Polymer Infused Porous Surfaces for Robust, Thermally Conductive, Self-Healing Coatings for Dropwise Condensation

Wicking assisted condenser platform with patterned wettability for space application

Vapor condensation is extensively used in applications that demand the exchange of a substantial amount of heat energy or the vapor-liquid phase conversion. In conventional condensers, the condensate removal from a subcooled surface is caused by gravity force. This restricts the use of such condensers in space applications or horizontal orientations. The current study demonstrates proof-of-concept of a novel plate-type condenser platform for passively removing condensate from a horizontally oriented surface to the surrounding wicking reservoir without gravity. The condensing surface is engineered with patterned wettabilities, which enables the continuous migration of condensate from the inner region of the condenser surface to the side edges via surface energy gradient. The surrounding wicking reservoir facilitates the continuous absorption of condensate from the side edges. The condensation dynamics on different substrates with patterned wettabilities are investigated, and their condensation heat transfer performance is compared. The continuous migration of condensate drops from a superhydrophobic to a superhydrophilic area can rejuvenate the nucleation sites in the superhydrophobic area, resulting in increased heat transport. The proposed condenser design with engineered wettability can be used for temperature and humidity management applications in space.

Ultra-resilient multi-layer fluorinated diamond like carbon hydrophobic surfaces

Seventy percent of global electricity is generated by steam-cycle power plants. A hydrophobic condenser surface within these plants could boost overall cycle efficiency by 2%. In 2022, this enhancement equates to an additional electrical power generation of 1000 TWh annually, or 83% of the global solar electricity production. Furthermore, this efficiency increase reduces CO2 emissions by 460 million tons /year with a decreased use of 2 trillion gallons of cooling water per year. However, the main challenge with hydrophobic surfaces is their poor durability. Here, we show that solid microscale-thick fluorinated diamond-like carbon (F-DLC) possesses mechanical and thermal properties that ensure durability in moist, abrasive, and thermally harsh conditions. The F-DLC coating achieves this without relying on atmospheric interactions, infused lubricants, self-healing strategies, or sacrificial surface designs. Through tailored substrate adhesion and multilayer deposition, we develop a pinhole-free F-DLC coating with low surface energy and comparable Young's modulus to metals. In a three-year steam condensation experiment, the F-DLC coating maintains hydrophobicity, resulting in sustained and improved dropwise condensation on multiple metallic substrates. Our findings provide a promising solution to hydrophobic material fragility and can enhance the sustainability of renewable and non-renewable energy sources.

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

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

Quasi-Liquid Surfaces for Sustainable High-Performance Steam Condensation

Sustainable high-performance steam condensation is critical to reducing the size, weight, and cost of water and energy systems. It is well-known that dropwise condensation can provide a significantly higher heat-transfer coefficient than filmwise condensation. Tremendous efforts have been spent to promote dropwise condensation by achieving a nonwetting state on superhydrophobic surfaces and a slippery state on liquid-infused surfaces, but these surfaces suffer from severe durability challenges. Here, we report sustainable high-performance dropwise condensation of steam on newly developed durable quasi-liquid surfaces, which are easily made by chemically bonding quasi-liquid polymer molecules on solid substrates. As a result, the solid/water interface is changed to a quasi-liquid/water interface with minimal adhesion and extraordinary durability. The quasi-liquid surface with ultralow contact angle hysteresis down to 1° showed a heat-transfer coefficient up to 70 and 380% higher than those on conventional hydrophobic and hydrophilic surfaces, respectively. Furthermore, we demonstrated that the quasi-liquid coating exhibited a sustainable heat-transfer coefficient of 71 kW/(m² K) at a heat flux of 420 kW/m² under a prolonged period of 39 h in continuous steam condensation. Such a quasi-liquid surface has the potential to sustain high-performance dropwise condensation of steam and address the long-standing durability challenge in the field.

A Lipid-Inspired Highly Adhesive Interface for Durable Superhydrophobicity in Wet Environments and Stable Jumping Droplet Condensation

Creating thin (<100 nm) hydrophobic coatings that are durable in wet conditions remains challenging. Although the dropwise condensation of steam on thin hydrophobic coatings can enhance condensation heat transfer by 1000%, these coatings easily delaminate. Designing interfaces with high adhesion while maintaining a nanoscale coating thickness is key to overcoming this challenge. In nature, cell membranes face this same challenge where nanometer-thick lipid bilayers achieve high adhesion in wet environments to maintain integrity. Nature ensures this adhesion by forming a lipid interface having two nonpolar surfaces, demonstrating high physicochemical resistance to biofluids attempting to open the interface. Here, developing an artificial lipid-like interface that utilizes fluorine-carbon molecular chains can achieve durable nanometric hydrophobic coatings. The application of our approach to create a superhydrophobic material shows high stability during jumping-droplet-enhanced condensation as quantified from a continual one-year steam condensation experiment. The jumping-droplet condensation enhanced condensation heat transfer coefficient up to 400% on tube samples when compared to filmwise condensation on bare copper. Our bioinspired materials design principle can be followed to develop many durable hydrophobic surfaces using alternate substrate-coating pairs, providing stable hydrophobicity or superhydrophobicity to a plethora of applications.

Enhancement of Boiling with Scalable Sandblasted Surfaces

Surface engineering has been leveraged by researchers to enhance boiling heat transfer performance, with benefits ranging from improved thermal management to more efficient power generation. While engineered surfaces fabricated using cleanroom processes have shown promising boiling results, scalable methods for surface engineering are still limited despite most real-world industry-scale applications involving large boiling areas. In this work, we investigate the use of sandblasting as a scalable surface engineering technique for the enhancement of pool boiling heat transfer. We vary the size of an abrasive Al₂O₃ sandblasting medium (25, 50, 100, and 150 μm) and quantify its effects on silicon surface conditions and boiling characteristics. The surface morphology and capillary wicking performance are characterized by optical profilometry and capillary rise tests, respectively. Pool boiling results and surface characterization reveal that surface roughness and volumetric wicking rate increase with the abrasive size, which results in improvements in the critical heat flux and the heat transfer coefficient of up to 192.6 and 434.3% compared to a smooth silicon surface, respectively. The significant enhancement achieved with sandblasted surfaces indicates that sandblasting is a promising option for improving boiling performance in industry-scale applications.

Robust and durable liquid-repellent surfaces

This review provides a comprehensive summary of characterization, design, fabrication, and application of robust and durable liquid-repellent surfaces.

Ultra-thin self-healing vitrimer coatings for durable hydrophobicity

Durable hydrophobic materials have attracted considerable interest in the last century. Currently, the most popular strategy to achieve hydrophobic coating durability is through the combination of a perfluoro-compound with a mechanically robust matrix to form a composite for coating protection. The matrix structure is typically large (thicker than 10 μm), difficult to scale to arbitrary materials, and incompatible with applications requiring nanoscale thickness such as heat transfer, water harvesting, and desalination. Here, we demonstrate durable hydrophobicity and superhydrophobicity with nanoscale-thick, perfluorinated compound-free polydimethylsiloxane vitrimers that are self-healing due to the exchange of network strands. The polydimethylsiloxane vitrimer thin film maintains excellent hydrophobicity and optical transparency after scratching, cutting, and indenting. We show that the polydimethylsiloxane vitrimer thin film can be deposited through scalable dip-coating on a variety of substrates. In contrast to previous work achieving thick durable hydrophobic coatings by passively stacking protective structures, this work presents a pathway to achieving ultra-thin (thinner than 100 nm) durable hydrophobic films.

Dropwise condensation: From fundamentals of wetting, nucleation, and droplet mobility to performance improvement by advanced functional surfaces

As a ubiquitous vapor-liquid phase-change process, dropwise condensation has attracted tremendous research attention owing to its remarkable efficiency of energy transfer and transformative industrial potential. In recent years, advanced functional surfaces, profiting from great progress in modifying micro/nanoscale features and surface chemistry on surfaces, have led to exciting advances in both heat transfer enhancement and fundamental understanding of dropwise condensation. In this review, we discuss the development of some key components for achieving performance improvement of dropwise condensation, including surface wettability, nucleation, droplet mobility, and growth, and discuss how they can be elaborately controlled as desired using surface design. We also present an overview of dropwise condensation heat transfer enhancement on advanced functional surfaces along with the underlying mechanisms, such as jumping condensation on nanostructured superhydrophobic surfaces, and new condensation characteristics (e.g., Laplace pressure-driven droplet motion, hierarchical condensation, and sucking flow condensation) on hierarchically structured surfaces. Finally, the durability, cost, and scalability of specific functional surfaces are focused on for future industrial applications. The existing challenges, alternative strategies, as well as future perspectives, are essential in the fundamental and applied aspects for the practical implementation of dropwise condensation.

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

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

Enhanced condensation heat transfer using porous silica inverse opal coatings on copper tubes

Phase-change condensation is commonplace in nature and industry. Since the 1930s, it is well understood that vapor condenses in filmwise mode on clean metallic surfaces whereas it condenses by forming discrete droplets on surfaces coated with a promoter material. In both filmwise and dropwise modes, the condensate is removed when gravity overcomes pinning forces. In this work, we show rapid condensate transport through cracks that formed due to material shrinkage when a copper tube is coated with silica inverse opal structures. Importantly, the high hydraulic conductivity of the cracks promote axial condensate transport that is beneficial for condensation heat transfer. In our experiments, the cracks improved the heat transfer coefficient from ≈ 12 kW/m² K for laminar filmwise condensation on smooth clean copper tubes to ≈ 80 kW/m² K for inverse opal coated copper tubes; nearly a sevenfold increase from filmwise condensation and identical enhancement with state-of-the-art dropwise condensation. Furthermore, our results show that impregnating the porous structure with oil further improves the heat transfer coefficient by an additional 30% to ≈ 103 kW/m² K. Importantly, compared to the fast-degrading dropwise condensation, the inverse opal coated copper tubes maintained high heat transfer rates when the experiments were repeated > 20 times; each experiment lasting 3–4 h. In addition to the new coating approach, the insights gained from this work present a strategy to minimize oil depletion during condensation from lubricated surfaces.

Microtube Surfaces for the Simultaneous Enhancement of Efficiency and Critical Heat Flux during Pool Boiling

Boiling is an essential process in numerous applications including power plants, thermal management, water purification, and steam generation. Previous studies have shown that surfaces with microcavities or biphilic wettability can enhance the efficiency of boiling heat transfer, that is, the heat transfer coefficient (HTC). Surfaces with permeable structures such as micropillar arrays, in contrast, have shown significant enhancement of the critical heat flux (CHF). In this work, we investigated microtube structures, where a cavity is defined at the center of a pillar, as structural building blocks to enhance HTC and CHF simultaneously in a controllable manner. We demonstrated simultaneous CHF and HTC enhancements of up to 62 and 244%, respectively, compared to those of a smooth surface. The experimental data along with high-speed images elucidate the mechanism for simultaneous enhancement where bubble nucleation occurs in the microtube cavities for increased HTC and microlayer evaporation occurs around microtube sidewalls for increased CHF. Furthermore, we combined micropillars and microtubes to create surfaces that further increased CHF by achieving a path to separate nucleating bubbles and rewetting liquids. This work provides guidelines for the systematic surface design for boiling heat transfer enhancement and has important implications for understanding boiling heat transfer mechanisms.