Superbiphilic Laser‐Microengineered Surfaces with A Self‐Assembled Monolayer Coating for Exceptional Boiling Performance

The rapid advancement of engineering systems has spurred the search for innovative thermal management solutions. Boiling, as a phase‐change heat transfer method, has shown promise in heat dissipation, but non‐functionalized surfaces struggle with increasing cooling demands. To improve heat dissipation efficiency across different heat loads, functionalized surfaces with tailored wettability have been proposed. Separately, superhydrophilic and superhydrophobic surfaces each offer benefits and drawbacks in boiling applications but combining them on a single “biphilic” surface simultaneously harnesses their advantages. In this study, laser‐functionalized copper surfaces with spatially tailored wettability are developed by combining two‐step laser texturing with a self‐assembled monolayer coating, while focus is placed on the impact of the size and pitch of superhydrophobic spots. The developed functionalized surfaces exhibit exceptional boiling performance with heat transfer coefficients up to 299 kW m−2 K−1, a 434% enhancement over untreated surfaces. Optimal ratios of superhydrophilic and superhydrophobic areas and optimal spot pitch are identified. Additionally, varying behavior at different heat flux levels is observed, emphasizing the importance of considering thermal loads when determining the optimal surface pattern. This advancement in performance, along with the rapid and cost‐effective functionalization process, represents a significant breakthrough for enhanced thermal management applications.


Introduction
Pool boiling is one of the most common processes for removing large amounts of heat and is continuously investigated in order to achieve performance breakthroughs in thermal management DOI: 10.1002/adfm.202310662 of nuclear power plants, [1,2] heat exchangers, [3,4] as well as highpower-density devices, such as microelectronics. [5,6]oiling enables very efficient heat transfer due to high latent heat consumed by the phase change from the liquid to the vapor state.Boiling performance is primarily determined by the critical heat flux (CHF) and heattransfer coefficient (HTC).The CHF denotes the upper limit of nucleate boiling and occurs when the population of bubbles on the surface becomes too high (at a high heat flux), neighboring bubbles extensively coalesce (i.e., merge on or very near the surface), and form an insulating vapor blanket that covers the heating surface, thereby significantly reducing heat transfer intensity. [7]The heat transfer coefficient (HTC) represents the ratio between the dissipated heat flux and the corresponding wall superheat, i.e., the temperature difference between the boiling surface and the bulk fluid.Although boiling heat transfer is one of the most intense heat transfer mechanisms, researchers in this field have extensively investigated numerous types of methods in the last few decades [8,9] to further improve the boiling performance.Studies range from enhancements based on boiling fluid modification, [9,10] such as with the addition of nanoparticles (i.e., nanofluids) [11,12] or with mixtures of different surface tensions, [13,14] to studies focusing on surface modification, [15,16] with an emphasis on changing the surface topography [17,18] and morphology [19,20] alongside with its wetting behavior. [21,22]n recent years, various technologies have been explored to tailor the surface topography and morphology to improve boiling performance. [1,23]The fabrication of micro/nanostructures, e.g., cavities, pores, and irregularities on surfaces, can generally enhance bubble nucleation and rewetting with liquid, thus improving boiling performance. [24,25]For example, sintering, [26,27] electrochemical (electroplating) processes , [28] and chemical vapor deposition, [29] have been used to produce various coatings on surfaces in order to enhance performance of boiling heat transfer.In addition, laser texturing [30,31] and wet/dry etching techniques [10,32] have been used to fabricate micro pin fins , [33] microcavities , [34] and nanowires [35,36] on boiling surfaces.Surfaces with tailored surface micro-and nanostructures exhibited significantly improved CHF values by augmenting the contact line behavior and exploiting capillary-fed rewetting, i.e., surface wicking. [37,38]Furthermore, surfaces can exhibit different wetting properties through tailored micro-and nanostructures, which can lead to improvement of liquid supply and thus CHF and HTC enhancement. [39,40]ettability has a strong influence on CHF, heat transfer coefficient and the onset of nucleate boiling.A (super)hydrophilic surface is able to provide a higher CHF value , [41] but delays the onset of nucleation due to a higher energy barrier with a higher overall surface superheat in the nucleate boiling region. [42,30]In contrast, (super)hydrophobic surfaces reduce the energy barrier of the phase change between liquid and vapor, resulting in a reduction in wall superheat required for the onset of nucleate boiling and an improvement in heat transfer coefficients, especially in low and medium heat flux regions. [43,44,45]However, such surfaces are unable to limit bubble coalescence, resulting in a lower CHF value compared to hydrophilic surfaces and untreated surfaces.Recently, Allred et al. [44] and Može et al. [34] demonstrated that significant overall enhancement of the boiling heat transfer performance is possible on surfaces that exhibited superhydrophobic properties (evaluated in air atmosphere, outside of boiling conditions) and increased vapor affinity in boiling conditions.Since wettability represents a key influence on boiling performance, it has been the subject of numerous studies on the improvement of boiling performance with the utilization of hydrophilic or hydrophobic surfaces as well as by surfaces with combine hydrophobic and hydrophilic regions. [43,46,47]The latter are typically denoted as "biphilic" surfaces as they exhibit heterogeneous wettability with juxtaposed hydrophilic and hydrophobic regions.Biphilic surfaces aim to simultaneously exploit the advantages of hydrophobicity and hydrophilicity by combining regions with opposite wetting behavior to manipulate nucleation behavior alongside with liquid and vapor flows.The nucleation of the bubbles takes place in the hydrophobic areas, while the hydrophilic areas serve to prevent coalescence of the bubbles and provide liquid supply through wicking toward hydrophobic areas.Additionally, Chinnov et al. [48] have underscored the significance of the sustained and enduring boiling performance characteristics of manufactured biphilc coatings on boiling interfaces.This emphasis on longevity is a relatively infrequent aspect within the broader community of researchers in the field of boiling.Consequently, it warrants heightened attention and consideration as a focal point for future studies.The findings of some of the most prominent studies in this area are summarized in Table 1.
The literature review shows different findings regarding the influence of spot size, spot pitch, spot shape, and distribution of hydrophobic spots on pool boiling performance.Among these parameters, the key finding is that smaller spot sizes can enhance boiling performance by promoting more efficient heat transfer and better nucleation.This is due to the local surface energy gradient being increased by a smaller spot size, which facilitates the formation of more nucleation sites and improves overall boiling performance. [62,63]First studies concluded that a capillary length criterion can serve as a starting point in the optimization procedure for achieving efficient boiling performance on a 2D patterned biphilic surface.However, recent studies suggested that the actual optimum should be determined based on the max-imization of active nucleation site density [31,57] where the distance between nucleation sites should not go significantly below expected bubble departure diameter.It was also found that the shape of hydrophobic spots has little influence on boiling performance . [64]Several studies reviewed in this article demonstrated a systematic approach to optimizing the geometry of biphilic patterns to achieve maximum boiling performance.They presented an optimal geometrical pattern of biphilic surfaces coupled with an optimal spot pitch.However, the presented optimal patterns were only valid for the specific case considered in the study.Despite the valuable contributions of these studies, there is still a lack of a precise description and systematic approach to defining the influence of geometric pattern factors, such as spot pitch, due to the large scatter in results obtained under various experimental conditions.
Therefore, this work presents a detailed characterization of the influence of spot size and spot pitch of the biphilic surface in order to develop an optimal surface pattern that improves heat transfer coefficient during pool boiling with reduced surface superheat.In addition, the study presents a low-cost, fast, and robust method for fabricating scalable superbiphilic copper surfaces.Superbiphilic surfaces are prepared using two-step nanosecond laser texturing in combination with application of a self-assembled monolayer of fluorinated phosphonic acid.Boiling heat transfer performance of surfaces with tailored wettability is evaluated by pool boiling tests with water, conducted under saturated conditions at atmospheric pressure.The effects of spot size, spot pitch, and superhydrophobic area fraction are evaluated using three different spot size values (0.2, 0.4, and 0.6 mm) and by varying spot pitch from 0.4 to 3 mm through 120 experimental runs.

Laser-Functionalized Superbiphilic Surfaces
Superbiphilic surfaces were prepared in three steps.Firstly, the entire surface is laser textured to rendered it homogeneously superhydrophilic and induce favorable micro-and nanostructure.Afterwards, a hydrophobic agent is applied to make the surface homogeneously superhydrophobic.Finally, local laser texturing is used to create a juxtaposed pattern of superhydrophilic and superhydrophobic areas, thus rendering the surfaces heterogeneously wettable, i.e., superbiphilic.
The first laser texturing step was based on a set of parameters with a higher laser pulse fluence of ≈55.6 J cm −2 and a variable spacing of laser beam scanning lines to create microcavities on the surface, which form through ablation and melting of material, followed by solidification.Previous studies showed that variable spacing ensures the fabrication of multiscale microcavities suitable for nucleating in boiling applications. [9,34]Multiscale microcavities significantly enhance the boiling process as they serve as a potential active nucleation site from which the vapor bubbles preferentially grow.Furthermore, the interaction of laser pulses with the material during laser-processing both moves and removes material from the surface via ablation, forming microstructures that ensure microroughness of the surface. [9,65]Meanwhile, oxidation of the material due to laser irradiation results in the growth of nanostructures, Betz et al. [ 46] 50-200 [μm] 40%−60% of spot pitch Hydrophobic area: 110°H ydrophilic area: 7-25°H ydrophobic spots facilitate nucleation.Hydrophilic surrounding area delays CHF incipience due to preventing a formation of an insulating vapor layer.Betz et al. [ 49] 10-800 [μm] 5-400 [μm] / Jo et al. [ 50] Relative pitch (1.2;2;4) Relative pitch = Spot pitch/ diameter of spot (RP = P/D) 50-1000 [μm] Hydrophobic area: 123°H ydrophilic area: 54°C HF on heterogeneous wetting surfaces was strongly dependent on the ratio of the area covered by hydrophobic regions to the heated area, but independent of the diameter of hydrophobic regions and the spot pitch.The diameter of the hydrophobic region, spot pitch, and the number of hydrophobic regions have a significant effect on the enhancement of boiling performance.
Rahman et al. [ 51] 90 [μm] 45 [μm] Hydrophobic area: >150°H The HTC of nanostructured surfaces increases with the addition of patterned low-surface-energy films.The HTC enhancement is due to increase nucleation sites and a decrease in ONB.Nanostructured surfaces with mixed-wettability reduce CHF due to the suppression of capillary wicking.
Yamada et al. [ 52] 1.5 and 3 [mm] 0.5 and 1 [mm] Hydrophobic area:140°Boiling performance deteriorated with decreasing pressure and increasing heat flux on biphilic surfaces with large hydrophobic spots.Better boiling performance was obtained on biphilic surfaces with smaller spot pitch and diameter of hydrophobic spots at the low pressure.
Motezakker et al. [ 54] 1 [mm] 50-1100 [μm] Hydrophobic area: 165°H ydrophilic area: 20°T he optimal fraction of hydrophobic areas is ≈38.46%.CHF and HTC increase with the increase of the fraction of hydrophobic areas up to the suggested optimal value.The decreasing trend in CHF and HTC enhancement on surfaces with a fraction of hydrophobic areas above the proposed optimal value.
Pontes et al. [ 55] / 1.5-5 (mm)_ singular spot Hydrophobic area: 150°H ydrophilic area: 82°T he size of the superhydrophobic regions affects bubble dynamics and the rate of evaporated mass.Patterns with smaller superhydrophobic areas are the most effective at removing heat through evaporation.Moderate horizontal bubble coalescence promotes additional liquid flow between the spots and increases the amount of dissipated heat.Small spot pitch can result in vapor blanketing of the surface.
Pontes et al. [ 56] (0. ože et al. [ 57] 0.5-2.5 [mm] 0.25-1 [mm] Hydrophobic area: 165°H ydrophilic area: <1°T he size of (super)hydrophobic spots does not have a major influence on the boiling performance when optimal spot pitch is used.The optimal fraction of (super)hydrophobic areas depends on the surface functionalization approach and spot pattern.
Lim et al. [ 58] 0- Pontes et al. [ 59] 1.5-3 mm 3.3 mm; 3.6 mm; 3.9 mm; / High heat dissipation is assigned to the well-established distance between superhydrophobic regions, of the order of the bubble departure diameter.Well-established spot pitch allows a controlled bubble coalescence and provides a better liquid supply toward the poorly wettable regions.
Santos et al. [ 60] 1.5 mm 0-3 mm Hydrophobic area: 156.5-160.7°Hydrophilic area: he heterogenous wettable pattern clearly promotes fluid flow within the superhydrophobic regions, as well as a controlled coalescence, which further promotes induced convection of fresh fluid near the surface.
Serdyukov et al. [ 61] 1.5 mm 10 mm Hydrophobic area: 124°H ydrophilic area: 70°N otable decrease in bubble departure diameters and a high increase in emission frequencies compared to bare surface.
providing nanoroughness of the surface and a hierarchical surface structure.The latter allows for superhydrophobicity of the surface to be achieved with the application of the hydrophobic agent.The nucleation-promoting ability of microcavities is additionally enhanced by applying a self-assembled superhydrophobic monolayer, which is formed from a fluorinated phosphonic acid solution via drop casting.While superhydrophobic surfaces tend to promote nucleation and improve the HTC, [56,66] superhydrophilic surfaces ensure liquid supply to the hot surface in the slug and column regime, resulting in higher CHF. [54,67]Therefore, the final functionalization step was aimed at fabricating superhydrophilic areas on the homogenously superhydrophobic surface using a second set of parameters (with lower laser pulse fluence of ∼9.8 J cm −2 and a crosshatch pattern) to locally remove the monolayer hydrophobic coating.Additionally, the second laser irradiation of the surface induced additional microroughness and fresh copper oxides.As the later exhibit high surface energy, the area exposed to the second laser-texturing step exhibit superhydrophilicity immediately after laser texturing.The collection of surfaces prepared and tested within the study is summarized in Table 2. Multiple replicas of each surface were prepared, and each surface was tested multiple times to verify both the fabrication repeatability and the stability of their boiling performance.Several surfaces were prepared to serve as the baseline of the experiment to enable an objective evaluation of enhancements obtained with superbiphilic surfaces.The references include a reference untreated surface (REF), a homogeneously superhydrophobic surface treated only with the first laser texturing step and hydrophobization (SHPO), and a homogeneously superhydrophilic surface treated twice in its entirety by the first and the second laser-texturing step (SHPI).The individual names of superbiphilic surfaces denote the size of the superhydrophobic spot and the spot pitch between adjacent spots (e.g., the sample a02p08 corresponds to a spot size of 0.2 mm and a spot pitch of 0.8 mm).
A comparison of SEM images of biphilic surfaces with a constant spot pitch value of 0.8 mm and different spot sizes (0.2, 0.4, and 0.6 mm) is shown in Figure 1, while additional SEM images of surfaces with different spot pitch can be found in Figures S1,S2 (Supporting Information) in the Section 1 of the Supporting Information.
Apparent static contact angles of these surfaces are listed in Table 2. Detailed information for static and dynamic contact angle measurements can be found in the Supporting Information, Section 2. Briefly, the superhydrophilic surface exhibits a low apparent static contact angle of 5.3°(±0.7°),while the measurements of the dynamic contact angle showed an advancing  contact angle of 167.2°(±0.8°)and a receding contact angle of 163.8°(±1.3°)were recorded on the superhydrophobic surface (hysteresis of 3.4°).

Pool Boiling Heat Transfer Performance on Reference Surfaces
Evaluation of the boiling performance of the reference surfaces within this study is shown in Figure 2 as a boiling curve comparison.The untreated surface represents the initial state of all surfaces prior to any functionalization treatments.The SHPI and SHPO surfaces represent the properties of superhydrophilic surrounding areas and superhydrophobic spots on superbiphilic surfaces, respectively, and were tested to allow for identification of any additional heat transfer performance benefits arising from their simultaneous inclusion on mixed wettability surfaces.The obtained critical heat flux for the untreated surface (REF) is 980 kW m −2 and its value slightly decreased during the three repetitions of the boiling test, which could be due to the occurrence of different surface oxide species due to the high temperatures of the sample when the CHF incipience occurs . [18]Laserengineered surface (sample SHPI-refers to the fully superhydrophilic surface with a contact angle of 5.3°) exhibited enhancement of the CHF by ≈17% compared to the REF surface for all three repetitions of the test.During the first experimental run of the testing, a decrease in the superheat value was observed on fully superhydrophilic sample.This is attributed to the transformation of copper oxides from CuO into CuO 2 with the following reduction of Cu(OH) 2 to CuO . [18]Moreover, secondary boiling effects (a "hook back" of the boiling curve) were observed on the SHPI sample and are attributed to activation of additional nucleation sites on the top of microstructures with high aspect ratios in high heat flux region as explained by Kruse et al. [68] Secondary boiling effects resulted in a significant reduction in the wall superheat temperature at high heat fluxes (near the CHF), which in turn led to a notable improvement in HTC.After the first CHF incipience, the boiling curves were shifted towards lower superheat values.This is consistent with previous research showing that the shift in boiling curves is due to changes in surface morphology and chemistry . [18]Hydrophobization of the laser-textured surfaces resulted in a superhydrophobic sample (SHPO) that exhibited excellent boiling performance with boiling curves shifted to exceptionally low superheats of ≈3 K, while the CHF value remained similar to the one observed on the REF sample.The stability of homogenously superhydrophilic and superhydrophobic surfaces coupled with the stability of surfaces with different superhydrophobic fractions used for finding optimal spot pitch are

Effect of Superhydrophobic Surface Area Fraction
To elucidate the effect of the superhydrophobic surface area fraction (A SHPO ) and spot pitch on the boiling behavior of heterogeneously wettable surfaces, different combinations of spot pitch values (0.4 to 3.0 mm) and spot sizes (0.2, 0.4, 0.6 mm) were tested.For a spot size of 0.4 mm, three samples were fabricated and tested for each spot pitch value.Figure 3a shows the average values of the heat transfer coefficients, where 0% superhydrophobic surface fraction refers to the homogeneously superhydrophilic surface (SHPI) and 100% superhydrophobic surface fraction refers to the homogeneously superhydrophobic surface (SHPO).The average HTC values recorded on the superhydrophilic surface during the first and third experimental run are shown separately in the graph, due to the large boiling curve shifts, discussed in the Supporting Information, Section S5.The average HTCs recorded on the SHPO/SHPI surfaces and surfaces with the different superhydrophobic surface fractions are compared at three heat fluxes corresponding to the low heat flux range (< 300 kW m −2 ), the medium heat flux range (300-1000 kW m −2 ), and the high heat flux range (> 1000 kW m −2 , i.e., close to the CHF).Multiple criteria were employed in the determination of low, medium, and heat flux ranges.A low heat flux boiling regime (up to 300 kW m −2 ) is characterized by smaller bubbles diameters and lower a bubble nucleation frequency.Bubbles may emerge intermittently, resulting in an incomplete cov-erage of the surface.Conversely, as heat flux exceeds 300 kW m −2 but remains below 1000 kW m −2 (medium heat flux regime), both bubble size and frequency increase, leading to detachment at larger sizes.Furthermore, in this regime, maximum number of nucleation sites is attained (i.e., the nucleate boiling is fully developed), thereby enhancing boiling performance.Approaching a heat flux of 1000 kW m −2 and beyond (high heat flux regime) is distinguished by the intensive coalescence of bubbles, followed by the forming of dry patches on the surface that is a result of a complete microlayer evaporation underneath the coalesced vapor regions. [69,70]he results presented in Figure 3a demonstrate that the boiling performance on surfaces with superbiphilic properties in low and medium heat flux regions increases as the area fraction of superhydrophobic spots (A SHPO ) increases.The superbiphilic surface with the highest A SHPO value of 65% (i.e., a06p08) also displayed the best performance, while the overall best performance was observed on a homogeneously superhydrophobic surface.To further investigate the impact of spot pitch and A SHPO , we analyzed the boiling performance for each tested spot size over different spot pitches, which is presented in the Supporting Information, Section 5 (Figures S11-S13, Supporting Information).Our findings show that surfaces with smaller spot pitch (i.e., a02p04, a04p06, a06p08) outperform others in the heat flux range up to 1000 kW m −2 , whereas in the high heat flux regions near and at critical heat flux (CHF), the surfaces with lower A SHPO (i.e., larger spot pitch) exhibit superior boiling performance.
In the region of high heat fluxes at or near CHF (not shown in Figures S11-S13), our data indicates that boiling performance increases up to an A SHPO of 1% (i.e., a04p125), after which it again decreases.For example, the average heat transfer coefficient at CHF for samples with a spot size of 0.4 mm increased from 188 kW m −2 K −1 to 282 kW m −2 K −1 when increasing the spot pitch from 0.6 to 1.25 mm, and then decreased to 170 kW m −2 K −1 when the pitch was further increased to 3.0 mm.The observed differences in boiling performance for surfaces with the same spot size but different spot pitch at different heat flux regions can be attributed to different bubble interaction in relation to the distance between the nucleation regions.It has been reported that as the operating heat flux increases, the bubble size and number of active nucleation sites also increase, and the interaction between them changes. [71,72]In the low heat flux region, the diameter of a bubble can be measured, whereas in medium and high heat flux the nucleation frequency and bubble coalescence is too high for accurate measurements.In the low heat flux region, the mean bubble departure diameter changed from ≈0.5 to 1.4 mm, when the spot pitch was increased from 0.6 to 3.0 mm on samples with a constant spot size of 0.4 mm.Previous studies have demonstrated that a spot pitch closely matching the bubble departure diameter can yield a higher density of nucleation sites, thereby promoting superior boiling performance . [57]Our experimental findings indicate that in the regions characterized by low to moderate heat flux, the bubble departure diameter (e.g., on a04p06) closely matches the spot pitch, thus adding to the already advantageous effects of the small spot pitch.Under these heat flux conditions, the interaction between bubbles allows the preferable liquid supply towards hydrophobic regions on surfaces with smaller spot pitches due to moderate lateral coalescence of bubbles, which leads to increased boiling performance.When the spot pitch is too large, such superbiphilic surfaces demonstrate inferior performance due to reduced jet impingement impact on the surface, which agrees with previous findings of Jaikumar et al. [73] In contrast, when the operating heat flux increases, the spacing between previously optimally distributed superhydrophobic spots becomes inadequate due to the formation of larger bubbles and the increase in the number of active nucleation sites.These larger bubbles can cause undesirable lateral coalescence over the superhydrophilic surface areas, resulting in the formation of local vapor films, which hinder liquid supply and cause a reduction in boiling performance. [73,74]However, when surfaces with a larger spot pitch (e.g., a04p125) are utilized in high heat flux regions, their boiling performance is enhanced, as the large bubbles have enough space between them to achieve only moderate lateral coalescence, resulting in increased liquid flow over the superhydrophilic regions and greater boiling heat transfer performance.
However, as observed in this study, if the spot pitch is further increased (i.e., a surface with A SHPO less than 11% -surfaces with spot pitch larger than 1.25 mm), the performance starts to worsen in the high heat flux region as well due to the distance between neighboring nucleation sites being too large, due to reduced jet impingement impact on the surface which was also shown in previous studies. [73,74]Moreover, in high heat flux regions near the CHF, the appearance of secondary boiling effects has also a notable impact on the improvement of boiling performance.The appearance of secondary boiling effects was already aforementioned and it is presented by the so-called "hook-back" of the boiling curve.The boiling performance improvement due to secondary boiling effects in the high heat flux regions is attributed to the appearance of additional nucleation sites in superhydrophilic regions, where nucleation only appears at sufficiently high superheat and heat flux values.The average boiling curves of all tested surfaces are shown in Figures S14-S16, Supporting Information, Section 5, where the aforementioned phenomena can be observed.Furthermore, statistical analysis (ANOVA) of the heat transfer coefficient dependence on the superhydrophobic area fraction (in the range 2%−65%) showed that differences in heat transfer coefficient versus the superhydrophobic surface fraction (in the range of 2% -65%) are statically significant at all three heat flux regions with p-values notably under the significance threshold.More detailed statistical analysis (ANOVA) information is presented in Supporting Information, Section 6.
The results discussed so far indicate that the optimal spacing between active nucleation sites on superbiphilic surfaces for achieving superior boiling performance is influenced by the heat flux at which the surfaces will operate.This gap in understanding may explain why various authors have reported different optimal distributions and spacing between active nucleation sites.The initial investigation into optimizing the distribution and spacing between active nucleation sites was conducted by Rahman et al. , [74] who suggested that the spacing between active nucleation areas should be determined based on the capillary length criterion for a given fluid and operating conditions.Their study involved the one-dimensional separation of active nucleation areas, and they found that the optimal spacing between nucleation areas was the capillary length of water (≈2.5 mm), as determined by the bubble departure diameters during the saturated pool boil-ing of water on plain copper.Later on, Zakšek et al. [31] discovered that the optimal spacing between active boiling areas distributed in one dimension was slightly larger than what the capillary length criterion predicted.They concluded that the capillary length criterion could serve as a starting point for the optimization process, but the actual optimal spacing should be based on the maximization of active nucleation site density.Furthermore, a recent investigation into optimization techniques for boiling applications indicates the emergence of a unified criterion resulting from instabilities observed in the close-range interactions among bubbles.This phenomenon can be conceptualized as a percolation process governed by three fundamental parameters of boiling: nucleation site density, average bubble footprint radius, and the product of average bubble growth time and detachment frequency.Consequently, this criterion offers a straightforward mechanistic principle for forecasting critical heat flux while concurrently striving to maximize the efficiency of boiling processes . [69]Može et al. [57] demonstrated that the optimal spacing between poorly wettable areas is not universal for each fabrication type and can be determined by the bubble departure diameter.Therefore, it can be concluded that the optimal value of spot pitch for the 2D distribution of active nucleation sites is not solely determined by the maximization of nucleation sites or the bubble departure diameter, or the fabrication type.
A comparison of average critical heat flux values with standard deviation versus superhydrophobic surface fraction is shown in Figure 3b.It has already been shown that the hydrophilic surface increases the liquid flow towards areas of nucleation, which leads to a delay in the critical heat flux due to water replenishment on dry spots. [42,51,75]It should also be noted that the superhydrophilic laser-textured surfaces exposed to the boiling process for several hours and high temperatures during the onset of CHF exhibit increased contact angles and show transition from the initial superhydrophilic state toward a hydrophobic state . [18]The larger percentage of transition to a hydrophobic wettable state with each hour of boiling leads to a decrease in CHF (observed for almost each subsequent run).In addition, it has already been shown that high temperatures and the onset of CHF lead to a change in surface morphology and chemistry, which also resulted in decrease of CHF . [18]Within the context of this observation and measurement uncertainty, the standard deviation shown in Figure 3b can be attributed to the randomness of the boiling process and not experimental uncertainty.The highest average CHF value was recorded on the sample with a superhydrophobic surface fraction of 12%, and up to this value there is no clear correlation between CHF and superhydrophobic surface fraction.From that point on, the average CHF value decreases with increasing superhydrophobic surface fraction and reaches the lowest value on a completely superhydrophobic surface.This is due to a decrease in the superhydrophilic surroundings and the fact that poorly wettable areas are usually unable to successfully limit bubble coalescence and tend to transition to film boiling at much lower heat flux values.

Effect of Spot Size
To further discuss the effect of the superbiphilic surface pattern geometry, several surfaces with spot sizes of 0.2, 0.4, and 0.6 mm  S17 in the Supporting Information, Section 6.In the low and medium heat flux regions, the boiling performance increases with increasing spot size for all tested spot pitch values.The superbiphilic surfaces with the best boiling performance in these two regions (for each tested spot pitch) are those with the highest superhydrophobic surface fraction (a06p08 -65%, a06p125 -27%, a06p20 -11%) This is consistent with the previous observations where the best boiling performance was observed on surfaces with the highest superhydrophobic surface fraction. [54,76]Additionally, these results are also consistent with the observations of previous studies, showing that the size of the hydrophobic spot does not have a major influence on the boiling performance. [54,56,76]In this context, spot pitch (correlated to the percentage of the superhydrophobic surface area), has the largest effect on boiling heat transfer performance, with the highest performance expected at the highest percentage of superhydrophobic area on superbiphilic surfaces.Finally, in high heat flux boiling regime near or at CHF, there is no established boiling performance trend with increase of the spot size which is opposite from spot pitch.This is attributed to the appearance of secondary boiling effects at high heat fluxes, especially above heat flux 1000 kW m −2 , near critical heat flux, which significantly obscures observation of trends concerning the superhydrophobic spots, as the bubble nucleation spreads to other parts of the surface, which in turn precludes any conclusions from being drawn from the results.

Stability of Superbiphilic Surface
The superbiphilic surfaces demonstrate a notable alteration in the boiling curve, with a shift toward lower superheats after the first onset of CHF.This shift can be attributed to a distinct transformation in surface chemistry and morphology that occurs as the copper oxides undergo a transition.This transition is observed in superhydrophilic and superhydrophobic regions of the laser-textured superbiphilic surface and is prompted by the occurrence of high surface temperatures during the first incipience of CHF.The resultant shift in the boiling curve gives rise to a discrepancy in superheat temperatures between the first and second experimental runs, thereby indicating a reduction in superheat.Figure 5. shows the reduction of superheat temperature within the critical heat flux region as a function of the superhydrophobic surface fraction.An inverse relationship between this reduction and the increase of superhydrophilic surface fraction (i.e., a decrease in superhydrophobic surface fraction) is observed.The larger shift of the boiling curve, i.e., greater reduction of superheat temperature, on superbiphilic surfaces with lower A SHPO can be attributed to alterations in the surface chemistry and morphology of the superhydrophilic region.Može et al. [18] observed changes in the surface chemistry and morphology of superhydrophilic laser-textured surfaces, leading to the shift of the boiling curve towards lower superheat temperatures after the critical heat flux incipience.After the literature review, it was found that Boinovic et al. [77] showed that transformation from Cu 2 O to CuO occurs in ambient air at ≈400 °C, while the thermal conversion of CuO to Cu 2 O takes place at high temperatures, i.e., above 1100 °C.On the other, Može et al. [18] demonstrated that the transformation of stable copper oxide forms (CuO) to more unstable forms (Cu 2 O), resulting from the high temperatures (in the region of 210-320 °C), i.e., low-temperature annealing during the transition from nucleate to film boiling regime, is thought to be the cause of this change.Nevertheless, the findings in this study show that the temperatures within the range of 210-320 °C, attained during the transition from nucleate to film boiling, as well as the duration of the vapor film until its disintegration (≈10 min), constitute conducive conditions for oxide and hydroxide transformations to occur.In conclusion, superbiphilic surfaces with lower superhydrophilic fractions exhibit greater stability in terms of boiling performance compared to those with higher superhydrophilic surface fractions.Additional information on the stability of superbiphilic surface is presented in Supporting Information, Section 3.

Enhancement Mechanism
A homogeneously superhydrophilic surface with microcavities (SHPI) provided favorable boiling performance with an HTC of 133 Kw m −2 K −1 at CHF, which can be attributed to the presence of microcavities on the surface.It was previously shown that microcavities are extremely effective in serving as preferential active nucleation sites during the nucleate boiling process. [34,78]However, the performance of the superhydrophilic surface is significantly lower than the boiling performance of its superhydrophobic counterpart (SHPO).Previous studies showed that properly degassed poorly wettable surfaces can initiate boiling from the Wenzel wetting regime instead of the Cassie-Baxter regime and by that provide highly efficient boiling performance. [44,79]Therefore, the occurrence of the Wenzel wetting state on the SHPO surface is favorable and is enabled by a three-hour degassing of the surface.Furthermore, the combination of superhydrophobic and superhydrophilic surfaces allowed for the fabrication of heterogeneously wettable surfaces where bubble nucleation occurs at hydrophobic spots, and the superhydrophilic surrounding serves to prevent bubble coalescence and provide liquid supply toward hydrophobic regions. [43,56,57]he boiling performance of superbiphilic surfaces was remarkably higher than the reference and superhydrophilic surfaces.In order to better understand and explain the exceptional boiling performance of superbiphilic surfaces, we observed bubble dynamics using a high-speed video camera.High-speed video (HSV) snapshots coupled with graphical representations of the ongoing phenomena are shown in Figure S18 (Supporting Information).The distinctive feature of superbiphilic surfaces was that during the detachment process of the bubbles on the superhydrophobic regions, a new bubble has formed immediately after the departure of the previous bubble.The remaining vapor facilitated nucleation, resulting in almost no waiting time in the bubble ebullition cycle.This phenomenon leads to an increased nucleation frequency, generating a greater number of smaller bubbles that detach rapidly and increase the available surface area for more efficient heat transfer between the heated surface and the surrounding fluid . [56]oreover, the findings of this investigation demonstrate that the superior boiling performance of superbiphilic surfaces is dependent on the heat flux region in which they are utilized.The principal factors that influence the distinct boiling behavior of these surfaces in diverse heat flux regions are attributed to the surface geometry pattern (spot pitch) and the interaction between bubbles that forms in superhydrophobic areas during boiling.The schematic diagram that illustrates the interaction of bubbles on surfaces with different spot pitches in different heat flux regions and their effect on heat transfer performance is depicted in Figure 6.
Our visual observations and quantitative measurements both indicate that an appropriate spot pitch in a given heat flux region facilitates moderate lateral coalescence of bubbles over the superhydrophilic surroundings, resulting in enhanced fluid supply towards the superhydrophobic spots and leading to exceptional boiling performance.In addition to the spot pitch, bubble size plays a significant role in achieving moderate lateral coalescence on the surface.The size of the bubbles increases as the operating heat flux increases. [71,72]Hence, our results show that at low and medium heat flux, surfaces with smaller spot pitch (e.g., smaller bubble sizes such as on surface a04p06) exhibit the best performance, facilitating moderate lateral bubble coalescence that increases liquid supply toward superhydrophobic regions.However, as heat flux increases (i.e., high heat flux region), bubble size also increases, leading to the unwanted formation of a vapor blanket over the superhydrophilic regions that prevents the liquid supply, resulting in poor boiling performance (see Figure 6e  and f).
Additionally, visual analysis of the results reveals that at low heat fluxes, the departure diameter of the bubble rises with increasing spot pitch up to a certain threshold (1.5 mm) and then remains relatively constant as the spot pitch further increases as shown in Figure 7.A detailed description of determining the bubble departure diameter is presented in Supporting information, Section 7. Previous studies showed that the ratio between the departure diameter of the bubble and the spot pitch has a large influence on the density of active nucleation sites, i.e., achieving superior boiling performances in low and medium heat flux regions.Bubble departure diameter values similar to the spot pitch are favorable for promoting nucleation (i.e., increased boiling performances), and vice versa if the departure diameter of the bubble is lower than the spot pitch, boiling performance is inferior.
Although surfaces with smaller spot pitch values are more favorable for promoting boiling performance at low and medium heat fluxes, surfaces with larger spot pitch (e.g., a04p125) exhibit the best boiling performance at high heat fluxes due to a greater ratio of superhydrophilic area that promotes liquid supply and increases the boiling performance of the surface (see Figure 6d).In cases when the bubble size is much smaller than the spot pitch, the liquid supply over superhydrophilic region is uninterrupted due to very poor interaction between bubbles.However, the surface with a much larger spot pitch than bubble size in all considered heat flux regions exhibits moderate boiling performance, resulting from reduced jet impingement impact [73] on the surface (see Figure 6a,b, and c).
Furthermore, the emergence of secondary boiling phenomena also has a significant influence on the extreme boiling performance achieved on superbiphilic surfaces at high heat flux regions.The change in boiling behavior due to secondary boiling effects is attributed to the appearance of additional nucleation sites on superhydrophilic surroundings, which only appear at high heat fluxes, when the temperature of this region is sufficiently high for nucleation to occur.The activation of additional nucleation sites on the superhydrophilic surroundings due to the occurrence of these phenomena enhances the boiling performance of the surfaces, as illustrated in Figure 8.
In summary, the significant increase in heat transfer performance on a laser-textured surface is attributable to the presence of microcavities that act as active nucleation sites.Moreover, to achieve superior boiling performance on superbiphilic surfaces, it is essential to consider not only the increasing number of active nucleation sites or the bubble departure diameter but also the operating heat flux in which the surface will be utilized.Thus, the appropriate geometry pattern of superbiphilic surfaces (i.e., spot pitch) cannot be defined solely based on these parameters but must also consider the specific operating heat flux in which the surface will be utilized.

Comparison with Literature-Reported Data
To put the achieved boiling performance of the developed surfaces into perspective, we compared the CHF and HTC values recorded on the developed superbiphilic surfaces with various reports from literature in Figure 9.The data shown in Figure 9, as well as additional data on HTCs in low, medium, and high heat flux regions, is summarized in Table S4, Supporting Information, Section 9.The results in Figure 9 show that the highest average heat transfer coefficient values recorded during this study are at least 10-20% higher than the next highest reported value.The highest average HTC calculated from HTCs obtained in mul- tiple runs at CHF for biphilic surfaces was recorded for samples with a spot size of 0.6 mm and spot pitch of 2.0 mm, while the highest HTC recorded during one experimental run was for a superbiphilic surface with a spot size of 0.4 mm and spot pitch 1.25 mm with a value of 466 kW m −2 K −1 .This is ≈90% higher than the next highest reported value.The comparison of CHF with data from the literature shows that CHFs recorded on superbiphilic surfaces are not the highest, but on the other hand, extreme values of HTC were recorded in this study at the time.The superbiphilic and superhydrophobic surfaces presented in this work are capable of achieving heat transfer coefficients in the range of 200-300 kW m −2 K −1 , when operating at heat fluxes above 200 kW m −2 .This makes them very suitable for sensitive cooling applications where a low temperature of the cooled component is required together with high heat dissipation.Furthermore, we also reviewed articles dealing with pool boiling heat transfer performance using biphilic and superbipilic surfaces.We normalized our enhamcnet of the heat transfer coefficient with the literature-reported data (i.e., their enhanced HTC values).Table 3 presents the relative enhancement achieved with our best-performing surfaces over the best-performing (super)biphilic surfaces, found in the literature.Positive values de-note superior performance of our surfaces in comparison with previous literature reports.

Conclusion
In this work, we demonstrate exceptional boiling performance of superbiphilic and superhydrophobic surfaces fabricated Table 3. Relative HTC enhancement was achieved with our best-performing surfaces over the best-performing (super)biphilic surfaces reported in the literature.

Authors HTC enhancement at different heat fluxes [%]
@ 250 kW m −2 @ 500 kW m −2 @ 750 kW m −2 @ 1000 kW m −2 @ CHF Betz et al. [ 80] 163 Rahman et al. [ 74] 21.3 12.9 23.9 42.3 100.8 Lim et al. [ 62] 233.9 using direct nanosecond laser texturing in combination with application of a self-assembled monolayer of fluorinated phosphonic acid.We analyzed pool boiling results and investigated the enhancement mechanisms with high-speed imaging.Compared to a smooth reference surface, the developed superbiphilic and superhydrophobic surfaces exhibit up to an order of magnitude higher heat transfer coefficients under pool boiling of water at atmospheric pressure, with values exceeding 300 kW m −2 K −1 .
The extreme boiling performance of a superhydrophobic surface is attributed to an abundance of microcavities serving as active nucleation sites.On the other hand, the superior boiling performance of superbiphilic surfaces is attributed to achieving moderate bubble coalescence over superhydophilic regions and increasing the liquid supply of poorly wettable regions.Importantly, we show that the optimal surface pattern of poorly wettable regions for achieving moderate bubble coalescence (and thus large boiling performance) on superbiphilic surfaces may not solely depend on the fraction of the superhydrophobic surface, but also on the heat flux for which the surface is designed.In the region of low and medium heat fluxes, the superior boiling performance is observed on surfaces with a higher fraction of superhydrophobic areas, which is attributed to the interplay between bubble departure diameter and the spot pitch together with the consequential bubble interaction through coalescence.Conversely, in regions of high heat fluxes (i.e., above 1000 kW m −2 ) the best boiling performance was exhibited by surfaces with a lower fraction of superhydrophobic areas (≈11%).This is ascribed to the increase in bubble size with increasing heat flux, which led to the moderate bubble coalescence (responsible for superior boiling performance) being achieved on surfaces with larger spot pitch values.Additionally, the extreme boiling performance in this region benefited from the appearance of additional nucleation sites in the superhydrophilic surroundings due to the secondary boiling effects.Overall, the developed surfaces exhibited some of the highest boiling heat transfer intensities recoded and reported to date.

Experimental Section
Samples: Samples were manufactured from copper rods of electrolytic purity (>99.9%Cu).The samples were cylindrical with a diameter of 14 mm and an overall height of 22 mm.The top flat face of the sample served as the boiling surface while the bottom face was flanged for mounting the sample onto the heating block.The boiling surface of each sample was sanded with P1200 and P2000 sandpaper to achieve surface roughness of approximately Ra = 0.2 μm and cleaned with 2-propanol prior to testing or subsequent treatments.Each sample was equipped with 3 Ktype thermocouples to record the axial temperature gradient within the sample and calculate the heat flux.A schematic representation of the sample including the important dimensions was shown in the Figure S19, Supporting Information, Section 10.
Nanosecond Laser Microengineering of Boiling Interfaces: Laser texturing of the boiling surfaces was performed with a nanosecond fiber laser (JPT Opto-electronics Co., Ltd., M7 30 W MOPA;  = 1064 nm).The laser beam was directed across the surface with an F-Theta lens with a working area of 70 × 70 mm 2 and a focal distance of 100 mm.The focused beam spot diameter was ≈25 μm, the laser beam quality M 2 < 1.3 (manufacturer data) and the maximal power of the laser source 30 W. Surface treatment patterns were designed in "ezCAD" software, which was also used to control the laser system during the laser treatment.
Two sets of laser texturing parameters were developed and used within the study.The first step produced microcavities on the surface, which were previously shown [18,78,79] to be extremely effective at serving as preferential active nucleation sites during the nucleate boiling process, especially in a hydrophobized state.To produce parallel grooves with ridges of porous resolidified material in between, the following texturing parameters were used: scanning speed 110 mm s −1 pulse frequency 110 kHz, pulse width 45 ns, average power of 30 W and average pulse fluence of 55.6 J cm −2 Parallel lines with variable spacing were used as the scanning pattern, with the spacing varying cyclically by 5 μm from 25 μm to 35 μm and back.
The second laser texturing step was performed after the entire surface was hydrophobized to locally remove the low-surface-energy coating and produce the surface with locally varied wettability (i.e., a biphilic surface).To achieve this, the following parameters were used alongside a crosshatch scanning pattern (45°between the first and second surface treatment and 90°between the and second pass during the second treatment; line spacing of 10 μm): scanning speed 500 mm s −1 pulse frequency 500 kHz, pulse width 30 ns, average power of 24 W and average pulse fluence of 9.8 J cm −2 Both laser texturing patterns were graphically explained in the Figures S20,S21, Supporting Information, Section 10.
Analysis of Surface Properties: Properties of functionalized surfaces were investigated through scanning electron microscopy and contact angle measurements.To observe the surface micro-and nanostructure, a scanning electron microscope (SEM; ThermoFisher Scientific Quattro S) utilizing both BSE and SE imaging at an accelerating voltage of 10 kV was used.Contact angles were recorded using a goniometer (Ossila), where measurements using water at room temperature were performed on the untreated surface, fully hydrophilic surface (i.e., after the first laser texturing and with no hydrophobization) and fully hydrophobic surface (i.e., after the first laser texturing and after surface hydrophobization).Furthermore, the persistence of superhydrophobicity of hydrophobized patches not exposed to the second step of laser treatment was verified experimentally as well as the reappearance of superhydrophilicity on the area subjected to the second laser texturing step.SEM image of surface pattern and their schematic representation were shown in Figure S23, Supporting Information, Section 9. Additionally, the surface roughness of superhydrophobic and superhydrophilic regions were presented in Table S5 (Supporting Information), while the structure obtained on four different spots for both regions were shown in Figure S24 in Section 11, Supporting Information.Briefly, the superhydrophobic regions exhibited a roughness of approx.Sa = 5.9±0.7 μm, Sz = 59.7±10.2μm and Sq = 7.8±1.4μm, while the superhydrophilic surrounding areas exhibited a roughness of approx.Sa = 6.3±2.8 μm, Sz = 56.9±14.6 μm and Sq = 8.4±3.1 μm.
Pool Boiling Performance Evaluation: Pool boiling performance was evaluated using a custom experimental setup shown schematically in Figure S25, Supporting Information, Section 12. Three holes (7 mm long, 0.8 mm diameter) were drilled at 5 mm intervals on the side of the sample, into which thin type K-type thermocouples (Figure S19, Supporting Information, Section 10) were embedded.The samples were mounted on a copper heating block and inserted into the boiling chamber through the lower stainless-steel flange.PEEK bushing, a ring of flexible epoxy glue, and a silicone O-ring were used to provide sealing, limit heat loss, and prevent parasitic boiling.The boiling chamber was constructed from a glass cylinder (inner diameter 60 mm) between two stainless steel flanges and filled with ≈200 mL of twice distilled water during the measurements.The cartridge heaters, positioned inside the heating block were used to generate heat and were also controlled by a variable transformer.All measurements were performed at atmospheric pressure, its stability confirmed by a stable saturation temperature.Vapor produced during the measurements was fed into the water-cooled glass condenser and returned to the boiling chamber.Two other type K-type thermocouples were immersed in the boiling chamber at different heights to measure the temperature of the working fluid.A detailed depiction of the experimental setup was shown in Figure S25, Supporting Information, Section 12. Measurements were repeated three times on every surface to evaluate their stability.During each experimental run, the heat flux was continuously increased at a rate of 2 kW m −2 s −1 .The chosen methodology of slow continuous increase of heat flux can be considered as quasi-stationary, as shown by Može et al. [82] The boiling curve was recorded until the CHF was reached.When an experimental run was finished, cartridge heaters were turned off and the sample was left to cool down on its own.KRYPTONi-8xTH DAQ device was used for collecting all temperature signals as raw voltages.Data from DAQ device were acquired using Dewesoft X3 software at a frequency of 10 Hz.
Data Reduction and Measurement Uncertainty: During the boiling experiments, five temperatures were monitored (three along the axis of the sample in the direction of heat conduction and two in the water).These were used to obtain instantaneous values of relevant heat transfer parameters via online calculations in DewesoftX software during measurements and in MathWorks MATLAB during post-processing of the results.While a detailed description of data reduction and measurement uncertainty calculations including all relevant equations were provided in the Supporting Information, Section 11, a brief description of the data reduction procedure was given here.
All data was collected at 10 Hz, from which 1 Hz average values were calculated.Average temperature of the sample was determined as the arithmetic mean of all three thermocouples.Using temperaturedependent value of sample's thermal conductivity, evaluated at its average temperature, heat flux was calculated from the spatial temperature gradient within the sample by assuming 1D conduction and using Fourier's law of conduction.Sample's surface temperature was then extrapolated using the temperature of the thermocouple closest to the boiling surface and the calculated heat flux.Surface superheat was determined as the difference between the sample's surface temperature and the mean temperature of the water in the boiling vessel.Finally, the heat transfer coefficient was calculated by dividing the heat flux with the surface superheat to obtain a metric describing the boiling performance and heat transfer intensity.
Evaluation of measurement uncertainty was performed following recommendations made in a previous study [82] Uncertainty of all three crucial heat transfer parameters (heat flux, surface superheat and heat transfer coefficient) changes with the operating point (i.e., with heat flux and surface superheat).Therefore, evaluation was performed at a low heat flux value of 250 kW m −2 and at a higher value of 1000 kW m −2 .At 250 kW m −2 , the relative heat flux uncertainty was 9-10%, the surface superheat uncertainty was 0.4 K and the relative heat transfer coefficient uncertainty measures 9.9-15.1%.At 1000 kW m −2 , the relative heat flux uncertainty was 4.8%, the surface superheat uncertainty was 0.8 K and the relative heat transfer coefficient uncertainty measures 8.0-24.6%.
All boiling performance evaluations were performed using the dynamic boiling curve measurement methods as used in the previous studies, which was proven [14] to provide accurate results regarding the heat transfer parameters and significantly speed up the measurements procedure.The latter has the benefit of reducing the effect of possible surface property changes on the measurement results while also allowing for acquisition of more boiling curves to determine the stability of the surface and its heat transfer performance.

Figure 1 .
Figure 1.SEM images of superbiphilic surfaces with constant spot pitch of 0.8 mm and different size of superhydrophobic area: a) spot size of 0.2 mm; b) spot size of 0.4 mm; c) spot size of 0.6 mm and d) SEM images of superhydrophilic region (bottom right) and superhydrophobic region (bottom left).

Figure 3 .
Figure 3. a) Heat transfer coefficients versus superhydrophobic surface fraction and b) Critical heat fluxes versus hydrophobic surface fraction.

Figure 4 . 4 .
Figure 4. a) Heat transfer coefficient on the samples with constant spot pitch 0.8 mm versus spot size of superhydrophobic region and b) Heat transfer coefficient on the samples with constant spot pitch 1.25 mm versus spot size of superhydrophobic region.

Figure 5 .
Figure 5.The reduction of surface superheat as a function of the superhydrophobic surface fraction within the critical heat flux region.

Figure 6 .
Figure 6.A graphical illustration of the interplay between bubbles on superbiphilic surfaces featuring diverse spot pitches under distinct heat flux regions, and its influence on the boiling performance.

Figure 7 .
Figure 7. Bubble departure diameter for samples with a spot size of 0.4 mm at two different superheat temperature 1.5 and 2 K.

Figure 8 .
Figure 8. Graphical representation of the appearance of additional nucleation sites and their effect on boiling performance.

Figure 9 .
Figure 9.Comparison of pool boiling performance of previously reported data of heat transfer coefficients against data of heat transfer coefficients presented in this study at critical heat flux.

Table 1 .
Summary of selected studies on surfaces with heterogeneous wettability for enhanced boiling performance.

Table 2 .
List of surfaces fabricated and tested within the study.