline to synergistically enhance the HTC and CHF of these surfaces. Immersion boiling tests in fluorinated and nonfluorinated fluids, coupled with wickability and elemental analysis, revealed that surface degeneration─caused by fluorine deposition forming C-F bonds with adventitious carbon─has minimal impact on CHF in fluorinated fluids. To further verify that surface degeneration is not responsible for CHF reduction, pool boiling experiments with cavity sizes from 1 to 5 μm identified the 5 μm cavity surface, AM-H(400)E(5), as achieving the highest HTC in both HFE-7100 and ethanol. However, CHF reductions of 30-50% were consistently observed, regardless of whether the surface transitioned to hydrophobicity or retained superhydrophilicity. Arising from this investigation, it is concluded that the increased nucleation site density on AM-H(400)E(5), which leads to the overcrowding of bubbles, is the primary cause of CHF reduction. To overcome these limitations, we devise a method of hierarchical addition of microstructures on macro-fins to simultaneously enhance HTC and CHF, creating a single-process two-tier hierarchical structure by leveraging on AM to fabricate the macrostructures. The two-tier macro/microstructure design has successfully enhanced HTC and CHF by 99 and 202.2%, respectively, compared to the best single-tier microstructured surface. This approach not only effectively delay undesirable vapor layer formation but also provides a robust guideline for enhancing boiling performance in other fluorinated fluids, including refrigerants R134a and R1234ze(E).

Understanding ultrafast free-rising bubble capturing on nano/micro-structured super-aerophilic surfaces

Rapid bubble capture is essential for collecting targeted gaseous media and eliminating floating impurities across aquatic environments. While the role of nanostructures during the collision of free-rising bubbles with super-aerophilic surfaces is well established, the fundamental contribution of microtextures in promoting initial capture, even before contact, has yet to be fully understood. We report the rising bubble-induced large deformation of the entrapped gas layer, rapidly thinning the liquid film to its rupture threshold and thus achieving an ultrafast bubble capture down to about 1 ms with an array of microcones, decorated with nanoparticles as a convenient example to obtain super-aerophilicity. This rapid capture is also very stable due to the hysteresis movement of three-phase contact lines that inspired a critical pressure criterion for ensuring gas-layer stability and capture efficacy. The present nano/microstructured surface supports prolonged, loss-free gas transport in challenging shear flow as well, providing robust bubble control strategies for diverse systems.

Enhancing Liquid-Vapor Phase-Change Heat Transfer with Micro/Nano-Structured Surfaces

Liquid-vapor phase-change heat transfer plays an important role in many industrial systems, ranging from power generation and air conditioning to water desalination, food processing, and thermal management of electronics and data centers. Recent advances in micro/nanofabrication have enabled desirable manipulation of multiscale dynamics governing droplet/bubble motion and capillary liquid flows for highly efficient phase-change heat transfer. However, there lacks a comprehensive review on the design and fabrication of micro/nanostructured surfaces with controlled morphology and wettability, to enhance the diverse phase-change heat transfer processes. Here, we review the advances in micro/nanostructuring for phase-change heat transfer applications. While traditional mechanical machining and sintering have commonly been used to manufacture structures down to sub-millimeter or micron scales, advanced micro/nanostructure fabrication methods such as laser texturing, oxidation, lithography-based etching, and spray coating are being utilized to manufacture surfaces with hierarchical structures or heterogeneous wettability. Droplets, bubbles, and liquid films generally experience a multiscale life cycle from nanometer scale to millimeter scale in the phase-change processes, including condensation, pool boiling, capillary-driven evaporation, and liquid film boiling. Micro/nanostructured surfaces need to be designed to coordinate different requirements of the surface wettability and morphology for the multiscale dynamics of droplets, bubbles, and films including increased nucleation, facilitated growth, accelerated transport, and departure. For active phase-change processes with pump-driven flow, including flow condensation, flow boiling, jet impingement boiling, and spray cooling, the enhancement strategies using functionalized micro/nanostructures focus on sustaining thin liquid films, strengthening thin film evaporation, promoting nucleate boiling, and regulating bubble departure within the convective liquid film. We conclude this review by a short discussion on the practical aspects of micro/nanoenabled phase-change heat transfer including reliability and scalability.

Three-Tier Hierarchical Porous Structure with Ultrafast Capillary Transport for Flexible Electronics Cooling

The development of flexible electronics needs efficient cooling devices. The porous wick, the key component in a heat pipe (HP) and vapor chamber (VC), is generally fabricated by sintering copper particles at high temperatures (>1000 °C), which makes it only formed on an inflexible substrate. In this work, one three-tier hierarchical porous structure (mesocrack, micropore, and nanopapillae) was fabricated via a low-temperature sintering method based on the utilization of self-reducing metal precursors (∼300 °C), which can be used as a flexible porous wick. The mesocrack, acting as the main water flow channel, efficiently decreases the flow resistance. The micropore, covered with densely distributed spore-like nanopapillae, creates a heterogeneous wetting surface. By harnessing the synergistic effect of hydrophobic drag reduction and hydrophilic driving force enhancement, the capillary performance is significantly improved. The obtained wick on the flexible substrate can overcome the dilemma between diminishing viscous resistance and strengthening capillary force at different length scales. It can achieve an ultimate wicking coefficient of 7.132 mm/s0.5, representing an enhancement of 9.1% compared to the best micro/nano wick structure in the previous works. Moreover, for the flexible light-emitting diode, the passive cooling approach utilizing the fluid transport and evaporation within the porous structure fabricated in this study, in comparison to the natural cooling, achieved a temperature decrease of 35.9 °C, resulting in a cooling effect of up to 35.1%. The proposed method resolves the challenge of fabricating a porous wick for flexible HP and VC, and it will open up a way for the cooling technique of flexible electronics.

High-Performance Boiling Surfaces Enabled by an Electrode-Transpose All-Electrochemical Strategy

High-performance boiling surfaces are in great demand for efficient cooling of high-heat-flux devices. Although various micro-/nano-structured surfaces have been engineered toward higher surface wettability and wickability for enhanced boiling, the design and fabrication of surface structures for realizing both high critical heat flux (CHF) and high heat transfer coefficient (HTC) remain a key challenge. Here, a novel "electrode-transpose" all-electrochemical strategy is proposed to create superhydrophilic microporous surfaces with higher dendrites and larger pores by simply adding an electrochemical etching step prior to the multiple electrochemical deposition steps. Enabled by the high nucleation density and high wicking capability, a high boiling performance is shown on such "etching-then-deposition" surfaces with simultaneously high CHF of 2,641 ± 10 kW m-2 and high HTC of 214 ± 6 kW (m2 K)-1, which are more than 2.5 and 4.3-fold enhanced from those on smooth surfaces, respectively. A very stable morphology and boiling performance of such surfaces subject to consecutive tests are also shown. Using this strategy, such superhydrophilic microporous layers are fabricated on curved surfaces with larger areas, both on spheres and slender cylinders, and demonstrate excellent boiling performance in quenching tests. This facile, geometry-adaptive, durable, and scalable strategy is very promising for making high-performance boiling surfaces for large-scale industrial applications.

Bridging Innovations of Phase Change Heat Transfer to Electrochemical Gas Evolution Reactions

Bubbles play a ubiquitous role in electrochemical gas evolution reactions. However, a mechanistic understanding of how bubbles affect the energy efficiency of electrochemical processes remains limited to date, impeding effective approaches to further boost the performance of gas evolution systems. From a perspective of the analogy between heat and mass transfer, bubbles in electrochemical gas evolution reactions exhibit highly similar dynamic behaviors to them in the liquid-vapor phase change. Recent developments of liquid-vapor phase change systems have substantially advanced the fundamental knowledge of bubbles, leading to unprecedented enhancement of heat transfer performance. In this Review, we aim to elucidate a promising opportunity of understanding bubble dynamics in electrochemical gas evolution reactions through a lens of phase change heat transfer. We first provide a background about key parallels between electrochemical gas evolution reactions and phase change heat transfer. Then, we discuss bubble dynamics in gas evolution systems across multiple length scales, with an emphasis on exciting research problems inspired by new insights gained from liquid-vapor phase change systems. Lastly, we review advances in engineered surfaces for manipulating bubbles to enhance heat and mass transfer, providing an outlook on the design of high-performance gas evolving electrodes.

Corrosive effect on saturated pool boiling heat transfer characteristics of metallic surfaces with hierarchical micro/nano structures

Surface modification is of critical interest to enhance boiling heat transfer in terms of heat transfer coefficient or critical heat flux (CHF), which is significantly affected by distinct surface morphology and wettability and it can improve the efficiency and safety of equipment. Furthermore, actual service environment may cause severe corrosion to the processed structured surfaces while its consequence on boiling heat transfer is still obscure. In this article, comprehensive researches are conducted to unravel corrosive effect on metallic samples made of stainless steel (SS) and Inconel materials with microstructures. Different constructions (i.e., microgroove, microcavity and micropillar array) and characteristic dimensions (∼20, 50 μm) of microstructure, various duration time (up to 300 days) and pH values (∼7.0-8.5) of corrosive environment are compared thoroughly. Conclusions can be drawn that not all microstructures can enhance pool boiling heat transfer characteristics, especially in terms of CHF values. More importantly, CHF value of SS microgroove sample firstly increases from 60.94 to 94.09 W·cm-2 in 50 days, then decreases to 47.77 W·cm-2 in 300 days, which can be attributed to competition result between formation of hierarchical micro/nano structure with enhancing wicking capability and chemistry condition with increasing contact angle. In addition, distinct bubble dynamics during pool boiling is also analyzed. The insights obtained from this article can be used to guide surface modification method and to reveal evolvement rule of engineered metallic surface in highly corrosive and harsh boiling heating transfer environment.

Aluminum Micropillar Surfaces with Hierarchical Micro- and Nanoscale Features for Enhancement of Boiling Heat Transfer Coefficient and Critical Heat Flux

The rapid progress of electronic devices has necessitated efficient heat dissipation within boiling cooling systems, underscoring the need for improvements in boiling heat transfer coefficient (HTC) and critical heat flux (CHF). While different approaches for micropillar fabrication on copper or silicon substrates have been developed and have shown significant boiling performance improvements, such enhancement approaches on aluminum surfaces are not broadly investigated, despite their industrial applicability. This study introduces a scalable approach to engineering hierarchical micro-nano structures on aluminum surfaces, aiming to simultaneously increase HTC and CHF. One set of samples was produced using a combination of nanosecond laser texturing and chemical etching in hydrochloric acid, while another set underwent an additional laser texturing step. Three distinct micropillar patterns were tested under saturated pool boiling conditions using water at atmospheric pressure. Our findings reveal that microcavities created atop pillars successfully facilitate nucleation and micropillars representing nucleation site areas on a microscale, leading to an enhanced HTC up to 242 kW m-2 K-1. At the same time, the combination of the surrounding hydrophilic porous area enables increased wicking and pillar patterning, defining the vapor-liquid pathways on a macroscale, which leads to an increase in CHF of up to 2609 kW m-2.

Droplet impact on doubly re-entrant structures

Doubly re-entrant pillars have been demonstrated to possess superior static and dynamic liquid repellency against highly wettable liquids compared to straight or re-entrant pillars. Nevertheless, there has been little insight into how the key structural parameters of doubly re-entrant pillars influence the hydrodynamics of impacting droplets. In this work, we carried out numerical simulations and experimental studies to portray the fundamental physical phenomena that can explain the alteration of the surface wettability from adjusting the design parameters of the doubly re-entrant pillars. On the one hand, three-dimensional multiphase flow simulations of droplet impact were conducted to probe the predominance of the overhang structure in dynamic liquid repellency. On the other hand, the numerical results of droplet impact behaviours are agreed by the experimental results for different pitch sizes and contact angles. Furthermore, the dimensions of the doubly re-entrant pillars, including the height, diameter, overhang length and overhang thickness, were altered to establish their effect on droplet repellency. These findings present the opportunity for manipulations of droplet behaviours by means of improving the critical dimensional parameters of doubly re-entrant structures.

Conduction-Dominated Cryomesh for Organism Vitrification

Vitrification-based cryopreservation is a promising approach to achieving long-term storage of biological systems for maintaining biodiversity, healthcare, and sustainable food production. Using the "cryomesh" system achieves rapid cooling and rewarming of biomaterials, but further improvement in cooling rates is needed to increase biosystem viability and the ability to cryopreserve new biosystems. Improved cooling rates and viability are possible by enabling conductive cooling through cryomesh. Conduction-dominated cryomesh improves cooling rates from twofold to tenfold (i.e., 0.24 to 1.2 × 105  °C min-1 ) in a variety of biosystems. Higher thermal conductivity, smaller mesh wire diameter and pore size, and minimizing the nitrogen vapor barrier (e.g., vertical plunging in liquid nitrogen) are key parameters to achieving improved vitrification. Conduction-dominated cryomesh successfully vitrifies coral larvae, Drosophila embryos, and zebrafish embryos with improved outcomes. Not only a theoretical foundation for improved vitrification in µm to mm biosystems but also the capability to scale up for biorepositories and/or agricultural, aquaculture, or scientific use are demonstrated.

Boosting water evaporation via continuous formation of a 3D thin film through triple-level super-wicking routes

Capillary-fed thin-film evaporation via micro/nanoscale structures has attracted increasing attention for its high evaporation flux and pumpless liquid replenishment. However, maximizing thin-film evaporation has been hindered by the intrinsic trade-off between the heat flux and liquid transport. Here, we designed and fabricated nanostructured micro-steam volcanoes on copper surfaces featuring triple-level super-wicking routes to overcome this trade-off and boost water evaporation. The triple-level super-wicking routes enable the continuous formation of a 3D thin film for highly efficient evaporation by continuous self-driven liquid replenishment and extending the thin-film region. The micro-steam volcanoes increased the surface area by 225%, improving the evaporation rate by 141%, with a rapid self-pumping water transport speed up to 80 mm s-1. A remarkable solar-driven water evaporation rate of 3.33 kg m-2 h-1 under one sun vertical incidence was achieved, which is among the highest reported values for metal-based evaporators. When attached to electric-heating plates, the evaporator realized an electrothermal evaporation rate of 12.13 kg m-2 h-1. Moreover, it can also be used for evaporative cooling with enhanced convective heat transfer, reaching a 36.2 °C temperature reduction on a heat source with a heat flux of 6 W cm-2. This study promises a general strategy for designing thin-film evaporators with high efficiencies, low costs, and multi-functional compatibilities.

Advances in micro and nanoengineered surfaces for enhancing boiling and condensation heat transfer: a review

Liquid-vapor phase change phenomena such as boiling and condensation are processes widely implemented in industrial systems such as power plants, refrigeration and air conditioning systems, desalination plants, water processing installations and thermal management devices due to their enhanced heat transfer capability when compared to single-phase processes. The last decade has seen significant advances in the development and application of micro and nanostructured surfaces to enhance phase change heat transfer. Phase change heat transfer enhancement mechanisms on micro and nanostructures are significantly different from those on conventional surfaces. In this review, we provide a comprehensive summary of the effects of micro and nanostructure morphology and surface chemistry on phase change phenomena. Our review elucidates how various rational designs of micro and nanostructures can be utilized to increase heat flux and heat transfer coefficient in the case of both boiling and condensation at different environmental conditions by manipulating surface wetting and nucleation rate. We also discuss phase change heat transfer performance of liquids having higher surface tension such as water and lower surface tension liquids such as dielectric fluids, hydrocarbons and refrigerants. We discuss the effects of micro/nanostructures on boiling and condensation in both external quiescent and internal flow conditions. The review also outlines limitations of micro/nanostructures and discusses the rational development of structures to mitigate these limitations. We end the review by summarizing recent machine learning approaches for predicting heat transfer performance of micro and nanostructured surfaces in boiling and condensation applications.

A Graphene Quantum Dot Film with a Nanoengineered Crack-Like Surface via Bubble-Induced Self-Assembly for High-Power Thermal Energy Management Applications

Films with micro/nanostructures that show high wicking performance are promising in water desalination, atmospheric water harvesting, and thermal energy management systems. Here, we use a facile bubble-induced self-assembly method to directly generate films with a nanoengineered crack-like surface on the substrate during bubble growth when self-dispersible graphene quantum dot (GQD) nanofluid is used as the working medium. The crack-like micro/nanostructure, which is generated due to the thermal stress, enables the GQD film to not only have superior capillary wicking performance but also provide many additional nucleation sites. The film demonstrates enhanced phase change-based heat transfer performance, with a simultaneous enhancement of the critical heat flux and heat transfer coefficient up to 169% and 135% over a smooth substrate, respectively. Additionally, the GQD film with high stability enables a performance improvement in the concentration ratio and electrical efficiency of concentrated photovoltaics in an analytical study, which is promising for high-power thermal energy management applications.

Meniscus-Induced Directional Self-Transport of Submerged Bubbles on a Slippery Oil-Infused Pillar Array with Height-Gradient

Directional manipulation of submerged bubbles is fundamental for both theoretical research and industrial production. However, most current strategies are limited to the upward motion direction, complex surface topography, and additional apparatuses. Here, we report a meniscus-induced self-transport platform, namely, a slippery oil-infused pillar array with height-gradient (SOPAH) by combining femtosecond laser drilling and replica mold technology. Owing to the unbalanced capillary force and Laplace pressure difference, bubbles on SOPAH tend to spontaneously transport along the meniscus gradient toward a higher elevation. The self-transport performances of bubbles near the pillars depend on the complex meniscus shape. Significantly, to understand the underlying transport mechanism, the 3D meniscus profile is simulated by solving the Young-Laplace equation. It is found that the concave valleys formed between the adjacent pillars can change the gradient direction of the meniscus and lead to the varied transport performances. Finally, by taking advantage of a water electrolysis system, the assembled SOPAH serving as a bubble-collecting device is successfully deployed. This work should not only bring new insights into the meniscus-induced self-transport dynamics but also benefit potential applications in the field of intelligent bubble manipulation.