Gavin Braithwaite - Thesis Abstract

   This thesis describes the design and development of a force sensing tool using the principles of the Atomic Force Microscope. It has been designed for the aim of investigating and understanding the interactions between a near-colloidal sized particle and a surface. The reasoning behind these choices, and the design constraints and problems encountered are outlined. Additionally the techniques and approaches that make the mounting of close to colloidal sized particles viable are discussed, along with the expected properties of such systems.
   The experimental section deals with two main polymeric systems. Initially Poly (ethylene oxide) adsorbed to glass surfaces is described as a model, well characterised, system. These results will be compared with theory and with data previously obtained on this system with the mica Surface Force Apparatus. A description is then made of the use of the modified AFM to study gelatin layers adsorbed to glass surfaces. Finally a discussion is presented of the use of the force microscope to determine the rheological (viscoelastic) properties of these adsorbed layers.
   In the poly (ethylene oxide) study the interaction distances are seen to be longer than those expected from SFA or theoretical work. This is due to the increased resolution of the force microscope technique. Bridging interactions are rarely seen, and this is attributed to the non-equilibrium nature of the AFM approach and the high scan speeds used. The gelatin results demonstrate denser adsorbed layers. The layer is also seen to be strongly electrolyte and pH sensitive, and the structure is observed to be a result of the past adsorption conditions as well as the current conditions. The preliminary results from the oscillatory technique yield generally qualitative data, but give a good indication of the transition from viscous to elastic properties as the probe moves from solvent to adsorbed polymer. In particular frequency dependent effects are apparent, and the problems with obtaining an equipment independent result are also outlined.

G. J. C. Braithwaite (1997) "Colloidal Interactions Measured Using a Modified Force Microscope" In Chemical Engineering, Imperial College of Science Technology and Medicine, London University, London, pp. 311.