Ian Wark Research Institute
ARC Special Research Centre
ARC Special Research Centre
Research Area: Physics, chemistry, chemical engineering
Degree: Honours
Supervisor: A/Prof Rossen Sedev
Significance of the Proposed Research: Wetting plays a decisive role in the success or failure of many industrial and natural activities. Photographic film production, pigment dispersion, mineral flotation, the movement of water in soils, printing, optical filters, cell membrane function and aspects of gene therapy are all controlled in large measure by wetting and dewetting processes. The liquid phase involved is most commonly, but not exclusively, water.
The control and optimization of the static and dynamic aspects of electrowetting, obtained through a fundamental study of model surfaces, is the principal objective of this project. For the majority of the wetting studies performed to date, equilibrium or static measurements have generally been the focus. Dynamic wetting and dewetting behavior has generally received far less attention, yet is arguably of greater practical relevance. Wetting is of course characterized by the contact angle, measured through the denser phase, that the tangent to the liquid-vapor interface makes with the solid surface at the contact line.
For either partially or fully wetting surfaces, such as water contacting a polymer surface or polysiloxanes wetting a silicon wafer, the emphasis has been on surfaces taken to be atomically smooth and chemically homogeneous. Even with such systems it has not been possible to vary the properties of the solid-liquid interface without influencing the contacting bulk liquid structure, apart from very recent pioneering work performed in our group, involving light sensitive surfaces and those which respond to an electrical stimulus. These "clever" surfaces, which are solids coated by very thin organic films, change their wettability in a reversible manner, as they are stimulated by light of different wavelengths, for example, in alternating cycles. We can also achieve a similar result by using an applied potential to cause the wettability of a surface to change. The process is called electrowetting and is an active research topic in the IWRI. It is the focus of this project. Electrowetting has many applications in industry. For example, it is possible to increase film coating speeds in the photographic industry by the clever use of electrowetting and there are useful applications in areas such as medical radiography where X-rays can be blocked by the rapid movement of liquid up a capillary array. This is known as a dynamic beam attenuation apparatus.
Approach: If a gold or other metal surface is coated with an insulator, that, in turn, is intrinsically hydrophobic or can be made so by an appropriate surface coating, and the gold surface is then
connected to an electrical circuit, major changes to wettability can be achieved through the application of potentials of the order of a few hundred volts. Contact angle changes of the order of 40 or 50 degrees can readily be achieved, an effect which can predicted moderately well with the aid of a Lippmann-type equation. Details can be found in the following publications. However relatively little is understood about the mechanism of the charging process, whether or not the same effects can be obtained at lower voltages for different materials, what happens when heterogeneous surfaces are involved and how electrowetting occurs in the presence of complex fluids such as emulsions, surfactant and polymer solutions, dispersions of small colloidal particles etc. We have suggested that one of the reasons for saturation in electrowetting (the point where any further increase in applied potential does not cause any more alteration of contact angle) could be that the solid-liquid effective interfacial tension is reduced to zero. If this is the case there are major implications for adsorption processes. Hence in this project we will explore:
- electrowetting of inorganic materials such as quartz, titanium dioxide, aluminium oxide, silicon nitride, titanium diboride coated with a smooth hydrophobic layer (eg a fluoropolymer). Homogeneous and patterned surfaces will be used.
- measurement of the microscopic contact angle and the potential distribution around the three phase contact line using scanning probe microsocopy. These measurements will be conducted in conjunction with external collaborators in Japan.
- electrowetting of tailored surfaces with complex fluids. This is completely novel and potentially will enable us to precisely control the disposition of material on a solid surface, to encourage droplets to selectively wet specific parts of a solid surface and to develop structured surfaces for applications as biosensors and other applications.
This project will involve extensive collaboration with other research groups in Japan and Europe.
References
JG Petrov, J Ralston and RA Hayes, "Dewetting Dynamics on Heterogeneous Surfaces. A Molecular-Kinetic Treatment", Langmuir, 15, No 9, 3365-3373 (1999).
M Schneemilch, RA Hayes, JG Petrov and J Ralston, "The Dynamic Wetting of a Low Energy Surface by Pure Liquids", Langmuir, 14, No 24, 7047-7051 (1998).
M Schneemilch, RA Hayes, JG Petrov and J Ralston, "Dynamic Wetting and Dewetting of a Low-Energy Surface by Pure Liquids � Part 2: Polar Liquids", in preparation for Langmuir.
M Schneemilch, WJJ Welters, RA Hayes and J Ralston, "Electrically Induced Changes in Dynamic Wettability", Langmuir, 16, No. 6, 2924-2927 (2000).
S Abbott, J Ralston, GD Reynolds and RA Hayes, "Reversible Wettability of Photoresponsive Pyrimidine Coated Surfaces", Langmuir, 15, No. 26, 8923-8928, (1999).
V. Peykov, A. Quinn and J. Ralston, "Electrowetting: A Model for Contact Angle Saturation", submitted to Colloid and Polymer Science (1999).
