Evaporator Wick Research Articles

Experimental study on the thermal performance of aluminum three-dimensional vapor chamber heat sink with a louvered-fin stacked evaporator wick for data center servers

Under increasing heat loads and stricter energy consumption limits for data center servers, traditional heat sinks are becoming inadequate. In this paper, a novel aluminum three-dimensional vapor chamber heat sink of air cooling is developed, in which the evaporator and condenser are connected internally to form a three-dimensional flow channel for working fluid so that two heat transfer methods, phase change and single-phase, are combined in one device. Furthermore, a novel louvered-fin stacked evaporator wick structure and its fabrication process are proposed, which greatly improve thermal performance and temperature uniformity. Thermal characteristics under different heat loads, fan input power, and tilt angles are studied experimentally. Results show that the heat sink has a maximum heat dissipation capability of more than 450W, at which the maximum evaporator surface temperature is within 75 °C. The fan input power can distinctly improve thermal performance, heat load impacts overall thermal resistance slightly and the minimum is 0.083 °C/W. The tilt angle impacts thermal resistance slightly but significantly affects both the evaporator surface and condenser fin temperature uniformity. Under a tilt angle of 90°, regardless of heat loads, the condenser fin temperature is much lower than other tilt angles.

Multi-Fidelity Design of Porous Microstructures for Thermofluidic Applications

Abstract As modern electronic devices are increasingly miniaturized and integrated, their performance relies more heavily on effective thermal management. In this regard, two-phase cooling methods which capitalize on thin-film evaporation atop structured porous surfaces are emerging as potential solutions. In such porous structures, the optimum heat dissipation capacity relies on two competing objectives that depend on mass and heat transfer. Optimizing these objectives for effective thermal management is challenging due to the simulation costs and the high dimensionality of the design space which is often a voxelated microstructure representation that must also be manufacturable. We address these challenges by developing a data-driven framework for designing optimal porous microstructures for cooling applications. In our framework, we leverage spectral density functions to encode the design space via a handful of interpretable variables and, in turn, efficiently search it. We develop physics-based formulas to simulate the thermofluidic properties and assess the feasibility of candidate designs based on offline image-based analyses. To decrease the reliance on expensive simulations, we generate multi-fidelity data and build emulators to find Pareto-optimal designs. We apply our approach to a canonical problem on evaporator wick design and obtain fin-like topologies in the optimal microstructures which are also characteristics often observed in industrial applications.

Elliptical pillars for the thermal performance enhancement of micro-structured evaporators

In this study, we propose and investigate the employment of elliptical micropillars as a novel evaporator wick structure to boost the performance of capillarity-driven two-phase heat spreaders through high-fidelity numerical modeling that accounts for all pertinent transport mechanisms, including thermocapillary convection. Results reveal that transforming cylindrical micropillars to elliptical ones significantly enhances the overall evaporator performance by improving the hydraulic performance associated with decreased flow resistance and boosted capillary pumping and by improving the thermal performance associated with increased contact (triple) line length. Among the scenarios tested and elliptical aspect ratios swept, it is demonstrated that the heat capacity and heat transfer coefficient can be increased by up to 350% and 52%, respectively. Overall, this study paves the way for the utilization of stretched geometries in the capillary flow direction of pillar-type evaporators by using elliptical micropillars as a demonstrator showcase.

An improved vapor chamber with enhanced two-phase transport by using structured surfaces

The novel type of vapor chamber (VC) with combined superhydrophilic/superhydrophobic surface holds great potential in hot spot cooling. In this study, an improved VC was developed by implementing structured surfaces on both condenser and evaporator. The thermal resistance, condenser temperature uniformity, and gravity independence were discussed. It was found that sintering an appropriate copper mesh on condenser could reduce the thermal resistance but would cause a slight deterioration in the temperature uniformity of the condenser. On the evaporator, the thermal performance of VC was further improved by introducing micro grooves on the wick in order to enhance bubble discharge of the boiling working fluid. Moreover, to strengthen the liquid recirculation on the evaporator, pore size gradient was adopted in the evaporator wick. Compared to the axial gradient of pore size, the radial gradient of pore size was more efficient in accelerating the working fluid recirculation. Among all the results reported in this study, the combination of sintering copper mesh on the condenser and implementing micro-grooves and pore size gradient on the evaporator wick yields optimal results, with the minimum thermal resistance of the VC reaching 0.041 K/W and the critical heat flux exceeding 213.3 W/cm2, which shows excellent thermal performance.

Surface evaporation enhancement using porous metasurfaces: 3-D multiscale, open-system wick evaporators

In open-system water-vapor production by direct heat supply, wick evaporators are promising for efficient use of the supplied heat. However, designing a suitable wick structure for achieving high evaporator performance had proven challenging. Here, novel, high-performance porous metasurfaces (unit-cell based capillary structures) are designed, fabricated and tested, using sintered copper powder. In addition, the wicks optimized for yet higher performance are presented. Vertically placed wicks are partially submerged in a pool of water, while heated through a copper substrate by Joule heating. The baseline wick is a monolayer wick, and three different particle diameters (78, 100, and 130 μm) are used. The next variations are bilayer wicks which improve the particle packing and the maximum capillary pressure. The most improvement is achieved by adding strips of capillary arteries (1 mm thick and 1 mm wide, with an interartery gap of 1 mm) on the monolayer. These unit-cell based 3-D porous metasurfaces allow for tailoring the surface design to achieve an optimal thermal-hydraulic performance. Analyses of the wick capillary-viscous dryout limit and the extended-surface heat transfer predict the performance of the fabricated wicks under the test conditions. Overall, an evaporation efficiency of unity and an evaporation enhancement by 50% are recorded.

Enhanced capillary and heat transfer performance of asymmetric micropost wicks

We developed asymmetric capillary wicks composed of slanted microposts using inclined photolithography. Then we investigated the effects of inclination angle and wicking direction on the capillary and the heat transfer performances. The working fluid accelerates when it flows in the slope direction of the structure (forward direction, FD) and decelerates when it flows in the opposite slope direction (rear direction, RD). We applied the scaling law to the capillary rise experiment data to verify that the inclination angle and the wicking direction affect the capillary performance. The capillary performance parameter was improved by up to ∼39% with FD case and decreased by 21.3% with RD. The heat transfer performance test showed that the wick-CHF (the enhanced critical heat flux due to the formation of the wick) of the asymmetric FD case was increased by 43.3% compared to symmetric ones while maintaining the heat transfer coefficient. This work shows that asymmetric evaporator wicks can enhance the critical heat flux without sacrificing the heat transfer coefficient, which can help develop high-performance thermal management solutions.

Heat and mass transfer analysis and optimization of passive interfacial solar still

Due to the fast thermal response and high solar energy efficiency, the passive interfacial solar still has become a reliable scheme for distributed water supply. In this paper, the thermodynamic performance of the interfacial solar still under different structures and operating conditions is studied experimentally, which provides an optimization direction for strengthening the distillation process. Firstly, based on the different Ra numbers, the calculation correlations for diffusion and convective mass transfer in the air interlayer were established to accurately predict the thermodynamic model of the interfacial distillation process. Then, the temperature distribution of the condenser and the growth process of condensate droplets were analyzed with the constant evaporation temperature. Simultaneously, the influence of different condensing structures and external variables on the heat transfer performance was compared. The results show that the interfacial still could obtain maximum evaporation heat under the vertical operation condition. When the evaporation temperature is 70 °C, the temperature difference between the wick evaporator and condenser can be increased by 49.5 %, after coupling a finned condenser with the aspect ratio of 7.2. In addition, a narrower interlayer spacing and a larger evaporation temperature could bring a higher equivalent heat transfer rate and evaporation efficiency. In a sunny weather, the interfacial solar still coupled with an extended condenser can reach a 4.81 kg/m2/day water yield and a 0.486 daily gained output ratio under 1 cm interlayer spacing.

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.

Thermal performance of an ammonia loop heat pipe using a rectangular evaporator with liquid-guiding channels

In order to enhance the heat transfer capacity of a stainless steel-ammonia loop heat pipe using a rectangular evaporator, liquid-guiding channels were adopted to significantly reduce the liquid flow resistacne in the evaporator wick. An experimental system was established, and extensive experimental studies have been implemented to investigate its startup characteristics, heat transfer capability and thermal resistance. To be specific, the effects of heat load, evaporator attitude and antigravity on the startup performance were evaluated. According to the experimental results, the loop heat pipe showed excellent startup performance, which could realize the startup at small heat loads for different attitudes of the evaporator. Both adverse evaporator attitude and anti-gravity can cause an increase of the operating temperature of the loop heat pipe, due to poor cooling effect of return liquid and increased heat leak from the evaporator respectively. The loop heat pipe could achieve a heat transfer capability up to 300 W, corresponding to a heat flux of 20 W/cm2. The minimum system thermal resistance reached as low as 0.048 K/W at the head load of 140 W. The present study contributes greatly to the future loop heat pipe applications in high heat flux electronics cooling.

Experimental studies on boiling heat transfer and friction characteristics in evaporator with double-layer micro/nano porous wick

High-performance boiling devices are widely adopted in various scientific and industrial applications. In this paper, in order to achieve the synergy between liquid supply and vapor escape in boiling heat transfer, the multi-scale double-layer porous wick evaporators were fabricated by sintering two different sizes of copper particles with surface modification through nano-structures. The boiling heat transfer and pressure drop characteristics were studied experimentally. The S-shaped flow boiling curves are shown with three heat transfer regimes, corresponding to pool boiling, secondary boiling and vapor convection heat transfer, respectively. The secondary boiling can enhance the heat transfer coefficient (HTC) around twice. The maximum critical heat flux (CHF) of 578 W/cm2 can be achieved in the evaporator with the smooth surface of copper particles of 57 μm in diameter. Because the liquid and vapor flow separately, according to the concept of process decomposition, the ultra-low pressure drop about 1 kPa is achieved in the double-layer porous wick, even to zero in some cases. The nano-structures on the copper particle surface can provide more nucleation sites, thus enhance HTC up to 250 kW/m2K, but the corresponding the pressure drop is increased to 10 kPa. This study is helpful to reveal the flow boiling mechanism in porous wick evaporators with multiscale structures, and provide a guideline for fabrication of high-performance porous wick evaporators.

A model based on analytical spreading relations for predicting the thermal performance of vapour chambers in thermal management solutions

Vapour chambers are two-phase heat transfer devices that are typically used for thermal spreading applications in electronic devices with high heat fluxes. In this work, a novel method for predicting thermal resistance of a vapour chamber was developed and integrated within the complete model for a thermal management system. The vapour chamber model combined three sub-models; the spreading sub-model (1) and one-dimensional sub-model (2) to estimate thermal resistances within the evaporator and condenser regions, respectively, and the effective thickness sub-model (3) to estimate the effective fluid distribution within the wick structures. Spreading mechanisms within the evaporator region were included via the analytical solution for thermal spreading. Model results were compared to experimental results for thermal resistance and showed acceptable agreement with the vapour chamber model. The vapour chamber thermal resistance contributed up to 40% of the systems total thermal resistance, which highlighted the need for a vapour chamber model. This model was further explored to provide insights into the heat transfer mechanisms within the vapour chamber. It was found that the spreading sub-model (evaporator region) contributed up to 90% of the vapour chambers thermal resistance. Its contribution decreased as heat load increased mainly due to the reduction in the effective thickness of the fluid layer in the evaporator wick. The one-dimensional sub-model (condenser region) was more significant at higher heat inputs, contributing up to 28% of the vapour chambers thermal resistance. The model could determine the thermal resistance of the vapour chamber based on its dimensions, materials and operating conditions. This could be useful for thermal engineers investigating the application of vapour chamber heat spreaders that require rapid and accurate predictions for vapour chamber performance.

Intra-Lid Multi-Core Vapor Chamber Architecture for Heterogeneous Electronic Packages: Technology Concept to Prototype Evaluation

Thermal management of future heterogeneous electronic packages with extreme heat fluxes relies on effective spreading of heat in the package lid. Intra-lid integration of vapor chambers is a promising strategy for simultaneous dissipation of large total heat loads and localized high-flux hot spots. However, conventional vapor chambers comprising a single vapor core require relatively thick evaporator wicks to prevent dry out at high total heat loads, thereby imposing a large temperature drop across the wick at the hot spot location. We recently proposed a cascaded multi-core vapor chamber (CMVC) architecture comprising a single-core vapor chamber stacked on an array of smaller footprint vapor cores having relatively thinner wicks. The multi-core array is designed to spread heat from arbitrarily distributed high-flux hot spots before they enter the top vapor chamber having a thicker wick. Then, the top vapor chamber spreads the high total heat loads to the base of the mounted heat sink. To evaluate the proposed CMVC technology, we make a weighted decision to identify an appropriate minimum viable prototype for subsequent design and testing. A reduced-order model is used to determine the dimensions and properties of the wick and vapor core that minimize the temperature drop across the down-selected multi-core vapor chamber architecture for a given power map, considering manufacturing process constraints. Finite element analysis (FEA) simulations are employed to decide a condenser wall thickness that avoids permanent deformation in the vapor chamber architecture under a mechanical load. A prototype is manufactured by a commercial vendor following these parameters and manufacturing constraints. As predicted by the thermal model, experimental characterization of this first-reported multi-core vapor chamber array prototype offers a notable reduction in temperature drop relative to a benchmark solid copper spreader, owing to attenuation of hot spots at a low temperature difference.

Topology optimization of vapor chamber internal structures consisting of evaporator and condenser

This paper studies the integrated topology optimization of vapor chambers consisting of evaporator and condenser. Firstly, the patterning is regarded as a fundamental access-maximization problem and classified into two general categories: (1) “area to point” (AP) flow problem for condenser wick design and (2) “area to line” (AL) flow problem for evaporator wick design. Then, the branch-patterned disc with first order assembly of flow channels is deduced, where the design domain is divided into 18 identical sectors for both condenser and evaporator. A fluid dynamic model is established to analyze the flow characteristics of the working fluid driven by capillary force. Based on this, a generative topology optimization model with a bifurcation rule is developed to produce the hierarchical capillary flow channels that minimize the global resistance for AP flow problem and AL flow problem, respectively. Unlike the commonly used Eulerian framework for topology optimization, the generative topology optimization used in this paper is carried out essentially in a Lagrangian framework, where capillary flow channels undergoing changes of volume, of positions, of inclined angles and of shapes are described by a set of parameterized level-set surfaces which can move, deform, overlap and merge freely upon the underlying FEM grids. The generated constructal wick structures exhibit dichotomous, hierarchical and branching characteristics. Numerical comparisons show that the new wick structures can contribute to the vapor chambers with better thermal performance.

Shape optimization of composite porous vapor chamber with radial grooves: A study on the minimization of maximum pressure drop

Composite porous vapor chamber (CPVC) with uniform radial grooves is a recently developed vapor chamber, exhibiting good temperature uniformity and excellent stability under high heat flux of 280 W/cm2 with the thermal resistance less than 0.15 °C/W. However, the maximum pressure drop in CPVC due to liquid flow is prone to exceed the limits of capillary pressure, and the shape design of the wicks lacks scientific and objective enough criteria. This paper proposes a shape optimization method to optimize the shape of the evaporator wick in CPVC, based on the study on the minimization of the maximum pressure drop. In this method, the discrete adjoint method is utilized to efficiently analyze the sensitivities and find the most rapid descendant directions for the minima problem. The shape of the evaporator wick is updated by the polynomial-based approach based on the sensitivity information. Results indicate that the bottom contours of the wick blocks’ surfaces surrounding the circular cavity contribute the highest sensitivities. Consequently, the mesh deformations near the contours change the shape of the lateral surfaces of wick blocks from rectangular to concave trapezoidal in both Cylindrical and Cartesian coordinate systems. This reduces the maximum velocities and flow resistance in CPVC, due to the smoother structural transition from the condenser wick to the evaporator wick, thus reducing the maximum pressure drop by about 18.5% and 23.3% in Cartesian and Cylindrical coordinate systems, respectively. The proposed method may pave a more objective and scientific alternative to design the shape of the wicks in CPVC.

Flow-boiling canopy wick capillary-viscous limit

It was predicted that the perforated, leveed flow-boiling canopy wick (FBCW) can significantly increase the hydrodynamic instability limit of the flow boiling [1,2]. The perforations represent and modulate the vapor columns in the boiling crises hydrodynamic stability theory. The lower capillary-viscous limit, governed by the evaporator wick maximum capillary pressure and the viscous drag within the three-dimensional composite wick, may control the actual dryout performance. Saturated-water flow-boiling (one atm) experiment results are presented employing an FBCW, created from sintered-copper particles, with and without the hydrodynamic-stabilizing levees. The bimodal bilayer evaporator wick allows for sufficiently large maximum capillary pressure and permeability, compared to the monolayer wick. The experimental results indicate that the levees can increase the critical heat flux (CHF) over that without levees, whereas the existing maximum capillary pressure controls disallow achieving this larger hydrodynamic heat flux limit. This capillary-viscous dryout limit is predicted by employing the minimum surface energy principles, network models, and computational fluid dynamics (CFD), in good agreement with experiments. By employing both CFD and flow visualizations, good agreement is also established between the predicted and observed two-phase hydrodynamics above the canopy wick. They indicate that although the local dryout commences downstream; however, dryout is experimentally observed when a critical fraction of the liquid supply tracks dry downstream. The measured large thermal conductance (heat transfer coefficient) is beneficial for the predictions, verifying the vapor-occupied space between the canopy and evaporator wicks.

Three-dimensional lattice Boltzmann investigation on pore scale liquid–vapor distribution patterns and heat transfer performance of a loop heat pipe heterogeneous porous wick evaporator

The pore scale numerical simulations on evaporation of a three-dimensional heterogeneous porous wick capillary evaporator used in a loop heat pipe (LHP) are realized through using a lattice Boltzmann method with liquid–vapor phase change. The dynamic liquid–vapor interface characteristics are obtained. The steady-state wick state and heat transfer performance as well as the influences of wick porosity and surface wettability are discussed. It is found that as heat flux increases, the wick state changes from the fully saturated state at low heat fluxes, to the partial dryout state at medium heat fluxes, and finally to the complete dryout state at high heat fluxes. The effective heat transfer coefficient of evaporator firstly grows slowly for the saturated wick, then increases rapidly and reaches a maximum value at partial dryout wick state, but afterwards drops sharply when the wick starts to be complete dryout. With the decreasing of wick porosity and contact angle, the heat flux to achieve the maximum effective heat transfer coefficient is increased and the complete dryout state is postponed. In the present range of parameters, the maximum effective heat transfer coefficient is improved by reducing wick porosity, while it is barely affected by contact angle.

Cascaded Multicore Vapor Chambers for Intrapackage Spreading of High-Power, Heterogeneous Heat Loads

A cascaded multicore vapor chamber (CMVC) is designed for dissipating heat from high-flux hotspots simultaneously with a high-total-power background. Current thermal management strategies rely on spreading high local heat fluxes by conduction in the lid of electronics packages. Embedding vapor chambers (VCs) within the lid is an attractive option to directly address intrapackage hotspots. We investigate the design of intralid VCs, for a generic device having a total heat load of 476 W having a background heat flux of 0.75 W/mm <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sup> , with hotspots of 8 W/mm <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sup> over a 1-mm <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sup> area. A conventional VC design, having a single vapor core, will require a thick evaporator wick to avoid the capillary limit for large total power. The necessity for a thick wick then imposes a large thermal conduction resistance when the VC is exposed to high heat flux hotspots. The proposed CMVC architecture aims to address this limitation. The cascaded architecture comprises a bottom-tier VC having an array of multiple small vapor cores for spreading heat from the small hotspots. These small vapor cores have short paths of liquid return to the evaporator, such that they can handle their footprint heat load while using thin wicks, resulting in a low hotspot thermal resistance. Furthermore, local dampening of the hotspots by the bottom tier then reduces the thermal conduction resistance across the necessarily thick wick in the top tier. Hence, the cascaded architecture has the potential to significantly reduce the overall thermal resistance, relative to a single tier. To substantiate this design rationale, experiments are performed to illustrate that the resistance of a commercial VC can be significantly reduced by interfacing the heat source with an intermediate heat spreader. Reduced-order models are then used to understand the effect of the wick properties (porosity and particle size) and geometric parameters on the thermal performance of the CMVC for the representative power map. The optimal CMVC design offers a thermal resistance (0.66 K/W) that is significantly lower compared to a conventional single-core VC (1.76 K/W) owing to a reduction in the conduction resistances across the internal wicks. That parametric optimization results demonstrate that the thermal resistance of the CMVC is more sensitive to the wick porosity compared to the particle diameter. Furthermore, there exists a wide range of wick properties and vapor core sizes for which near-optimum thermal performance can be attained, which is particularly attractive from the standpoint of flexibility in design and manufacturing.

Vapor chamber with two-layer liquid supply evaporator wick for high-heat-flux devices

Modern electronic devices with high heat flux increasingly exceed the capability of thermal management method. Two-phase thermal dissipation devices such as vapor chambers offer effective method for high heat flux cooling. Various new structural designs of vapor chambers are used as effective thermal control methods for high heat flux devices. However, the optimization criteria for the capillary wick structures are different. The combinations between the structural surface parameters and pore size of the capillary wick are different, while their influence on the heat transfer performance need to be studied. In this work, it is reported that the fabrication and thermal performance of vapor chambers with multi-layer sintered wicks and multi-artery wick design for liquid supply. We numerically and experimentally characterize the two types of sintered multi-artery wick for replenishing liquid and escaping vapor effectively with high heat flux. Three kinds of combinations of different copper particle sizes were applied to the sintered wicks in order to balance the liquid collecting rate and flow resistance. The copper particles of large particle size (60 μm/100 μm/150 μm) in the upper layer wick and copper particles of small particle size (30 μm/50 μm/100 μm) the lower layer wick were applied in the vapor chambers. In the present study, as an evaluation criteria for capillary wick designs, the relationship between surface porosity, surface area and critical heat flux was analyzed to obtain the experimental optimization of structural design and pore size combination as 50/100 μm. It was found that with similar multi-artery structure wicks, higher surface area porosity can effectively increase the critical heat flux of the vapor chambers. Judging from the experiment results, the minimum through-plane direction heat transfer resistance of the vapor chamber is 0.1426 K/W, the minimum in-plane direction heat transfer resistance is 0.016 K/W, while the critical heat flux of the vapor chamber is 262 W/cm2.

Recent advances in the optimization of evaporator wicks of vapor chambers: From mechanism to fabrication technologies

For the challenge of cooling hotspots in the burgeoning electronics/optoelectronics equipment, vapor chambers provide a promising solution based on a distinctive passive two-phase cooling technology. However, as the most pivotal component in vapor chambers, evaporator wicks involve complex multi-physical and multi-scale processes in porous structures, which has attracted extensive attention. This work comprehensively reviews the fundamental understanding of the inherent mechanisms of evaporator wicks, optimization strategies of macro/micro-scale structures, as well as fabrication technologies and their characteristics. Further, exploration reveals that the trend of improving the thermal performance of evaporator wicks can be described as: relying on advanced fabrication technologies to synergistically optimize capillary wicking capacity, critical/dryout heat flux and thermal resistance by considering multi-scale effects, such as patterned structures, hierarchical structures, and nano-textured surfaces. Finally, this review concludes its findings and put forward prospects for future research directions.

Bridging‐Droplet Thermal Diodes

AbstractPhase‐change thermal diodes effectively transport heat unidirectionally, but are currently constrained by either a gravitational dependence, a 1D configuration, or poor durability. Here, a novel bridging‐droplet thermal diode which uniquely bypasses all of these existing constraints is developed. The diode is comprised of two opposing copper plates separated by an insulating gasket of micrometric thickness; one plate contains a superhydrophilic wick structure while the other is smooth and hydrophobic. In the forward mode of operation, water evaporates from the heated wicked plate and condenses on the hydrophobic plate. The large contact angle of the dropwise condensate enables bridging across the gap to replenish the wicked evaporator, providing sustained phase‐change heat transfer. Conversely, in the reverse mode, the heat source is now on the hydrophobic plate, resulting in dryout and excellent thermal insulation across the gap. An orientation‐independent heat transfer ratio (i.e., diodicity) as high as 85 was measured.