| dc.description.abstract | 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. | en |
| dc.relation.hasversion | Sawiak, L., Bailes, K., Harbottle, D., and Clegg, P. S.. \Mixing Time, Inversion and Multiple Emulsion Formation in a Limonene and Water Pickering Emulsion." Frontiers in Chemistry 6: (2018) 132. https: //www.frontiersin.org/article/10.3389/fchem.2018.00132. | en |
