Dynamic Dewetting: Designing and Breaking Novel Morphologies of Liquid Films

When a small droplet is deposited on a smooth surface it spreads across the surface until it reaches an equilibrium droplet shape or until it becomes a film. This allows the dynamic wetting process to be studied at a fundamental level enabling the fluid mechanics of contact line motion to be understood. This understanding is important in many industrial processes, such as printing. However, often a process starts with a liquid film, rather than a droplet, and a change of the environment or some other parameter, can initiate a process of de-wetting, i.e. the recoil or break up of a film on a surface into one or more droplets. The initial film state and its de-wetting from a surface are important for industrial processes, such as spin coated films used in lithography, painting/coatings, printing, heat exchangers, etc. One difficulty in understanding the de-wetting is that it is extremely challenging to initiate the breakup of a film of liquid on a surface in a controlled manner that leads to an ideal droplet state. Dewetting usually leads to a mixture of droplets and puddles making it difficult to study the dynamics of the process or to control the final droplet state. In a recent paper (Science Advances, 2016) we showed a new method using a non-uniform electric field to force a liquid to wet a non-wetting surface. By quenching the electric field, a controlled dewetting into a single droplet state can then be initiated.

In this project, we use electric-field induced film formation to study non-naturally occurring film morphologies (e.g. triangular, square and ring droplets) and their de-wetting dynamics into single droplets in a manner, which has never previously been possible. We investigate liquid-in-liquid systems with order of magnitude contrasts in viscosity ratios (from droplets-in-air to liquids-in-liquids to bubbles-in-liquids) thereby elucidating the fundamentals of the fluid mechanics of contact line motion. We also investigate the combination of individually programmable film morphologies into fully programmable arrays of wetting patterns. An ability to finely control liquid films has potential for industrial applications from printing to displays. Finally, we establish a new concept of electric field stabilised surface-localised 2D emulsions where the arrays of droplets or bubbles can be detached from the surface and reattached in a controlled manner.

(1) Knowledge and Techniques
Being able to reversibly induce a liquid film on a non-wetting surface using a voltage will establish a technique with broad applicability across a range of industrial sectors. This ability has relevance to coatings, films, adhesion and icing properties of surfaces, which can be modified by altering the wetting properties of a surface. For example, one industrial process of interest to one of our collaborators, Huntsman Ltd, is the release of objects from moulds without using a barrier release agent.

Our techniques also have greater flexibility than simply controlling wetting globally across an entire surface. Designing individual shaped areas to become wetting on the application of a voltage extends the potential industrial applications to those that require specific patterns, e.g. printing. Moreover, with the ability for reversible switching of these shaped areas between wetting states, the applicability potentially extends to displays. This ability is similar to electrowetting, which has been the basis for spin-out companies for displays, liquid lenses and digital microfluidics (e.g. Liquavista - now an Amazon company, Varioptic, and Advanced Liquid Logic - now part of Affymetrix).

Beyond the droplet-liquid film control techniques, the project also develops a method to reversibly induce bubble-gas film transitions and to control the position and motion of bubbles and arrays of bubbles along surfaces. Bubble and gas film formation during heat transfer creates an insulating barrier and so is an important industrial problem. The knowledge and techniques developed may inform industrial processes where nucleate boiling and critical heat flux are important.

(2) Economic Impact
This project is at Technology Readiness Level (TRL) 1 generating basic knowledge and developing new techniques. It combines ideas on liquid dielectrophoresis with ideas on surface patterning of wettability and the dynamics of moving contact lines and the hydrodynamics of internal and external flows in wedges and corners. The ability to voltage controllably pattern wettability is a new platform technology. The principles are scalable across large surface areas and the experimental work is fundamental, but includes a scoping element to enable future application oriented projects at lower TRLs. This project may also enable the re-engineering of existing processes (e.g. printing, optics, electronic paper and displays). Ultimately, it will generate enabling knowledge with the potential for high value disruptive technology fundamental to UK economic success.

Two industrial research laboratory collaborators are providing advice on leveraging industrial benefits. These companies have industrial networks and supply chains providing a wider understanding of potential commercial applications. The international basis of these companies means new techniques developed within the UK could have global relevance. Intellectual property will be protected by Collaborative Research Agreements and Invention Disclosures. Annual IPR reviews with transfer to industry will be supported by a Business Development Manager using patents and licences. As appropriate, demonstrator or industrial R&D projects may be initiated.

(3) People Pipeline
The UK skills base would benefit from the training of two multidisciplinary postdocs having skills relevant to high value manufacturing due to the project including work in (i) heat/mass transfer (bubbles/gas films), (ii) instrumentation/measurement, (iii) test-rig construction/operation, (iv) microfabrication/cleanrooms (v) computational modelling. They will also have received public communication training and taken part in our Nature's Raincoats Outreach (http://www.naturesraincoats.com/). Undergraduate summer work placements will promote interest in research thereby helping to ensure a continuing pipeline of future PhD students.