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Biphilic surfaces, namely surfaces comprising hydrophilic areas with a (super)hydrophobic background, are used in nature and engineering for controlled dropwise condensation and ...liquid transport. These, however, are highly dependent on the surface temperature and subcooling.
Here, biphilic surfaces were cooled inside a rotatable environmental chamber under controlled humidity. The condensation dynamics on the surface was quantified, depending on the subcooling, and compared to uniform superhydrophobic (USH) surfaces. Rates of condensation and transport were analyzed in terms of droplet number and size, covered area and fluid volume over several length scales. Specifically, from microscale condensation to macroscale droplet roll-off.
Four phases of condensation were identified: a) initial nucleation, b) droplets on single patches, c) droplets covering adjacent patches and d) multi-patch droplets. Only the latter become mobile and roll off the surface. Cooling the surface to temperatures between T = 2–16 °C shows that lowering the temperature shortens some of the condensation parameters linearly, while others follow a power law, as expected from the theory of condensation. The temperature dependent condensation dynamics on (super)biphilic surfaces is faster in comparison to uniform superhydrophobic surfaces. Nevertheless, within time intervals of a few hours, droplets are mostly immobile. This sets guiding lines for using biphilic surfaces in applications such as water collection, heat transfer and separation processes. Generally, biphilic surfaces are suitable for applications in which fluids should be collected, concentrated and immobilized in specific areas.
•We use graphene nanoplatelets (GNPs) of varied wettability to enhance nucleate boiling.•Hydrophobic GNPs enhances boiling heat flux up to 219% as compared to uncoated case.•A profound heat transfer ...coefficient enhancement of 230% is obtained.•A drastic temperature drop of 36.1 °C on the heated surface is achieved by using GNPs.•Hydrophobic GNPs manifests as a natural biphilic coating favorable to nucleate boiling.
Miniaturized electronics components require effective heat removal to ensure their reliability and long-lifespan. Here, the importance of surface wettability of graphene nanoplatelets (GNPs) coatings in the thermal performance enhancement of subcooled pool boiling is elucidated. The boiling enhancement of superhydrophilic, hydrophobic and superhydrophobic GNPs coatings, is compared and benchmarked with that of the uncoated copper surface. The hydrophobic GNPs case manifests extraordinarily enhanced boiling performance. When compared with the uncoated copper surface, the boiling heat flux and the heat transfer coefficient are enhanced up to 219%, and 230%, respectively. A significant temperature decrease up to 36.1 °C and an effectiveness of 3.19 as compared to the uncoated case are obtained. Besides the nanoporosity of GNPs coatings which is favourable to bubble nucleation, the unique nanostructure of the hydrophobic GNPs, which possesses a combination of hydrophilic and hydrophobic characteristics provide synergetic effect on the vapor bubble growth and departure rates. The hydrophobic GNPs is a single coating endowed with biphilicity naturally. This study incorporates facilely fabricated GNPs coatings while showcasing the importance of the wettability of GNPs in nucleate pool boiling. We envisage that the intrinsically biphilic GNPs coating has an immense potential in the highly demanding thermal management of electronics devices.
Water vapor condensation is a ubiquitous process in nature and industry. Over the past century, methods achieving dropwise condensation using a thin (<1 µm) hydrophobic “promoter” layer have been ...developed, which increases the condensation heat transfer by ten times compared to filmwise condensation. Unfortunately, implementations of dropwise condensation have been limited due to poor durability of the promoter coatings. Here, thin‐film condensation which utilizes a promoter layer not as a condensation surface, but rather to confine the condensate within a porous biphilic nanostructure, nickel inverse opals (NIO) with a thin (<20 nm) hydrophobic top layer of decomposed polyimide is developed. Filmwise condensation confined to thicknesses <10 µm is demonstrated. To test the stability of thin‐film condensation, condensation experiments are performed to show that at higher supersaturations droplets coalescing on top of the hydrophobic layer are absorbed into the superhydrophilic layer through coalescence‐induced transitions. Through detailed thermal‐hydrodynamic modeling, it is shown that thin‐film condensation has the potential to achieve heat transfer coefficients approaching ≈100 kW m−2 while avoiding durability issues by significantly reducing nucleation on the hydrophobic surface. The work presented here develops an approach to potentially ensure durable and high‐performance condensation comparable to dropwise condensation.
Thin‐film condensation on hydrophobic‐coated nickel inverse opal structures enables heat transfer performance approaching that of dropwise condensation while achieving higher robustness by confining the condensate film and reducing nucleation on the hydrophobic layer.
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.
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 as an innovative thermal management solution. These “biphilic” surfaces exhibit intentionally varied local superhydrophobicity and superhydrophilicity and exceptional pool boiling performance with heat transfer coefficients up to 299 kW m‐2 K‐1, a 434% enhancement over untreated surfaces.
Benefitting from the coalescence-induced droplet jumping on superhydrophobic surfaces, the condensing droplets on heat exchangers can be removed efficiently, significantly improving the condensation ...heat-transfer performance of various thermal applications. However, the enhancement of droplet jumping height and self-removal to further improve the condensation heat-transfer performance of the thermal applications remains a challenge due to considerable interfacial adhesion caused by the inevitable partial-Wenzel state condensing droplets on superhydrophobic surfaces. In this study, a biphilic nanostructure is developed to effectively improve the droplet jumping height by decreasing the interfacial adhesion with the formation of Cassie-like droplets. Under atmospheric conditions, ∼28% improvement of droplet jumping height is achieved on a biphilic surface compared to that of a superhydrophobic surface. Additionally, the droplet contact electrification on biphilic surfaces discovered in this work allows the droplets to jump ∼137% higher compared with that under atmospheric conditions. Furthermore, the droplet jumping and electrification mechanisms on the biphilic surface are revealed by building a theoretical model that can predict the experimental results well. Apart from being a milestone for the droplet jumping physics development on biphilic nanostructures, this work also provides new insights into the micro-droplet discipline.
•Explore the boiling mechanisms while using biphilic surfaces.•Alter biphilic geometrical parameters to enhance heat transfer capabilities.•Use non-intrusive synchronized high-speed imaging and ...thermography.•Smaller superhydrophobic regions provide a higher evaporated mass flux ratio.•Ideal distance between regions is the one that allows slight bubble coalescence.
Recognizing the relevance of wettability in pool boiling heat transfer, few authors have reported significantly enhanced heat transfer coefficients using the so-called biphilic surfaces, i.e. hydrophilic surfaces with hydrophobic regions. However, the development of these patterns is still scarcely reported in the literature and many studies rely on a trial and error approach. In this context, the present work addresses a systematic analysis of the effect of the geometry of biphilic patterns on bubble dynamics and consequently on the heat transfer processes occurring in pool boiling. Geometric representative quantities such as the size of the superhydrophobic regions and their relative position are systematically varied and their effect is analyzed in detail in both bubble dynamics and heat transfer processes, using synchronized high-speed video and time-resolved thermography. The results show that the size of the superhydrophobic regions affects bubble dynamics and the rate of evaporated mass, thus influencing the heat flux term associated to the latent heat of evaporation. In this context, patterns with smaller superhydrophobic areas are the most effective at removing heat through evaporation. Regarding the distance between the superhydrophobic areas, the results support the use of the minimum distance between superhydrophobic areas, that is, in the limit to promote coalescence. For this distance there is still no significant interaction between the bubbles sites, but the controlled coalescence promotes the occurrence of a periodic induced flow between the superhydrophobic regions, which contributes to cool the surface. Also, heat flux calculations confirm that heat flux peaks coincide with bubble departure occurrences in the superhydrophobic areas, thus confirming the heat transfer enhancement promoted by this kind of surfaces.
•Synchronized high-speed video and thermography for pool boiling characterization.•Biphilic surfaces enhance pool boiling heat transfer.•Smaller superhydrophobic regions lead to higher evaporation ...mass transfer rates.
This study concerns the detailed description of the fluid dynamics and heat transfer mechanisms occurring during single bubble nucleation on biphilic surfaces (superhydrophilic/hydrophilic surfaces surrounding hydrophobic/superhydrophobic isles). A high-speed video camera is synchronized with a high-speed thermographic camera to relate the temporal evolution of bubble dynamics, from generation to detachment. The results allowed identifying several stages of bubble growth in all the biphilic surfaces tested, regardless of the size of the superhydrophilic regions. Bubble dynamics is affected by the size of the superhydrophobic regions. Since bubbles are constrained to the superhydrophobic region boundaries, which alter their base diameter, smaller superhydrophobic regions tend to promote a regular and stable bubble generation, due to the action of stronger surface tension forces acting on the boundary region. As the base diameter increases, the surface tension effects, which only act at the boundary with the hydrophilic region, are lessened and only affect the bubble at larger volumes. Consequently, smaller superhydrophobic regions are associated to higher evaporation mass transfer rates. Temperature gradients are larger at the hydrophilic/superhydrophobic boundaries and despite being small amplitude differences, they clearly promote induced convection of the cold liquid between superhydrophobic regions, as observed in the high-speed thermographic images.
Абстракт: To study the process of boiling on a solid heater surface, a hybrid model based on lattice Boltzmann method and heat transfer equation is presented. The process of formation and rise of a ...single bubble during boiling over a single lyophobic zone located on a smooth lyophilic surface was studied. Dependences of the bubble departure frequency and bubble departure diameter on the width of the lyophobic zone and the wall superheat were obtained. It is shown that the bubble departure diameter increases with the width of the lyophobic zone, and the frequency of bubble departure increases with the wall superheat. Based on the obtained data, the optimal size of the lyophobic zone on the lyophilic surface was determined from the point of view of heat transfer enhancement.
Flow boiling is one of the most effective mechanisms in heat transfer thanks to the latent heat of vaporization. Surface modifications such as mixed-wettability have a considerable effect on the ...boiling heat transfer performance in terms of enhancement in boiling heat transfer as well as critical heat flux. This study introduces a new method of fabrication of biphilic surfaces, where C
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(Octafluorocyclobutane) islands are surrounded by silicon. Two different biphilic surfaces were fabricated and compared with the entirely uniform hydrophobic surface taken as a reference,. Each of the biphilic surfaces has three different sections, namely inlet, middle and outlet regions. The first region is mainly hydrophobic (inlet), while the third region is mainly hydrophilic (outlet). The heat transfer coefficients were obtained at different heat fluxes. Compared to the entirely uniform hydrophobic surface, the results show that biphilic surfaces enhance the boiling heat transfer performance by up to 50%. The visualization results revealed that the biphilic surfaces lead to more nucleation sites in the bubbly flow regime and break up the elongated bubbles in the slug flow regime.
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