Influence of Roughness on Non-wetting Behaviour of Aluminium Surfaces

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Project Code: 32894

Start date and original planned duration: January 2019, 12 months

Objectives

Establish a methodology that enables the chemical and topographic influences on wetting to be decoupled.
Identify the key topographic parameters behind highly repellent surfaces.
Provisionally assess the abrasion resistance of candidate repellent surfaces.
Measure the ice repellence of candidate structured surfaces.

Project Outline

There are a number of theories explaining adhesion, typically focusing on two key parameters: (i) Molecular forces or interactions at the interface between the two adherends; and (ii) Topographic features. To achieve the strongest joint it is necessary to maximise the chemical compatibility and interaction whilst also ensuring the greatest interfacial area.

The converse of adhesion is abhesion. To maximise abhesion it is necessary to reduce the chemical compatibility between two surfaces and also to minimise the interfacial contact area. The most widely repellent surfaces are based on fluoropolymers and siloxanes, because the electrochemical natures of the fluorocarbon and hydrocarbon bonds make them very stable and minimise electrostatic interactions with their neighbours. Sessile drop assessment of planar repellent coatings measure contact angles of 105-110° with water droplets and ~80° with diiodomethane.

The influence of roughness on minimising wetting of liquids onto solid surfaces has long been known and the lotus leaf is often quoteds as an example of highly repellent behaviour. This behaviour is frequently attributed to a dual-scale roughness that provides re-entrant features that trap air and minimise the actual interfacial contact area. While there is a consensus regarding the importance of topographic features to promote non-wetting, there is little guidance in the literature regarding which roughness parameters are important. There is a need for both practical and theoretical studies to identify the key roughness parameters.

This project aims to decouple the influence of surface chemistry and roughness. Different topographic profiles will be generated via laser machining and the deposition of thermally sprayed aluminium (TSA) on aluminium substrates. The samples will be treated with a low viscosity, conformal coating based on polysiloxane chemistry incorporating functionalised nanosilica additives. The conformal coating will provide a uniform inherently repellent surface chemistry. The ice adhesion reduction factor (ARF) of the most promising candidate surfaces will be measured.

Industry Sectors

Aerospace
Power
Surface Transport (Automotive, Marine, Rail)

Benefits to Industry

Establishing a technological direction towards a passive highly repellent surface technology via a top-down engineering approach will provide a route towards products and structures for extreme environments where icing specifically currently prevents operation. A design process for the fabrication of highly repellent aluminium surfaces will have benefits in enhancing the longer term corrosion resistance of TSA.

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