The wettability of materials is important in many natural and industrial processes. In this thesis, the wettability of chemically heterogeneous surfaces was investigated with respect to the size, shape and orientation of individual defects. Heterogeneous surfaces were structured by photolithography, using self-assembled monolayers (SAMs) of diverse functionality and, thus, wettability. In order to maximise any possible departure from theory, the wettability of the high-and low-energy regions of these heterogenous surfaces was chosen to differ substantially. The purity of the defects was optimized by studying the SAM formation and the patterning processes, whilst the influence of roughness was minimized. The focus of this work is therefore on chemical heterogeneity. The Wilhelmy plate method was employed to ascertain the wetting behaviour of individual high- and low-energy defects. Simultaneous measurement of the capillary force and the plate position allowed full characterization of the wettability at the defect boundaries. In addition, integration of the Wilhelmy trace enabled the work associated with advancing or receding a liquid over these defects to be quantified. / The defect boundary orientation was of critical importance to the wetting behaviour. Wetting boundaries perpendicular to the liquid front did not result in any deviation from theoretical predictions. Wetting boundaries that were arranged parallel to the liquid front, however, caused contact line pinning which, in turn, caused contact angle hysteresis. Therefore these boundaries are directly responsible for the departure from wetting theory for heterogeneous surfaces (i.e. the Cassie Equation). These observations are consistent with earlier studies of wetting hysteresis by Johnson and Dettre (1964) and Neumann and Good (1972). Extending their work, this thesis examined the extent of wetting hysteresis at individual rectangular defects with respect to the defect dimensions. The nature of wetting hysteresis was studied quantitatively and, as a result, a simple model for hysteresis was proposed. This model predicts that, for a high-energy defect, the work associated with an advancing liquid will always be less than the theoretical value due to capillary rise within the effect. However, the work associated with a receding liquid will be equal to the theoretical prediction (the opposite is true for low-energy defects). The proposed model was validated for two different liquids (water and ethylene glycol) rectangular and circular defects of macroscopic dimensions. For these surfaces, the empirical data and the proposed model showed excellent agreement for both high- and low-energy defects. This agreement is strong evidence that high- and low-energy defects induce distinctly different wetting behaviour on heterogeneous surfaces. / The proposed model for hysteresis was qualitatively applied to heterogeneous surfaces containing micro- and nanoscopic defects. For micropatterns of high- and low-energy defects, of identical composition, the wettability was entirely different. High-energy defects induced a deviation from theory for only the advancing contact angles, whilst low-energy defects only influenced the receding angles. These observations were qualitatively consistent with the outcomes predicted by the proposed model for hysteresis. For nanoscopic defects, however, the advancing contact angles were consistent with Cassie's law within reasonable error. The derivation of Cassie's equation assumes that there is no capillary rise within the chemical defects. The capillary rise within a nanoscopic defect must be extremely small, according to extrapolation of measurements performed on macro and microscopic defects. Therefore, consistency between the wettability of nanoscopic defects and Cassie equation predictions might be expected. The hysteresis mechanism outlined within this thesis can be quantitatively applied to macroscopic defects, whereas its application to micro and nanoscopic defects is qualitative. / Finally, several applications of this fundamental research, which are directly related to real systems, have been outlined. These include mineral flotation, oil recovery, liquid movement, directed crystallization, Secondary Ion Mass Spectrometry for flotation analysis and patterning of inorganic surfaces. / Thesis (PhDApSc(MineralsandMaterials))--University of South Australia, 2004.
| Identifer | oai:union.ndltd.org:ADTP/267545 |
| Creators | Priest, Craig Ian |
| Source Sets | Australiasian Digital Theses Program |
| Language | English |
| Detected Language | English |
| Rights | copyright under review |
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