Effect of roughness on emulsification physics and phase inversion
Sawiak, Laura Jane
Emulsions, a kinetically stable dispersion of droplets within a second, immiscible fluid, are industrially important. Phase inversion is the process by which the dispersed phase becomes continuous and vice versa. This is exploited in industry, for example in churning cream to butter in the food industry, or use of the phase inversion temperature emulsification method in the transport and production of asphalt for roads. This thesis considers emulsions stabilised by solid particles, known as Pickering emulsions. Whether the dispersed phase is oil or water is thought to be governed by particle wettability. It has previously been demonstrated that emulsions comprised of limonene, water and fumed silica particles exhibit complex emulsification behaviour as a function of composition and the duration of the emulsification step. Most notably the system can invert from being oil-continuous to being water-continuous under prolonged mixing. Here, we investigate this phenomenon experimentally for the regime where water is the majority liquid. We prepare samples using a range of different emulsification times and we examine the final properties in bulk and via confocal microscopy. We use the images to quantitatively track the sizes of droplets and clusters of particles. We find that a dense emulsion of water droplets forms initially which is transformed, in time, into a water-in-oil-in-water multiple emulsion with concomitant changes in droplet and cluster sizes. In parallel we carry out rheological studies of water-in-limonene emulsions using different concentrations of fumed silica particles. We unite our observations to propose a mechanism for inversion based on the changes in flow properties during emulsification. An in-depth explanation of image processing and analysis techniques is given, including code from matlab. It has been demonstrated repeatedly that the variation in surface wettability with roughness is non-monotonic. Historic studies are based on measuring maximum capillary pressure of Pickering emulsions stabilised by particles of different roughnesses before collapse or contact angles of fluid droplets for rough surfaces. In this thesis, pendant drop tensiometry was carried out for vesicles of variable roughness. The vesicles are self-assembled triblock copolymers. The roughness is dependent upon the length of one of the blocks, which undergoes nano-phase separation in the vesicle wall. A single droplet of oil was suspended in water, before particles were added to the bulk phase. Four oils were tested, and all suggest that wettability indeed has a non-monotonic change with roughness. Finally, this work turns to emulsification properties of an industrial stabiliser, with which a range of emulsions with 20% water volume fraction were fabricated with different oils. For various combinations of oil and water, studies were made on changes in emulsion type and droplet size with particle concentration. Two classes of behaviour were found. For dodecane and water, the resulting emulsion was always a high internal phase emulsion (HIPE), and changing the concentration of particles reduced the droplet size. These emulsions were dyed and viewed under confocal fluorescence microscopy, where droplets were visualised with aggregated material at their interface. In combination with fast dynamics seen through pendant drop tensiometry, it was concluded that adsorption has a strong molecular component, with material sitting on the fluid-fluid interface. With other oils, low concentrations of stabiliser result in small water-in-oil droplets. Increasing the powder concentration results in progressively more oil-in- water droplets until a phase inversion occurs, and the emulsion becomes an oil-in- water HIPE. Confocal fluorescence microscopy of these samples showed fibre-like structures within the continuous phase. Supporting rheological experiments upon dispersion of stabiliser-in-water at various concentrations showed a sudden jump in viscosity after a critical concentration. The corresponding flow curve showed shear thinning as shear rate increased, suggesting gel-formation. These observations suggest that within the continuous phase a percolating network can form with suffcient stabiliser, immobilising droplets. This is a completely different stabilisation mechanism compared to the previous case. Different behaviour for different oils may be due to their differing physical and chemical properties, for instance their polarity or viscosity.