Ultrathin Durable Organic Hydrophobic Coatings Enhancing Dropwise Condensation Heat Transfer

Abstract

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.

Figure 1

(a) Schematic of the iCVD tool used to deposit the PFDA polymer coating on copper substrates. (b) Advancing contact angle and contact angle hysteresis (inset) of water droplets on hydrophobic copper substrates.

Figure 2

Water condensation on test substrates observed using ESEM: (a) silane-coated copper, (b) thiol-coated copper, and (c) PFDA-coated copper using iCVD (scale bar, 100 μm; all images have the same magnification).

Figure 3

Heat flux (a), (c) and heat-transfer coefficient (HTC) (b), (d) for all of the substrates vs. subcooling at steam velocities of 3 m s^–1 (a and b) and 9 m s^–1 (c and d) and steam saturation pressure of 1.42 bar.

Figure 4

Temporal evolution of condensation on (a) thiolated and (b) PFDA-coated copper surfaces. The regions with the orange dashed line show the localized FWC on the PFDA-coated surface. (c) Heat-transfer coefficient (HTC) plotted vs. time during the durability experiment to observe the trend. Whenever we shut down the steam flow at the end of the experiment, there is an obvious decrease in HTC between shutdown and restart.