Dynamic Dewetting: Designing and Breaking Novel Morphologies of Liquid Films

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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.