Sprayable Thin and Robust Carbon Nanofiber Composite Coating for Extreme Jumping Dropwise Condensation Performance

Abstract

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

Exploiting radiative cooling for uninterrupted 24-hour water harvesting from the atmosphere

“…Therefore, it is important to reduce the critical size of water droplets falling from the dewing surface via surface engineering, because water droplets attached to the surface hinder an efficient heat transfer and suppress dew formation. A superhydrophobic condenser surface design, such that a gravity-independent jumping mechanism of the droplets emerges ( 48 , 64 ) and dew is passively removed, allows to operate fully autonomously and passively. When small droplets (~10 to 100 μm) coalesce on these nanostructured superhydrophobic surfaces, the reduction in surface energy leads to a coalescence-induced jumping of the droplets without any additional energy requirement ( 48 ).…”

“…The condenser surface with favorable wettability for droplet detachment is a thin, hierarchically micro-/nanostructured superhydrophobic coating that can be spray-coated onto the bottom side of our selective emitter ( 64 ), as illustrated in Fig. 4A .…”

“…For reasons of heat transfer efficiency and ease of application, we opted to use a state-of-the-art thermally conductive, superhydrophobic carbon nanofiber (CNF), polymer composite ( 64 ). We used a dispersion of CNF and polytetrafluoroethylene (PTFE), featuring minimal thermal resistance and outstanding heat transfer performance compared to uncoated control metallic surfaces ( 64 ). The thermal conductivity of the CNF coating is estimated to be 0.30 to 5.37 W m −1 K −1 ( 64 ).…”

“…We used a dispersion of CNF and polytetrafluoroethylene (PTFE), featuring minimal thermal resistance and outstanding heat transfer performance compared to uncoated control metallic surfaces ( 64 ). The thermal conductivity of the CNF coating is estimated to be 0.30 to 5.37 W m −1 K −1 ( 64 ). As this sprayable coating is fabricated to be very thin (coating thickness is ~2 μm; see fig.…”

“…There is a range of superhydrophobic coatings promoting dropwise condensation (49)(50)(51)(52). For reasons of heat transfer efficiency and ease of application, we opted to use a state-of-the-art thermally conductive, superhydrophobic carbon nanofiber (CNF), polymer composite (64). We used a dispersion of CNF and polytetrafluoroethylene (PTFE), featuring minimal thermal resistance and outstanding heat transfer performance compared to uncoated control metallic surfaces (64).…”