Elucidating the Mechanism of Condensation-Mediated Degradation of Organofunctional Silane Self-Assembled Monolayer Coatings

Abstract

Dropwise condensation is favorable for numerous industrial and heat/mass transfer applications due to the enhanced heat transfer performance that results from efficient condensate removal. Organofunctional silane self-assembled monolayer (SAM) coatings are one of the most common ultrathin low surface energy materials used to promote dropwise condensation of water vapors because of their minimal thermal resistance and scalable synthesis process. These SAM coatings typically degrade (i.e., condensation transitions from the efficient dropwise mode to the inefficient filmwise mode) rapidly during water vapor condensation. More importantly, the condensation-mediated coating degradation/failure mechanism(s) remain unknown and/or unproven. In this work, we develop a mechanistic understanding of water vapor condensation-mediated organofunctional silane SAM coating degradation and validate our hypothesis through controlled coating synthesis procedures on silicon/silicon dioxide substrates. We further demonstrate that a pristine organofunctional silane SAM coating resulting from a water/moisture-free coating environment exhibits superior long-term robustness during water vapor condensation. Our molecular/nanoscale surface characterizations, pre- and post-condensation heat transfer testing, indicate that the presence of moisture in the coating environment leads to uncoated regions of the substrate that act as nucleation sites for coating degradation. By elucidating the reasons for formation of these degradation nuclei and demonstrating a method to suppress such defects, this study provides new insight into why low surface energy silane SAM coatings degrade during water vapor condensation. The proposed approach addresses a key bottleneck (i.e., coating failure) preventing the adoption of efficient dropwise condensation methods in industry, and it will facilitate enhanced phase-change heat transfer technologies in industrial applications.

A Lipid-Inspired Highly Adhesive Interface for Durable Superhydrophobicity in Wet Environments and Stable Jumping Droplet Condensation

“…Although past work has conclusively demonstrated that silane monolayers are poor hydrophobic promoters for dropwise condensation due to the lack of durability, , they act as functional adhesion promoters. When a silane is directly exposed to water, it becomes chemically unstable and desorbs due to the hydrolysis reaction between its silanol group and water. ,− Past studies have experimentally demonstrated that a direct deposition of silane monolayers on metals results in rapid (within hours) degradation during steam condensation. , In our structure, having an additional polymer layer on top of the silane layer prevents direct contact between the water and silane, thus protecting the silane layer from reaction and desorption. Hence, we do not expect delamination to occur at the self-assembled monolayer (SAM)–substrate interfaces.…”

“…15,39−41 Past studies have experimentally demonstrated that a direct deposition of silane monolayers on metals results in rapid (within hours) degradation during steam condensation. 15,41 In our structure, having an additional polymer layer on top of the silane layer prevents direct contact between the water and silane, thus protecting the silane layer from reaction and desorption. Hence, we do not expect delamination to occur at the self-assembled monolayer (SAM)−substrate interfaces.…”

Stability of Octadecyltrimethoxysilane-Based Coatings on Aluminum Alloy Surface

“…Hydrophobic and superhydrophobic surfaces have been a subject of active research for several decades as they present interest for both fundamental science and industry [ 1 , 2 , 3 , 4 , 5 , 6 , 7 , 8 , 9 , 10 , 11 , 12 , 13 , 14 , 15 , 16 , 17 ]. The water-repellent properties of materials are known to be either provided by their intrinsically low surface energy or by depositing a thin top-layer with low energy.…”

“…Self-assembled ultrathin monolayers of organosilanes, which are one of the most commonly used low-surface energy coatings (often referred to as ‘hydrophobizers’), have been applied on metal, oxide, ceramic and Si substrates [ 1 , 8 , 14 , 16 , 17 , 18 , 19 , 20 , 21 , 22 , 23 , 24 , 25 , 26 , 27 , 28 , 29 , 30 , 31 , 32 ]. For several decades, various organosilanes, including fluorinated alkylsilanes (FASs), have been used as hydrophobizing agents deposited either via immersion in liquid bath [ 1 , 8 , 14 , 21 , 26 , 27 , 32 , 33 , 34 , 35 ] or from gas phase [ 13 , 27 ].…”

“…Self-assembled ultrathin monolayers of organosilanes, which are one of the most commonly used low-surface energy coatings (often referred to as ‘hydrophobizers’), have been applied on metal, oxide, ceramic and Si substrates [ 1 , 8 , 14 , 16 , 17 , 18 , 19 , 20 , 21 , 22 , 23 , 24 , 25 , 26 , 27 , 28 , 29 , 30 , 31 , 32 ]. For several decades, various organosilanes, including fluorinated alkylsilanes (FASs), have been used as hydrophobizing agents deposited either via immersion in liquid bath [ 1 , 8 , 14 , 21 , 26 , 27 , 32 , 33 , 34 , 35 ] or from gas phase [ 13 , 27 ]. Apart from water repellency, which is normally expected in case of alkylsilanes, organosilane layers were also reported as part of anti-ice (or ice-phobic) surfaces [ 4 , 8 , 14 , 16 , 17 , 36 , 37 ], as well as part of anticorrosive coatings [ 21 , 22 , 23 , 27 , 32 , 33 , 38 , 39 , 40 ].…”