Wetting of Auxetic Metamaterials

It is a common conception that when a material is stretched it becomes thinner. However, when an auxetic material is stretched it becomes wider (it possesses a negative Poisson's ratio). Since materials are never unbounded, they have surfaces.

And surfaces inevitably come into contact with liquids. However, surprisingly, there is no published research that considers how the unusual properties of auxetic materials change the solid-liquid interaction and influence how liquids spread on the solid surface or the impact and rebound behaviour of droplets. It is even more surprising because expanded polytetrafluoroethylene (ePTFE), the microporous superhydrophobic waterproof and breathable layer in GoreTex, can be easily converted to an auxetic form.

When auxetic materials expand their surface area, it is by an increase in the space within the solid frame of a lattice. We can therefore expect changes to the wetting properties of the surface. This type of change will cause the solid surface (Cassie) fraction of the surface to decrease as the space between solid components increases.

This balance between solid and liquid surface area is critical to how the surface chemistry properties are amplified by the surface topography/texture into super-liquid repellence, hemi-wicking and other wetting properties. It also controls whether the surface is one to which a liquid will stick or one which appears slippery to a liquid. An auxetic material has unusual impact resistance properties because material flows towards, rather than away, from the area of an impact. This offers a new approach to materials and surfaces for droplet impact and rebound.

In this project we therefore focus on the enabling materials-science to create a new class of wetting materials, which we refer to as "Auxetic Wetting Metamaterials". The project considers a range of techniques to fabricate auxetic materials with a focus on how the balance between solid surface area fraction and space area fraction change with strain.

The project considers how this solid surface area fraction change with strain combines with hydrophilic and hydrophobic properties to manipulate the overall wetting properties of the surfaces and the wetting state transitions of these new materials. It will do so experimentally and be supported by a theoretical programme and by modelling and simulation. The work will also consider how strain can be used to control liquid friction and droplet rebound.