The surface force apparatus (SFA) has been an important technique for making direct force measurements and has contributed enormously to our understanding of colloidal interactions. The conventional SFA has been limited to measuring forces between solid surfaces, until recently when a modified SFA was developed at the Ian Wark Research Institute [1]. A fluid drop (mercury) is introduced into the apparatus which allows a range of deformable surfaces to be studied in the SFA. This project is an extension of this technique. Interactions between a mica sheet and a mercury drop are studied, including the modification of mercury with self-assembled monolayers (SAMs) of thiol surfactants, and the drop deformation due to non-equilibrium adsorption effects and hydrodynamic forces. / SAMs can form spontaneously onto a surface by immersing the substrate into an appropriate surfactant solution. They have been used, generally formed on gold surfaces, for biosensors, chemical sensors, micro-electronics and detection of DNA and protein adsorption. In our study, mercury was chosen as the substrate, for its defect-free, renewable and molecularly smooth surface. The additional advantage of being an ideal polarisable electrode allows a potential to be applied to the mercury, and hence control of the surface forces. The charging behaviour of the mercury is changed by introducing a SAM onto the surface. An uncharged SAM (11-mercapto-1-undecanol or 11-mercaptoundecane) modifies the dipole potential of the mercury by replacing the water molecules oriented on the surface, whereas a charge SAM (11-mecapto-1-undecanoic acid) brings additional charges to the surface. / Drop deformation is an important factor when deformable surfaces are involved in colloidal systems, e.g. emulsions, foams, in mineral flotation and in biological systems. Drop profiles of a mercury surface which is already close ( ̃50 nm) to a flat mica sheet, with or without a SAM, were measured using the SFA technique. For the SAM-modified mercury, the negatively charged functional group (-COO⁻) yields a repulsion against mica, and a thin film is formed between the surfaces. When the applied potential was scanned negatively, desorption of thiols occurred at certain potentials, increasing the local solute concentration in the solution. The restricted flow of the solute within the small gap creates an excess osmotic pressure in the thin film compared to the bulk solution. As a result, the film pressure exceeds the internal pressure of the drop, inverting the drop curvature and forming a dimple. We propose that the drainage of the dimple is a diffusion-controlled process, which is supported by the comparison of the data with a simple model calculation. / For the bare mercury drop, a negative potential was applied to the mercury to provide a repulsion to form a thin film. Mica was then driven towards the mercury with an abrupt step. Beyond certain step sizes, a rippled shape - which we dub a “wimple” - was observed before it evolved into a classical hydrodynamic dimple. At small step sizes, no wimple was observed, but curiously the film in the central part thickens before eventually thinning out. This shows that fluid first flows towards the central axis before reversing its flow direction and flowing radially outwards. / Thesis ([PhDApSc(MineralsandMaterials)])--University of South Australia, 2005.
| Identifer | oai:union.ndltd.org:ADTP/267386 |
| Creators | Clasohm, Lucy Y. |
| Source Sets | Australiasian Digital Theses Program |
| Language | English |
| Detected Language | English |
| Rights | copyright under review |
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