Condensation of water on metallic surfaces is critical for multiple energy conversion processes. Enhancement in condensation heat transfer efficiency often requires surface texturing and hydrophobicity, usually achieved through coatings, to maintain dropwise condensation. However, such surface treatments face conflicting challenges of minimal coating thermal resistance, enhanced coating durability and scalable fabrication. Here we present a thin (~ 2 μm) polytetrafluoroethylene -carbon nanofiber nanocomposite coating which meets these challenges and sustains coalescence-induced jumping droplet condensation for extended periods under highly demanding condensation conditions. Coating durability is achieved through improved substrate adhesion by depositing a sub-micron thick aluminum primer layer. Carbon nanofibers in a polytetrafluoroethylene matrix increase coating thermal conductivity and promote spontaneous surface nano-texturing to achieve superhydrophobicity for condensate microdroplets. The coating material can be deposited through direct spraying, ensuring economical scalability and versatility for a wide range of substrates. We know of no other coating for metallic surfaces that is able to sustain jumping dropwise condensation under shear of steam at 111 ℃ flowing at ~ 3 m s -1 over the surface for 10 hours and dropwise
Liquid transport (continuous or segmented) in microfluidic platforms typically requires pumping devices or external fields working collaboratively with special fluid properties to enable fluid motion. Natural liquid adhesion on surfaces deters motion and promotes the possibility of liquid or surface contamination. Despite progress, significant advancements are needed before devices for passive liquid propulsion, without the input of external energy and unwanted contamination, become a reality in applications. Here we present an unexplored and facile approach based on the Laplace pressure imbalance, manifesting itself through targeted track texturing, driving passively droplet motion, while maintaining the limited contact of the Cassie–Baxter state on superhydrophobic surfaces. The track topography resembles out-of-plane, backgammon-board, slowly converging microridges decorated with nanotexturing. This design naturally deforms asymmetrically the menisci formed at the bottom of a droplet contacting such tracks and causes a Laplace pressure imbalance that drives droplet motion. We investigate this effect over a range of opening track angles and develop a model to explain and quantify the underlying mechanism of droplet self-propulsion. We further implement the developed topography for applications relevant to microfluidic platform functionalities. We demonstrate control of the rebound angle of vertically impacting droplets, achieve horizontal self-transport to distances up to 65 times the droplet diameter, show significant uphill motion against gravity, and illustrate a self-driven droplet-merging process.
Bacterial colonization poses significant health risks, such as infestation of surfaces in biomedical applications and clean water unavailability. If maintaining the surrounding water clean is a target, developing surfaces with strong bactericidal action, which is facilitated by bacterial access to the surface and mixing, can be a solution. On the other hand, if sustenance of a surface free of bacteria is the goal, developing surfaces with ultralow bacterial adhesion often suffices. Here we report a facile, scalable, and environmentally benign strategy that delivers customized surfaces for these challenges. For bactericidal action, nanostructures of inherently antibacterial ZnO, through simple immersion of zinc in hot water, are fabricated. The resulting nanostructured surface exhibits extreme bactericidal effectiveness (9250 cells cm –2 h –1 ) that eliminates bacteria in direct contact and also remotely through the action of reactive oxygen species. Remarkably, the remote bactericidal action is achieved without the need for any illumination, otherwise required in conventional approaches. As a result, ZnO nanostructures yield outstanding water disinfection of >99.98%, in the dark, by inactivating the bacteria within 3 h. Moreover, Zn 2+ released to the aqueous medium from the nanostructured ZnO surface have a concentration of 0.73 ± 0.15 ppm, markedly below the legal limit for safe drinking water (5–6 ppm). The same nanostructures, when hydrophobized (through a water-based or fluorine-free spray process), exhibit strong bacterial repulsion, thus substantially reducing bacterial adhesion. Such environmentally benign and scalable methods showcase pathways toward inhibiting surface bacterial colonization.
Lubricant-infused surfaces (LIS) are highly efficient in repelling water and constitute a very promising family of materials for condensation processes occurring in a broad range of energy applications. However, the performance of LIS in such processes is limited by the inherent thermal resistance imposed by the thickness of the lubricant and supporting surface structure, as well as by the gradual depletion of the lubricant over time. Here, we present an ultrathin (∼70 nm) and conductive LIS architecture, obtained by infusing lubricant into a vertically grown graphene nanoscaffold on copper. The ultrathin nature of the scaffold, combined with the high in-plane thermal conductivity of graphene, drastically minimize earlier limitations, effectively doubling the heat transfer performance compared to a state-of-the-art CuO LIS surface. We show that the effect of the thermal resistance to the heat transfer performance of a LIS surface, although often overlooked, can be so detrimental that a simple nanostructured CuO surface can outperform a CuO LIS surface, despite filmwise condensation on the former. The present vertical graphene LIS is also found to be resistant to lubricant depletion, maintaining stable dropwise condensation for at least 24 h with no significant change of advancing contact angle and contact angle hysteresis. The lubricant consumed by the vertical graphene LIS is 52.6% less than that of the existing state-of-the-art CuO LIS, also making the fabrication process more economical.
Organic hydrophobic layers targeting sustained dropwise condensation are highly desirable but suffer from poor chemical and mechanical stability, combined with low thermal conductivity. The requirement of such layers to remain ultrathin to minimize their inherent thermal resistance competes against durability considerations. Here, we investigate the long-term durability and enhanced heat-transfer performance of perfluorodecanethiol (PFDT) coatings compared to alternative organic coatings, namely, perfluorodecyltriethoxysilane (PFDTS) and perfluorodecyl acrylate (PFDA), the latter fabricated with initiated chemical vapor deposition (iCVD), in condensation heat transfer and under the challenging operating conditions of intense flow (up to 9 m s –1 ) of superheated steam (111 °C) at high pressures (1.42 bar). We find that the thiol coating clearly outperforms the silane coating in terms of both heat transfer and durability. In addition, despite being only a monolayer, it clearly also outperforms the iCVD-fabricated PFDA coating in terms of durability. Remarkably, the thiol layer exhibited dropwise condensation for at least 63 h (>2× times more than the PFDA coating, which survived for 30 h), without any visible deterioration, showcasing its hydrolytic stability. The cost of thiol functionalization per area was also the lowest as compared to all of the other surface hydrophobic treatments used in this study, thus making it the most efficient option for practical applications on copper substrates.
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