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Details of Grant 

EPSRC Reference: EP/P026613/1
Title: Self-Propelled Droplet Motion on Gradient Slippery Liquid Infused Porous Surfaces (G-SLIPS)
Principal Investigator: Wells, Dr GG
Other Investigators:
Researcher Co-Investigators:
Project Partners:
Cellix Ltd Durham, University of Jaguar Land Rover Limited
Department: Fac of Engineering and Environment
Organisation: Northumbria, University of
Scheme: First Grant - Revised 2009
Starts: 01 December 2017 Ends: 31 May 2019 Value (£): 99,037
EPSRC Research Topic Classifications:
Complex fluids & soft solids Fluid Dynamics
EPSRC Industrial Sector Classifications:
Healthcare Transport Systems and Vehicles
Related Grants:
Panel History:
Panel DatePanel NameOutcome
09 Feb 2017 Engineering Prioritisation Panel Meeting 9 and 10 February 2017 Announced
Summary on Grant Application Form
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.

Key Findings
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