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 the wetting (and thence coating) of heterogeneous surfaces, 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, or by an applied potential. Details may be found in the accompanying references. The behaviour of pure liquids wetting solid surfaces is reasonably well understood for both the dynamic and static cases. However this is not true for complex fluids such as surfactant and polymer solutions, emulsions, aqueous solutions at high salt concentrations and colloidal dispersions. In industry, the wetting and coating of fibres, paper, capillaries, porous media, membranes and beads by complex fluids is of great significance, providing considerable stimulus for this work.
Approach: The intention here is to examine the wetting of model heterogeneous surfaces, with known and controlled physical and chemical roughness. The surfaces will be in the form of flats, rods and fibres. The surfaces can be patterned chemically at the micron scale and above by photomasking techniques and at the sub-micron level by self-assembly processes.
The solid surfaces of interest will include quartz, glass and silicon wafers. The patterns can be varied considerably and individual "regions" can be tailored with photochromic molecules as desired. Physical roughening can be controlled by the introduction of grooves, pits etc. The intention is to:
- examine the static wetting behaviour of these tailored surfaces in the presence of complex fluids. Do components of the fluid show a preference for a given region? What is the nature of the microscopic contact angle here? Can we induce the specific interaction between a component of the complex fluid and a given surface region, for example by the interaction of an emulsified oil droplet with a hydrophobic surface site, or by the interaction with an aqueous droplet with a hydrophilic site? From a colloidal dispersion of small particles, could we encourage the small particles to selectively adsorb on some sites and not others, perhaps stimulated by irradiation? Can we obtain surfaces with contact angles approaching 180 degrees?
- use dynamic wetting techniques (Wilhelmy plate apparatus and a moving rod/fibre) to study how dynamic wetting occurs on these tailored surfaces in the presence of complex fluids. Particular emphasis will be placed on the molecular kinetic and hydrodynamic aspects of wetting and how they might be modified to account for the dynamic behaviour of complex fluids on tailored solid surfaces.
References
1) JG Petrov, J Ralston and RA Hayes, "Dewetting Dynamics on Heterogeneous Surfaces. A Molecular-Kinetic Treatment", Langmuir, 15, No 9, 3365-3373 (1999).
2) 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).
3) 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.
4) M Schneemilch, WJJ Welters, RA Hayes and J Ralston, "Electrically Induced Changes in Dynamic Wettability", Langmuir, 16, No. 6, 2924-2927 (2000).
5) S Abbott, J Ralston, GD Reynolds and RA Hayes, "Reversible Wettability of Photoresponsive Pyrimidine Coated Surfaces", Langmuir, 15, No. 26, 8923-8928, (1999).
6) V Peykov, A Quinn and J Ralston, "Electrowetting: A Model for Contact Angle Saturation", Colloid and Polymer Science, 278, 789-793, (2000).
