Relationships between surface chemistry, nanotopography, wettability and ice adhesion in epoxy and SU-8 modified with fluoroalkylsilanes from the vapor phase

Abstract

The objective of this work was to elucidate how ice adhesion to solid substrates is related to surface wettability, which, in turn, results from surface chemical composition and nanotopography. The study was carried out using microscopically smooth surfaces of epoxy and SU-8, a negative epoxy-based photoresist. Epoxy resins are known for their outstanding physicochemical properties. They are often used as surface coatings for outdoor devices and can face harsh environmental conditions, such as icing. SU-8 is extensively used in microfabrication of surface topographical structures. Some of these structures are designed to be superhydrophobic and, in some cases, icephobic. In the present study, we focused on the role of surface physicochemical modifications of these materials in controlling their wetting and icing properties. A homologous series of fluoroalkyl silanes (FAS), which contained 3, 8, 10, and 12 carbon atoms in a molecule, was used to make epoxy and SU-8 surfaces hydrophobic. The FAS deposition process was carried out from vapor phase at atmospheric pressure (AP-VPD). The modified substrates exhibited a significantly increased surface nanoroughness, as a result of etching in oxygen radio frequency (RF) plasma and subsequent silanization. The coatings obtained by the AP-VPD contribute to surface roughness because the FAS vapors and atmospheric water are abundant during deposition processes. Such conditions favor vertical polymerization and formation of clustered deposits. The resulting nanoroughness was shown to affect wettability of modified surfaces. The influence of FAS backbone chain length on resulting surface nanotopography and how it affects surface wettability was investigated. Finally, the relationship between wettability and ice adhesion was examined. The strength of adhesion between frozen sessile droplets and modified substrates was investigated in tensile mode. A linear correlation between the ice-substrate adhesion strength and the macroscopic work of water-substrate adhesion was found.