Abstract
It has previously been demonstrated that particle-stabilized emulsions comprised of limonene, water and fumed silica particles exhibit complex emulsification behavior 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 and the availability of particles during emulsification.
Highlights
That emulsions can turn “inside out” with prolonged mixing is a phenomenon which has been exploited in butter making for millennia
As will be confirmed using fluorescence confocal microscopy below, the sample is full of water droplets indicating that it is a particle-stabilized high internal phase emulsion (HIPE)
Confocal microscopy and rheology we have explored the behavior of Pickering emulsions comprised of water, limonene and fumed silica as a function of mixing time
Summary
That emulsions can turn “inside out” with prolonged mixing is a phenomenon which has been exploited in butter making for millennia. The process is quite complex, involving very many components some of which are lost to the buttermilk. The same phenomenon has been noted much more recently, under high shear, for particle-stabilized emulsions. These emulsions are simpler: they incorporate only three components, most commonly none of them is close to its solidification temperature. In spite of its simplicity this type of inversion process is not yet fully understood. Without this understanding predictive control of emulsification using particles is not possible
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