Atmospheric Pressure Plasma Surface Processing of Metals for Water Condensation

Water vapor condensation on solid surfaces is a well-known phase-change phenomenon essential in a wide range of applications such as in thermal power plants, chemical industries, water harvesting, refrigeration, electronics cooling, and seawater desalination [1]. In the last decades, several processes have been optimized to modify metal surfaces in terms of chemical composition, morphology and wettability, in order to modulate their water condensation performances [2].<br/>In this contribution we propose the use of low-temperature atmospheric pressure plasma (APP) for fine tuning the surface properties of aluminum (Al) samples with the aim of developing novel materials for enhanced water condensation. In particular, various APP deposition processes, including plasma-enhanced chemical vapor deposition (PECVD) and aerosol-assisted plasma deposition (AAPD), were used to coat Al samples with both smooth hydrophobic polymer thin films (i.e., hydrocarbon and fluorocarbon films) and superhydrophobic organic-inorganic nanocomposite coatings [3,4]. The chemical composition, morphology and wettability of the different samples was studied by various techniques, such as Fourier-transform infrared spectroscopy, X-ray photoelectron spectroscopy, scanning electron microscopy, white-light vertical scanning interferometry and water contact angle goniometry. The condensation mode, the droplet growth kinetics and the condensate removal mechanism were investigated performing experiments in a purposely designed condensation chamber under controlled conditions (e.g., relative air humidity and subcooling temperature).<br/>Preliminary results revealed that water vapor condensation on hydrophilic pristine samples occurs via filmwise mechanism. In contrast, on hydrophobic plasma-coated samples a stable dropwise condensation mode is obtained. On the other hand, flooding condensation is observed on the superhydrophobic samples coated with nanocomposite films. Here, large pinned droplets form due to condensation within the film micro/nanostructures, leading to surface flooding. Interestingly, by depositing on the nanocomposite coating a thin fluorocarbon layer, coalescence-induced jumping-droplet condensation is achieved. In this case small droplets merge and undergo a self-propelled out-of-plane jumping induced by the surface energy release upon droplet coalescence. Overall, these findings provide novel insights into the use of plasma-assisted deposition processes for the tuning of the water condensation performances of metal surfaces towards industrially relevant applications.<br/><br/>Acknowledgements:<br/>This research was supported by the Italian Ministry for University and Research (MUR) under grant ARS01_00849.<br/><br/>References:<br/>[1] B. El Fil et al., International Journal of Heat and Mass Transfer 160, 120172 (2020)<br/>[2] A. Goswami et al., Surface and Interfaces 25, 101143 (2021)<br/>[3] A. Uricchio et al., Applied Surface Science 561, 150014 (2021)<br/>[4] A. Uricchio, F. Fanelli, Processes 9, 2069 (2021)