Stabilising effect of topography on thin film flows for coating applications

Thin liquid films flowing over an inclined solid substrate have the propensity to form large-amplitude free-surface waves propagating with a coherent shape and characteristic speed - a commonly observed and well-known instability. Not surprisingly, the appearance of such wave patterns, on the surface of a fluid layer, is an unwanted feature in many technological applications involving coating operations connected to print manufacturing processes; for example the manufacture of solar cells or paper production.

Existing experimental data shows that the interaction of the film flow with substrate that contains repeating topographic features is able to delay the on-set of instability resulting in a higher topography-dependent critical condition for the onset of instability. However, while there are theoretical models for flow over periodic topography available in the literature, until now no evidence has appeared showing that any of them are able to predict and capture the experimentally observed stabilisation effects.

The purpose of the project is therefore to develop theoretical models capable of predicting the conditions leading to the onset of free-surface instability for as wide a range as possible of the governing parameters of interest. By developing such models, to understand the interaction of the parameters involved and to identify safe operating windows for defect free coating, there is the potential to either eliminate completely or to minimise their detrimental effects in an industrial context, by enabling manufactures to predict and utilise operating windows and conditions that guarantee the production of a metered steady film. To this end, the project will involve collaboration with representatives from both academia and industry.

There are several industrial, societal and economic impacts envisaged from this project.
1. Advancement in knowledge of the instability of thin film flows and the development of the associated manufacturing techniques targeting a stable reliable low-cost coating throughput over large-area functional surfaces. The range of applications benefiting from these techniques vary from printed electronics and aerospace industry to health diagnostics and tissue engineering. There are also three direct industrial beneficiaries, i.e. the Centre for Process Innovation, RKPrintCoat and Solaris Photonics, that will be actively involved with the project as collaborators.
2. Development of the applications in renewable energy, in particular reduction of the manufacturing costs of thin film solar cells, and transforming the UK electricity production technologies towards renewable sustainable energy supplies.
3. Strengthening of the international profile and leading position of the UK in the fundamental physics of fluid flows, and in particular the area of hydrodynamic instabilities.
4. Development of numerical methods and solvers for the efficient modelling of free-surface thin film flows over periodic topography. The software developed will be made free open source licensed under the GNU licence.