How liquids wet solid surfaces is of fundamental importance to a wide-range of scientific disciplines and technological applications from creating thin films on semiconductor wafers, through adhesion and coating of surfaces, to effective droplet deposition and mixing on DNA microarrays. Electrostatic fields can alter how effectively a liquid wets a solid surface. In recent years uniform electric fields have been used to control and manipulate droplets of conducting (ion containing) liquids, typically a salt solution, by using the liquid-solid contact area as one electrode in a capacitive structure - so called electrowetting. This has led to new voltage controlled variable focus liquid lenses, liquid-based electronic paper and droplet-based microfluidic systems. The key to electrowetting is the ability of an applied voltage to reversibly increase the effective hydrophilicity of a solid surface and reduce the contact angle of the droplet without altering the surface chemistry. However, many liquids of interest are not conducting and the need for a sandwich-style capacitive structure and direct physical contact to the liquid limits its range of applicability.

In this project we create a new method of controlling hydrophilicity and oleophilicity of materials by using the dielectric properties of liquids, but with the effects localized to an interface. Unllike electrowetting which focuses on the ions, our method focuses on the dipoles in a liquid. Using a non-uniform electric field generates unequal forces on the two ends of the dipole. The resulting dielectrophoretic force can result in movement and redistribution of the liquid into the areas of highest field gradient. The basis of our project is the understanding that when the liquid has solid-liquid, liquid-vapor or liquid-liquid interfaces, dielectric energy changes can be coupled to surface free energy changes. With a suitable decaying electric field, the effects of liquid dielectrophoresis can be confined to either the solid-liquid interface or to the liquid-vapor (or liquid-liquid) interface and can be used with a non-conducting liquid.

By using microfabricated interdigitated electrodes a decaying, and hence non uniform, electric field can be created above a solid surface. For a droplet thicker than the decay length of the electric field, the major change of the surface energy compensating liquid dielectrophoretic energy changes is via a change in the contact area with a solid and so this can be a method of reversibly controlling the contact angle and, hence, the hydro- and oleo- philicity of a surface. For a thin liquid film the major change of the surface energy compensating liquid dielectrophoretic energy changes is via a change in the shape of the liquid-vapor (or liquid-liquid) interface and so, in this case, it becomes a method for shaping a liquid surface.

In this method of localizing the effects of liquid dielectrophoresis to an interface the contrast to electrowetting is that,

1. the electric fields are non-uniform;
2. the electric fields are generated by surface microfabricated co-planar rather than sandwich electrode structures;
3. the forces act upon the dipoles in the liquids, which can therefore be non-conducting (or conducting), rather than upon ions of conducting liquids;
4. the method does not suffer from saturation of the contact angle and so can be used to produce liquid films.

The research in this project seeks to establish an approach to wetting that allows conducting and non-conducting liquids to be manipulated using electric fields in a manner complementary to electrowetting. The project will provide the understanding needed to allow future development of novel droplet microfluidic, liquid microactuation, liquid-based optics and displays. The project includes industrial partners who have expertise in the development and commercialisation of microfluidic liquid handling, lab-on-chip devices, display devices and optofluidic systems.

Potential Impact:
(1) Knowledge and Techniques
Being able to localize the effect of liquid dielectrophoresis to an interface ("dielectrowetting") would establish a new voltage controllable method of tuning the interaction of a liquid with a surface. Since interactions at the solid-liquid interface are common in engineering and physical sciences disciplines and across a range of industrial sectors, the long term benefits could impact widely. Over the last fifteen years the extension of knowledge of wetting to electrowetting has led to new techniques for lab-on-a-chip, optical and displays applications. The principle of electrowetting has led to white light based electronic paper (Liquavista a spin-out from Philips Eindhoven and now part of Samsung), fast variable focus liquid lenses (Varioptic, France), high intensity edge wave-guide colour display (Extreme Photonix, USA) and digital microfluidics (Advanced Liquid Logic, USA) to name just a few. Dielectrowetting has similar potential, but with the ability to use a wider range of liquids and without needing direct electrical contact to the liquid.

(2) Economic Impact
A key priority for the UK economy is the development of high-value and specialist manufacturing. Our first industrial collaborator Merck is an example of a company based on specialist chemicals and materials - often for liquid crystal displays companies located in the Far East. Their success, like the UK's, depends on identifying emerging technologies at an early stage. The theme of emerging technologies and advanced manufacturing is also that of our two other industrial collaborators. L-3 TRL Technology is a high-tech UK company dealing with defence systems. The Dolomite Centre Ltd specialize in translating research concepts in microfluidics from the lab to products, solutions and instrumentation for industry. These collaborators provide a direct route to leveraging industrial benefits from any IPR identified. They are also well-placed in industrial networks to direct the project team to the wider UK industry as potential partners where opportunities less relevant to them develop during the research. The international extent of these companies means a new approach developed within the UK could provide inward investment from overseas and/or, as in the case of Merck, a base to provide specialist liquids/materials to overseas based industries.

This project may also provide potential for re-engineering of the approaches to some applications, such as lab-on-chip diagnostics, optics, electronic paper and displays. As a technology, control using dielectrowetting has the potential to provide simple device fabrication and low power consumption with the accompanying environmental benefits arising from the manufacturing process and device usage. Where there are no concerns relating to IPR/NDAs, we will seek to publicize work via presentations at KTN events, publication of research, and presentation at national and international conferences.

(3) People Pipeline
The UK skills base would benefit directly from the training of a multidisciplinary postdoctoral researcher having skills relevant to high value manufacturing due to the project including work in (i) microfluidics, (ii) liquid-based optics, (iii) electrostatics, (iv) instrumentation and (v) device design and fabrication. Theey will also have received public communication training and have taken part in the our Nature's Raincoats Outreach work (http://www.naturesraincoats.com/), including at the 2013 British Science Festival. Undergraduate summer research bursary student involvement in the project will promote interest in research thereby helping to ensure a continuing pipeline of future PhD students (particularly important at a time of increasing undergraduate fees). In addition, the project team have included the creation of a "Kit-in-a-Kase" (a portable schools/outreach demonstrator experiment) based upon dielectrowetting for use with our Nature's Raincoats activity.