CBET-EPSRC Dynamic Wetting & Interfacial Transitions in Three Dimensions: Theory vs Experiment

The spreading of liquids over solid objects is a familiar and every day occurrence. For example: raindrops smashing into windscreens; stones being thrown into ponds; a chocolate fountain coating a strawberry. In all these cases, there is a maximum speed at which the liquid can traverse (or 'wet') the object and going beyond this speed creates easily observable effects such as the the disintegration of the raindrop into smaller drops or a patchy coating of the strawberry. Remarkably, despite the seemingly innocuous nature of these everyday phenomena, at present there exists no theory or computational model capable of predicting, and hence controlling, the maximum speed of wetting.

In addition to the academic curiosity of these events, they form the basis of a remarkable array of technological applications and natural processes. In particular, the coating of thin layers of liquid which subsequently solidify is a ~$100 billion (and ever-increasing) market which is key to the manufacture of products ranging from solar cells, to alleviate energy and environmental crises, to emerging capabilities to print electronic circuits. In these industries, an ability to create optimal designs is currently limited by our knowledge of the underlying physics.

This project will underpin exploration of the aforementioned phenomena and innovation within industry by exploiting a synergy between computational models embedded within software and cutting-edge experimental analysis. The computational and experimental aspects are particularly ambitious as (a) the wetting of solids is a strongly multiscale problem, requiring resolution from almost-molecular scales right up to engineering application scales, and (b) the process is inherently three-dimensional, meaning that simplifications leading to reductions in computational complexity are impossible and high performance computing techniques must be implemented. This project exploits recent advances in (a), by the Investigators, in order to tackle the problems associated with (b) for the very first time.

New knowledge of how liquids spread over solid surfaces will be initially focussed on industrial coating problems, where the challenge is to wet a solid with a liquid as fast as possible without entraining air. Initial progress will be guided and enhanced by a collaboration with 3M (famous for products such as Post-it and Scotchgard), a multinational corporation with ~$30 billion sales annually from manufacturing solar cells, paints, anti-reflective coatings, adhesives, etc. For them, a computational model provides a fast and cost-effective way to achieve understanding of the physical mechanisms at play in order to optimise the coating process.

Breakthroughs achieved in this project will have impact within related fields of research. Within industry, this involves working with Trijet, a leading consulting firm on emerging drop-based technologies, who will translate our advances to improve the control of inkjet printing technologies that are being used in everyday applications of fluids, e.g. in the automotive industries and in the printing of high-value metallic inks such as silver for printed electronics. Furthermore, our advances could have impact in other fields, such as climate science, where similar flow structures are observed when a liquid drop impacts a bath of the same liquid, as occurs when a raindrop impacts the ocean. Here, our understanding of how trapped gas between the drop and the ocean is entrained into the latter could feed into climate models, where this is a key parameter.

This research project matches cutting-edge fundamental research with real-world importance, to ensure impact across academic (see Academic Beneficiaries), industrial, environmental and societal sectors. New capabilities and techniques for the computational modelling and experimental analysis of technologically critical instabilities in dynamic wetting and related flows will have immediate impact on our partners' operations, as initial end-users, and longer term far reaching impact for colleagues in academia, industry and, ultimately, members of society.

Industrial Impact
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Our main focus on liquid coatings is motivated by the manifold applications of our work to this field, which has a market size currently estimated at ~$100 billion and ever-increasing. The potential for impact within this sector is evidenced by the support of project partner 3M, a multinational corporation who use liquid-based coatings in the manufacture of photovoltaics, printed electronics, paints, anti-reflective coatings, adhesives, etc. Their Letter of Support states: "3M [...] has ~$30 billion in sales annually, and employs 90,000 people around the world. Liquid-applied coatings play a significant role in 3M's business. Dynamic wetting failure and the subsequent air entrainment in coating flows is one of the primary obstacles to improving upon current manufacturing speeds. Thus, 3M is naturally very interested in the work you propose."

Advances in computational modelling will be exploited to address challenges in related fields. Here, Trijet, a consultancy firm specialising in drop-based technologies, are interested in our work because they: "strongly believe that any simulation capabilities for rapidly exploring new formulations, supported by underlying experimental analysis, would be of great interest to the industrial inkjet and spray coating world."

Initial impact with industrial end users is through our partners (3M, Trijet & Sandia National Laboratories). They will also help us to secure longer term impact, in agreeing to "facilitate collaboration with industrial partners interested in coating technologies, including Dow, 3M, and Canon Nanotechnologies" (Sandia) and "engage with our clients such as Tecglass Spain (worldwide leader in digital printers for glass) and Fenzi group, Italy (a company in the field of speciality chemicals for glass processing)" (Trijet).

In addition to exploiting pathways to impact through project partners, industries interested in new user-friendly design-by-simulation capabilities will be engaged through a variety of routes, including: (a) presenting at industrially attended conferences; (b) the organisation of an Industry Workshop; and (c) the organisation of Short Courses, facilitated by Sandia, to showcase industry-relevant advances.

Environmental
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Advancing our fundamental understanding in the area of interfacial transitions could have significant benefits for the environment, including:
- the development of 'greener' coating processes that waste less materials and energy,
- improvement of the economic waste-free production of robust solar cells, to generate energy more sustainably and meet carbon reduction targets,
- knowledge of air-sea gas exchange key for climate science, which relies on understanding instabilities occurring when raindrops impact the ocean.

Societal
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This project will have clear societal impact in the US and UK through the following routes:
- creation of high-skilled jobs for the national workforces, enabled by supporting fundamental advances in high value manufacturing,
- development of an Anglo-American collaboration, including people and knowledge exchange, at a time of increasing international isolationism for the two nations,
- training of postgraduate and postdoctoral researchers, who will support future innovation in this area, in a world-leading, international and interdisciplinary research endeavour.