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.

Economic Relevance:
This project has relevance to a range of materials-based industries, including textiles, filters, coatings and packaging. For example, the 2018 Innovate UK KTN report "The UK Packaging Industry - A Strategic Opportunity: A Landscape Review and Technology Roadmap Report" notes that the UK Packaging Manufacturing Industry has annual sales of £11B and employs 85k people. The 2016 KTN report "A State-of-the-Art Review of Smart Materials: A Review of Auxetics in the UK" noted exponential growth in patent applications for auxetic technology most notably in the medical, aerospace and automotive sectors. The same report, however, noted a rapid rise in overseas competition recently, highlighting a strategic need to invest in the UK to ensure it benefits from the long-term impact in the materials/engineering sectors. The global auxetic materials market segments into structural and functional materials classified by whether they improve the mechanical or physical properties or detect events or stimuli. Their properties lead to use in fields such as biomedical (e.g. dilation, implants, prosthesis), filters and auxetic fibres. Auxetic materials impact resistance is important in manufacturing crash helmets, body armour, and sports clothing, as they provide resistance against cracks and dents (source: "Auxetic Materials Market - Global Industry Analysis, Size, Share, Growth, Trends and Forecast 2017-2025"). Understanding the interaction of auxetic materials with liquids at their surfaces and leveraging that understanding into new designs would broaden the application field. Doing so would provide the UK with unique innovation - a key objective of the UK Industrial Strategy.

Pathways to Economic Impact:
Invention records will be filed, IP workshops and IP reviews conducted, and these will be considered by the Universities' Innovation and Impact Teams. Patents will be filed if judged appropriate and the Research England Connected Capabilities Funded "The Northern Accelerator" project can provide Proof of Concept commercialisation projects, and seed funding for spin-out company proposals. We will leverage Knowledge Transfer Partnership and licensing opportunities working with a Business Development Manager and create links with companies through i) A North-East Coatings Consortium, ii) Industry Network Presentations on Smart Surfaces & Coatings, iii) the ERDF-funded Sheffield Innovation Programme, iv) Project Partners (CPI and Schlumberger) and v) existing auxetics industry links. We will contribute to the annual Knowledge Transfer Network Metamaterials Showcase Event. An end of project workshop with a target of 40 participants including 30% industrial industrialists will be held.

Training of Skilled People and People Pipeline:
The PDRFs will gain high level skills relevant to new materials and high value manufacturing via work on (i) surface coatings, (ii) measurement/instrumentation, (iii) experimental design and implementation, (iv) microfabrication techniques and (v) data analysis and interpretation. The PDRFs will also gain project, time and budget management experience and take part in the University Researcher and Career Development programme. They will attend a bespoke media and communications training residential programme. Two casual labour paid summer projects for undergraduate students will develop their interest in progression to PhD study.

Public Understanding/Outreach:
The PDRFs will take part in our Nature's Raincoats Outreach Exhibition (e.g. Big Bang Fairs, etc) and the Sheffield Festival of Science and Engineering, work with the NUSTEM team in developing research-based Schools experiments and assist the Think Week Summer Schools (focused on children from Schools - primary and secondary - in areas of deprivation) thereby contributing to the pipeline of the future STEM workforce. NUSTEM targets under-represented groups, with a focus on gender, and progression into STEM careers.