Smoothening Perfluoroalkylated Surfaces: Liquid-Like Despite Molecular Rigidity?

Abstract

The rational design of surfaces at the molecular level is essential toward realizing many engineering applications. However, molecular-scale defects affect processes such as triboelectrification, scaling, and condensation. These defects are often detectable via contact angle hysteresis (CAH) measurements. Liquid-like surfaces exhibit extremely low CAH (≤5°) and rely on the use of highly flexible molecular species such as long-chain alkyls or siloxanes. Their low glass transition temperatures lead to the so-termed self-smoothing behavior, reducing sensitivity to defects formed during fabrication. However, utilizing rigid molecular species such as perfluoroalkyl chains often results in higher hysteresis (10° to 60°) as defects are not self-smoothed after fabrication. Consequently, state-of-the-art perfluoroalkylated surfaces often show sub-optimal interfacial properties. Here, a customizable chemical vapor deposition process creates molecularly-thick, low-defect surfaces from trichloro(1H,1H,2H,2H-perfluorooctyl)silane. By implementing moisture-exposure controls, highly homogenous surfaces with root-mean-square roughness below 1 nm are fabricated. CAH is achieved down to ≈4° (average: 6°), surpassing the state-of-the-art by ≈5°. Reduction of CAH (26° to 6°) results in condensation suppression, decreasing surface droplet density by one order and surface droplet coverage by 40%. This work guides the synthesis of high-quality surfaces from tri-functional perfluoroalkylsilanes with liquid-like properties despite their molecular rigidity.