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.

Particulate-Droplet Coalescence and Self-Transport on Superhydrophobic Surfaces.

Particulate transport from surfaces governs a variety of phenomena including fungal spore dispersal, bioaerosol transmission, and self-cleaning. Here, we report a previously unidentified mechanism governing passive particulate removal from superhydrophobic surfaces, where a particle coalescing with a water droplet (∼10 to ∼100 µm) spontaneously launches. Compared to previously discovered coalescence-induced binary droplet jumping, the reported mechanism represents a more general capillary-inertial dominated transport mode coupled with particle/droplet properties and is typically mediated by rotation in addition to translation. Through wetting and momentum analyses, we show that transport physics depends on particle/droplet density, size, and wettability. The observed mechanism presents a simple and passive pathway to achieve self-cleaning on both artificial as well as biological materials as confirmed here with experiments conducted on butterfly wings, cicada wings, and clover leaves. Our findings provide insights into particle-droplet interaction and spontaneous particulate transport, which may facilitate the development of functional surfaces for medical, optical, thermal, and energy applications.

Ultrascalable Surface Structuring Strategy of Metal Additively Manufactured Materials for Enhanced Condensation.

Metal additive manufacturing (AM) enables unparalleled design freedom for the development of optimized devices in a plethora of applications. The requirement for the use of nonconventional aluminum alloys such as AlSi10Mg has made the rational micro/nanostructuring of metal AM challenging. Here, the techniques are developed and the fundamental mechanisms governing the micro/nanostructuring of AlSi10Mg, the most common metal AM material, are investigated. A surface structuring technique is rationally devised to form previously unexplored two-tier nanoscale architectures that enable remarkably low adhesion, excellent resilience to condensation flooding, and enhanced liquid-vapor phase transition. Using condensation as a demonstration framework, it is shown that the two-tier nanostructures achieve 6× higher heat transfer coefficient when compared to the best filmwise condensation. The study demonstrates that AM-enabled nanostructuring is optimal for confining droplets while reducing adhesion to facilitate droplet detachment. Extensive benchmarking with past reported data shows that the demonstrated heat transfer enhancement has not been achieved previously under high supersaturation conditions using conventional aluminum, further motivating the need for AM nanostructures. Finally, it has been demonstrated that the synergistic combination of wide AM design freedom and optimal AM nanostructuring method can provide an ultracompact condenser having excellent thermal performance and power density.

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.

Life Span of Slippery Lubricant Infused Surfaces.

Since their discovery a decade ago, slippery liquid infused porous surfaces (SLIPSs) or lubricant infused surfaces (LISs) have been demonstrated time and again to have immense potential for a plethora of applications. Of these, one of the most promising is enhancing the energy efficiency of both thermoelectric and organic Rankine cycle power generation via enhanced vapor condensation. However, utilization of SLIPSs in the energy sector remains limited due to the poor understanding of their life span. Here, we use controlled conditions to conduct multimonth steam and ethanol condensation tests on ultrascalable nanostructured copper oxide structured surfaces impregnated with mineral and fluorinated lubricants having differing viscosities (9.7 mPa·s < µ < 5216 mPa·s) and chemical structures. Our study demonstrates that SLIPSs lose their hydrophobicity during steam condensation after 1 month due to condensate cloaking. However, these same SLIPSs maintain nonwetting after 5 months of ethanol condensation due to the absence of cloaking. Surfaces impregnated with higher viscosity oil (5216 mPa·s) increase the life span to more than 8 months of continuous ethanol condensation. Vapor shear tests revealed that SLIPSs do not undergo oil depletion during exposure to 10 m/s gas flows, critical to condenser implementation where single-phase superheated vapor impingement is prevalent. Furthermore, higher viscosity SLIPSs are shown to maintain good stability after exposure to 200 °C air. A subset of the durable SLIPSs did not show change in slipperiness after submerging in stagnant water and ethanol for up to 2 weeks, critical to condenser implementation where single-phase condensate immersion is prevalent. Our work not only demonstrates design methods and longevity statistics for slippery nanoengineered surfaces undergoing long-term dropwise condensation of steam and ethanol but also develops the fundamental design guidelines for creating durable slippery liquid infused 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.

A Deep Learning Perspective on Dropwise Condensation.

Condensation is ubiquitous in nature and industry. Heterogeneous condensation on surfaces is typified by the continuous cycle of droplet nucleation, growth, and departure. Central to the mechanistic understanding of the thermofluidic processes governing condensation is the rapid and high-fidelity extraction of interpretable physical descriptors from the highly transient droplet population. However, extracting quantifiable measures out of dynamic objects with conventional imaging technologies poses a challenge to researchers. Here, an intelligent vision-based framework is demonstrated that unites classical thermofluidic imaging techniques with deep learning to fundamentally address this challenge. The deep learning framework can autonomously harness physical descriptors and quantify thermal performance at extreme spatio-temporal resolutions of 300 nm and 200 ms, respectively. The data-centric analysis conclusively shows that contrary to classical understanding, the overall condensation performance is governed by a key tradeoff between heat transfer rate per individual droplet and droplet population density. The vision-based approach presents a powerful tool for the study of not only phase-change processes but also any nucleation-based process within and beyond the thermal science community through the harnessing of big data.

Fabrication Optimization of Ultra-Scalable Nanostructured Aluminum-Alloy Surfaces.

Aluminum and its alloys are widely used in various industries. Aluminum plays an important role in heat transfer applications, where enhancing the overall system performance through surface nanostructuring is achieved. Combining optimized nanostructures with a conformal hydrophobic coating leads to superhydrophobicity, which enables coalescence induced droplet jumping, enhanced condensation heat transfer, and delayed frosting. Hence, the development of a rapid, energy-efficient, and highly scalable fabrication method for rendering aluminum superhydrophobic is crucial. Here, we employ a simple, ultrascalable fabrication method to create boehmite nanostructures on aluminum. We systematically explore the influence of fabrication conditions such as water immersion time and immersion temperature, on the created nanostructure morphology and resultant nanostructure length scale. We achieved optimized structures and fabrication procedures for best droplet jumping performance as measured by total manufacturing energy utilization, fabrication time, and total cost. The wettability of the nanostructures was studied using the modified Cassie-Baxter model. To better differentiate performance of the fabricated superhydrophobic surfaces, we quantify the role of the nanostructure morphology to corresponding condensation and antifrosting performance through study of droplet jumping behavior and frost propagation dynamics. The effect of aluminum substrate composition (alloy) on wettability, condensation and antifrosting performance was investigated, providing important directions for proper substrate selection. Our findings indicate that the presence of trace alloying elements play a previously unobserved and important role on wettability, condensation, and frosting behavior via the inclusion of defect sites on the surface that are difficult to remove and act as pinning locations to increase liquid-solid adhesion. Our work provides optimization strategies for the fabrication of ultrascalable aluminum and aluminum alloy superhydrophobic surfaces for a variety of applications.

Scalable Corrosion-Resistant Coatings for Thermal Applications.

Corrosion of metallic substrates is a problem for a variety of applications. Corrosion can be mitigated with the use of an electrically insulating coating protecting the substrate. Thick millimetric coatings, such as paints, are generally more corrosion-resistant when compared to nanoscale coatings. However, for thermal systems, thick coatings are undesirable due to the resulting decrease in the overall heat transfer stemming from the added coating thermal resistance. Hence, the development of ultrathin (<10 µm) coatings is of great interest. Ultrathin inorganic silicon dioxide (SiO2) coatings applied by sol-gel chemistries or chemical vapor deposition, as well as organic coatings such as Parylene C, have great anticorrosion performance due to their high dielectric breakdown and low moisture permeability. However, their application to arbitrarily shaped metals is difficult or expensive. Here, we develop a sol-gel solution capable of facile and controllable dip coating on arbitrary metals, resulting in a very smooth (<5 nm roughness), thin (∼3 µm), and conformal coating of dense SiO2. To benchmark our material, we compared the corrosion performance with in-house synthesized superhydrophobic aluminum and copper samples, Parylene C-coated substrates, and smooth hydrophobic surfaces functionalized with a hydrophobic self-assembled monolayer. For comparison with state-of-the-art commercial coatings, copper substrates were coated with an organo-ceramic SiO2 layer created by an elevated temperature and atmospheric pressure metal organic chemical vapor deposition process. To characterize corrosion performance, we electrochemically investigated the corrosion resistance of all samples through potentiodynamic polarization studies and electrochemical impedance spectroscopy. To benchmark the coating durability and to demonstrate scalability, we tested internally coated copper tubes in a custom-built corrosion flow loop to simulate realistic working conditions with shear and particulate saltwater flow. The sol-gel and Parylene C coatings demonstrated a 95% decrease in corrosion rate during electrochemical tests. Copper tube weight loss was reduced by 75% for the sol-gel SiO2-coated tubes when seawater was used as the corrosive fluid in the test loop. This work not only demonstrates scalable coating methodologies for applying ultrathin anticorrosion coatings but also develops mechanistic understanding of corrosion mechanisms on a variety of functional surfaces and substrates.

Laplace Pressure Driven Single-Droplet Jumping on Structured Surfaces.

Droplet transport on, and shedding from, surfaces is ubiquitous in nature and is a key phenomenon governing applications including biofluidics, self-cleaning, anti-icing, water harvesting, and electronics thermal management. Conventional methods to achieve spontaneous droplet shedding enabled by surface-droplet interactions suffer from low droplet transport velocities and energy conversion efficiencies. Here, by spatially confining the growing droplet and enabling relaxation via rationally designed grooves, we achieve single-droplet jumping of micrometer and millimeter droplets with dimensionless jumping velocities v* approaching 0.95, significantly higher than conventional passive approaches such as coalescence-induced droplet jumping (v* ≈ 0.2-0.3). The mechanisms governing single-droplet jumping are elucidated through the study of groove geometry and local pinning, providing guidelines for optimized surface design. We show that rational design of grooves enables flexible control of droplet-jumping velocity, direction, and size via tailoring of local pinning and Laplace pressure differences. We successfully exploit this previously unobserved mechanism as a means for rapid removal of droplets during steam condensation. Our study demonstrates a passive method for fast, efficient, directional, and surface-pinning-tolerant transport and shedding of droplets having micrometer to millimeter length scales.

Stable Dropwise Condensation of Ethanol and Hexane on Rationally Designed Ultrascalable Nanostructured Lubricant-Infused Surfaces.

Vapor condensation is a widely used industrial process for transferring heat and separating fluids. Despite progress in developing low surface energy hydrophobic and micro/nanostructured superhydrophobic coatings to enhance water vapor condensation, demonstration of stable dropwise condensation of low-surface-tension fluids has not been achieved. Here, we develop rationally designed nanoengineered lubricant-infused surfaces (LISs) having ultralow contact angle hysteresis (<3°) for stable dropwise condensation of ethanol (γ ≈ 23 mN/m) and hexane (γ ≈ 19 mN/m). Using a combination of optical imaging and rigorous heat transfer measurements in a controlled environmental chamber free from noncondensable gases (<4 Pa), we characterize the condensation behavior of ethanol and hexane on ultrascalable nanostructured CuO surfaces impregnated with fluorinated lubricants having varying viscosities (0.496 < µ < 5.216 Pa·s) and chemical structures (branched versus linear, Krytox and Fomblin). We demonstrate stable dropwise condensation of ethanol and hexane on LISs impregnated with Krytox 1525, attaining about 200% enhancement in condensation heat transfer coefficient for both fluids compared to filmwise condensation on hydrophobic surfaces. In contrast to previous studies, we use 7 h of steady dropwise condensation experiments to demonstrate the importance of rational lubricant selection to minimize lubricant drainage and maximize LIS durability. This work not only demonstrates an avenue to achieving stable dropwise condensation of ethanol and hexane, it develops the fundamental design principles for creating durable LISs for enhanced condensation heat transfer of low-surface-tension fluids.