Ian Wark Research Institute
ARC Special Research Centre
ARC Special Research Centre
Research Area: Physics, chemistry, colloid and interfacial science, chemical engineering, surface engineering
Supervisors: Laureate
Prof John
Ralston, A/Prof Daniel
Fornasiero and Dr Rossen
Sedev
Description: Thin liquid films often separate bulk phases and the stability of these films dictates whether or not the respective bulk phases will contact one another. Practical examples include lubricants such as synovial fluids in hip joints, an aqueous film separating an air bubble and particle during froth flotation and the surfactant or polymer-containing film between two soap bubbles.
Extensive experimental research on the interaction between dissimilar interfaces will be carried out using scanned probe microscopy, and other related techniques, including single bubble-particle experiments. The initial studies with well-defined surfaces will be extended to designed interfaces of increasing complexity, mimicking those found in nature and industry. The outcome of these model experiments will provide the basis for developing and testing adequate theoretical models.
Our main concern will be with aqueous wetting films, with several key experiments in non-aqueous systems. The first task is to follow in detail the behaviour of films on smooth, chemically homogeneous surfaces. Such surfaces (SiO2, TiO2, Al2O3) can be produced and, owing to their different isoelectric point pH values (2, 6 and 9) will provide the opportunity to study the effect of pH, which largely controls the electrostatic charge at the S/L and L/V interfaces. The electrostatic interaction can be modified by addition of ionic surfactants. Surface active ions may neutralize the interfacial charge or, at higher concentration, induce charge reversal at one or both interfaces. By increasing the ionic strength the influence of the electrostatic forces is effectively suppressed and the contribution of other surface forces (van der Waals, steric, etc) can be investigated. The above experiments will also provide a detailed picture of the transition from stable to metastable and rupturing films.
Special attention will be paid to the influence of dissolved gas on the stability of thin wetting films. Force-distance and thickness measurements will be performed as a function of dissolved gas concentration, emphasising high ionic strengths, where electrostatic contributions are minimized, offering the opportunity to test model predictions for van der Waals interactions alone. We will also study more complex fluids, where droplets will be included.
The next step is to conduct analogous experiments with increasingly rough and heterogeneous surfaces. Smooth titania surfaces will be made rough by adsorbing titania particles of different size (20 nm-2 µm). This system will permit us to investigate the influence of roughness for both hydrophilic and hydrophobic surfaces (moderate hydrophobicity can be achieved by visible light irradiation of titania; the sample can be rendered very hydrophobic by depositing a thin layer of amorphous fluoropolymer). Decorating silica plates with titania particles will provide an intriguing example of a hydrophilic surface coated with particles which can be reversibly altered from hydrophilic to hydrophobic by irradiation with light of an appropriate wavelength.
Chemical heterogeneity will be modelled with mixed self-assembled monolayers (SAM) on gold. Patches of thiols terminated with different terminal groups (-OH, -COOH, -CH3, -CF3) will be created. The patch size will be in the micron range (1-10 µm) and less. The area fraction as well as the size, shape and distribution of the components will be varied systematically. We will thus construct a model of how thin liquid films behave on micro and nanostructured surfaces. We will deal with plates, fibres and particles. Our research has application to:
- cosmetics (how do shampoos interact with hair fibres?);
- mineral flotation (the film rupture is critical to bubble-particle capture and selectivity);
- electronics and nano/micro fluidics;
- dewetting and drying processes when surfaces are coated.
This project is connected to a major advanced coatings project, with ten European and four Australian partners.
References
1. D Hewitt, D Fornasiero and J Ralston, “Bubble Particle Attachment”, J. Chem. Soc. Faraday Trans., 91 (13), 1997-2001 (1995).
2. ML Fielden, RA Hayes and J Ralston, “Surface and Capillary Forces Affecting Air Bubble-Particle Interactions in Aqueous Electrolyte”, Langmuir 12 No. 15, 3721-3727 (1996).
3. JG Petrov, J Ralston and RA Hayes, “Dewetting Dynamics on Heterogeneous Surfaces. A Molecular-Kinetic Treatment”, Langmuir, 15, No. 9, 3365-3373 (1999).
4. 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).
5. DRE Snoswell, J Duan, D Fornasiero and J Ralston, “Colloid Stability and the Influence of Dissolved Gas”, J.Phys.Chem.B., 107, 2986-2994 (2003).
6. N Mishchuk, J Ralston and D Fornasiero, “The Influence of Dissolved Gas on van der Waals Forces between Bubbles and Particles”, Journal of Physical Chemistry 106, No. 4, 689-696, (2001).
Funding: All students should apply for an IWRI fully-funded scholarship.
International students should also apply for an International Postgraduate Research Scholarship (IPRS) and a UniSA President’s Scholarship (UPS). To be eligible for UPS, applicants must have a supervisor willing to nominate them for consideration.
Australian students should also apply for an Australian Postgraduate Award (APA) and a UniSA Australian Postgraduate Research Award
(USAPRA).
International travel and collaboration will be involved in this project, and students should be prepared to travel overseas for short periods of focused research.
