Microscopic study of the freeze-thaw stability of particle-stabilised emulsions
Dickinson, Katy Louise
Particle-stabilised, or Pickering, emulsions are widely used in the food and personal care industries and are present in common household products such as homogenised milk and food spreads. Pickering stabilisation has also been suggested as a method of increasing product shelf life over conventional surfactant stabilisation. It is therefore important to characterise the behaviour and stability of Pickering emulsions subjected to processes used in industry and daily life. These processes have the potential to alter the taste of food products or the eﬀectiveness of personal care or pharmaceutical products. Freezing is one process of interest as it is used for product transport and in order to increase shelf life. Some products are also used when frozen, such as ice cream and frozen desserts. In this thesis, confocal ﬂuorescence microscopy is used to study the microscopic behaviour of model water-in-oil Pickering emulsions subjected to freeze-thaw cycling of the continuous phase. Hexadecane oil and poly(methyl methacrylate) particles are used and both are ﬂuorescently labelled, allowing all three emulsion components to be distinguished individually. Initially, we study the behaviour of the oil alone, both experimentally and in simulation, under uniform and non-uniform cooling. In non-uniform cooling, the sample is cooled more rapidly from one end, whereas in the uniform setup the sample is cooled evenly. Simulation results show that the two setups lead to diﬀerent temperature and ﬂuid ﬂow proﬁles throughout the sample: the nonuniform setup causes a temperature gradient through the oil which is essentially eliminated in the uniform setup. Experiments conﬁrm the presence/absence of these gradients and show that the temperature gradient aﬀects the crystal growth in the oil. Dilute emulsions are studied under two diﬀerent non-uniform conﬁgurations. In the ﬁrst, droplets interact with crystals growing from many directions. Deformation of the droplets is observed as growing crystals progressively trap the droplets, stretching them towards parts of the sample where the oil is still liquid. The droplet deformation is characterised via image analysis to compare the droplet shape before and after freeze-thaw cycling. Irreversible shape changes are explained by the behaviour of the particles on the interface. Particles initially jammed on the interface become unjammed as the interfacial area increases during droplet deformation in the freezing process. Upon thaw, the particles rejam at a lower packing fraction meaning that droplets are permanently deformed. In the second conﬁguration, the droplets interact with a moving solidiﬁcation front in which crystal growth is unidirectional. Again deformation of the droplets is observed as droplets are trapped by the front. In some cases particles are trapped and pulled oﬀ the droplet interface, temporarily releasing the droplet and leaving it with a lower surface coverage of particles. In both cases, particle behaviour at the interface is key in determining the droplet behaviour. Finally undiluted emulsions are subjected to freeze-thaw cycles under both uniform and non-uniform conditions. In both cases droplets are deformed by the crystal growth, but during non-uniform freezing biliquid foam regions are formed which are not present in the uniformly frozen samples. The eﬀects of droplet size and cooling rate are also explored for uniform freezing. In both cases, irreversible changes are observed in the emulsion structure as a result of the process. As in the single droplet case, these results suggest that particles at the interface become unjammed during the freeze cycle and upon thawing re-jam at a diﬀerent packing fraction. Here deformation is enhanced by partial coalescence which promotes jamming in a deformed state through reduction of interfacial area at ﬁxed volume. In the absence of particles, similar freeze-thaw cycling of surfactant emulsions does not cause the same irreversible change to the emulsion structure as in the particle-stabilised emulsions. In addition, under certain conditions, Pickering emulsions after a freeze-thaw cycle exhibit some bicontinuity, suggesting that freeze-thaw cycling may be a promising route to producing bicontinuous emulsions. Collectively, these results improve our understanding of the behaviour of particlestabilised emulsions under freeze-thaw cycling, which is an industrially relevant process. They show the importance of the interfacial particles in controlling emulsion structure and they provide insight into the interaction between soft materials, like droplets, and hard materials, such as ice crystals, at the microscopic level. Although freezing can prolong shelf life, and Pickering stabilisation can enhance emulsion stability, the combination of these two must be treated with care as together they can cause irreversible structural damage, reducing rather than enhancing product stability.