Self-Propelled Droplet Motion on Gradient Slippery Liquid Infused Porous Surfaces (G-SLIPS)

The ability to control how liquid droplets interact with surfaces is a topic of wide scientific interests and has many practical applications. From self-cleaning surfaces for building facades or domestic appliances, right through to medical applications and self-diagnostics, there is an increasing and invaluable call for smart materials capable of a myriad of tasks. Recently there has been an increasing interest in biomimetic systems such as super hydrophobic surfaces based on the Lotus leaf effect, and more recently on Slippery Liquid Infused Porous Surfaces (SLIPS) based on the nepenthes pitcher plant. Such surfaces have been the subject of much interest in terms of their capabilities for water shedding and anti-icing properties as well as their ability to control wetting in applications such as coating and inkjet printing.

When dealing with liquids on a microscale, the ratio of a droplet's surface area to volume becomes ever larger and frictional forces must be overcome during motion. To induce motion of droplets on a surface, static frictional forces, due to contact angle hysteresis, must also be overcome.

When a sessile droplet sits on a SLIPS it exhibits a wetting ridge around its base. This wetting ridge is due to the balance of forces at the three phase contact line of the droplet. Usually drawn as a Neumman Triangle, the shape of this ridge is dependent on the properties of the liquids involved. In 2015, with co-workers, (Langmuir, 31(43), 2015) I showed that the height of the wetting ridge around a droplet on a surface is also dependent on the nature and texture of the underlying surface. We used such a surface to show that one can measure an apparent contact angle, for a sessile droplet, much like that of a droplet on a solid surface and showed that this apparent contact line is highly mobile with very little hysteresis. A droplet placed on such a surface can move or be moved very easily because of the lubricating nature of SLIP surface. Subsequently I have developed the new idea that, if one introduces a gradient into the underlying topography of the SLIP surface, it is possible to generate autonomous motion in a droplet.

This project will create surfaces which give low contact angle hysteresis, low friction and are capable of imparting motion onto droplets without the need for any pumping mechanism. I will use gradients in the roughness of an underlying surface texture, along with an infused liquid lubricant to create a Gradient Slippery Liquid Infused Porous Surface (G-SLIPS). I will use this variation of a SLIP surface and results from preliminary experiments to perform a full study into the mechanisms of motion on such surfaces. I will experimentally examine how parameters such as the type of lubricating liquid or the underlying surface topography create and control droplet motion. Finally, I will create and test specific surfaces capable of prescribed motion commensurate with specific microfluidic tasks. This project will ultimately add new techniques to the future development of pump free microfluidic systems and create surfaces that overcome both contact-line pinning and reduce viscous friction using SLIP Surfaces.

1) Knowledge and Techniques.
Work I have recently co-published (Evaporation of sessile droplet on Slippery Liquid Infused Porous Surfaces SLIPS) showed that the apparent contact angle of a droplet on these surfaces is highly mobile and shows very low contact angle hysteresis. My later research (Publication in preparation) shows that, under the conditions of a highly mobile contact line, a droplet becomes very easy to move and seeks out well-defined equilibrium states. Along with my preliminary data showing that motion can be generated on a Gradient Slippery Liquid Infused Porous Surface this project focuses on understanding and creating new surfaces, with the ability to create droplet motion, with potential uses in surface coatings and microfluidics.

2) Economic impact.
According to a report by the Centre for Process Innovation, (www.uk-cpi.com/wp-content/uploads/2014/10/SEAC-SIG-Final-Report-.pdf) , The Surface Engineering and Advanced Coatings Special Interest Group (SEAC SIG) community has created an £11bn business, which plays a key role in producing over £140bn of products with applications in auto motive, healthcare and the built environment . In a strategic workshop in January 2014 the group found that the UK has the potential to provide a full supply chain for Surface Engineering and Advanced Coatings (SEAC). In micro fluidics, "The global microfluidics market is valued at $3 Billion in 2015 and is poised to grow at a compound annual growth rate of 19% during the forecast period (2015 to 2020)". (www.globenewswire.com (2015)) The design, manufacture and use digital microfluidics has grown dramatically in the past two decades with over 200 companies involved in areas from bio applications to optical devices (http://fluidicmems.com). My proposal has potential for exploitation in both of these markets along with possible others such as heat exchange or inkjet printing. It will offer new materials and new methods to an important market.

Intellectual property will be protected by a collaborative research agreement, invention disclosures, and IPR reviews with transfer to industry supported by a Business Development Manager using patents and licences. These possibilities will be discussed with the industrial partners, Cellix Ltd and Jaguar Land Rover, in regular advisory board meetings. Any potential further projects will be identified and opportunities for commercialization and exploitation will be advised upon by the project collaborators.

3) People pipeline and public engagement
The project will contribute directly to my development as an independent researcher by broadening my research expertise in an exciting and industrially-relevant field. The UK skills base will also benefit from the training of a postdoctoral researcher having skills in high value manufacturing techniques such as (i) microfluidics, (ii) Photolithography (iii) experimental prototyping, (iv) instrumentation, (v) device design and manufacture. They will also receive public communications training (The Royal Society) and take part in Nature Raincoats. (www.natureraincoats.com) The project will also result in the creation of a primary level workshop which will be delivered in two partner schools as part of the ongoing long term intervention.