Establishing the role of surface chemistry and topography in determining wettability and the development of a novel assessment methodology for repellent surfaces

Abstract

Coatings and surfaces with repellence to a range of liquids can find application in aerospace, marine, construction, energy industries and many more. The reported research provides understanding of the relative roles of surface chemistry and surface roughness on repellence and has allowed the development of a new methodology for assessing wettability. A review on surface treatments and how they affect solid-liquid interaction by measuring the static and dynamic contact angle with a variety of polar and non-polar probe liquids has been presented. During this research, eleven coating systems (including fluorinated and nonfluorinated ones) were assessed for promoting repellence on planar/smooth surfaces and on substrates grit blasted to micro-level roughness (roughness average of 1 μm to 4 μm ). To assess the impact of nano-scale and dual-scale roughness on repellence, the functionalised silica nanoparticles (fumed and synthesised by sol-gel) were incorporated into the coating system to build up the desired nano-scale topography (roughness average of 56 nm). This approach was undertaken in efforts to decouple the effects of surface roughness/topography on repellence from the surface chemistry contributions. The nano-scale topography provided high static contact angles, 128° and 93° with water and diiodo-methane respectively as probe liquids, this scale of roughness however, also exhibited high roll-off angles/or no roll-off even at 80° tilt. The micro-scale topography provided similar results. The combination of both nano and micro-scale topographies provided both high static contact angles (above 150°), low contact angle hysteresis and low roll-off tilts (below 10°) for water. However, this same combination of surface characteristics does not satisfy the conditions to achieve super repellence for probe liquids with lower surface tensions and different surface tension components (polar part/disperse part). The results in this study show that a high static contact angle with a probe liquid does not guarantee the abhesive behaviour. A novel assessment methodology has been proposed for the evaluation of repellence of surfaces. This approach helps to classify coatings and surface roughness characteristics according to their ability to repel various liquids not only in terms of static contact angles but also in terms of contact angle hysteresis, roll off tilt and film forming behaviour. It is proposed that droplet diameter is used as an indicator of a tendency of specific liquids to film formation. Whilst the critical parameters to achieving omniphobicity are still unclear, this work sheds light on the parameters that have to be considered and the methods to elucidate them.