Structure and properties of tunable Pickering emulsions : a thesis presented in partial fulfilment of the requirements for the degree of Doctor of Philosophy in Chemistry, Massey University, Manawatū, New Zealand
The ability to design soft-materials with targeted rheological properties is a vital part of the modern world. One type of soft-materials that are important across a range of industries from food and consumer goods, to paints and oil products are emulsions. Generally speaking, emulsions are mixtures of oil and water, with a stabiliser which controls the interactions between the droplets. Pickering emulsions are a subset of emulsions which utilise a solid nanoparticle stabiliser to increase droplet stability. Pickering emulsions are becoming increasingly attractive due to the wide, and varied, range of stabilisers available, along with the remarkable stability that these systems can have. Recently a number of workers have demonstrated the ability to tune the interactions between high volume fraction emulsions (with modifications prior to emulsification), resulting in the changes to the bulk strength of the emulsion systems and the droplet size distribution. Additionally, some unique yielding behaviour has been uncovered in certain situations, presenting an area which can be further investigated. The work presented in this thesis has developed low volume fraction emulsion systems with interactions between the droplets that were tuned post-emulsification. This was carried out through two distinct processes, modification of the Debye length with the addition of salt, and modification of the surface charge of the Pickering emulsifier by changing the pH of the aqueous phase. The results of this have demonstrated low volume percentage emulsions with interactions ranging from highly attractive through to repulsive between the droplets while maintaining a consistent droplet size. These new systems have demonstrated interesting rheological properties, with the attractive systems demonstrating significantly higher strength than anticipated. in certain cases these low volume percentage emulsion systems were demonstrated to show multi-stage yielding behavior, something that has previously only been seen for higher volume fraction systems. In addition to these properties, this work is thought to present the first case of a titania stabilised Pickering emulsion system with tunable interactions, demonstrating a new material for future development.