Mixing and transport of particles and droplets co-suspended in a liquid-continuous phase

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Every day we encounter systems containing droplets and particles. The combination of the two is needed for inkjet printers; the mouth-feel and stability of many food products, like salad dressing or yogurt also depend on them, as does the preparation of many medicines. In these cases, the particles are very small relative to the size of the droplets, so they collect on the droplet surfaces, forming "suits of armour" that make the droplets highly stable. In cases where the particles are larger than the droplets, the behaviour of a system can be totally different, and in fact cannot yet be well predicted. This is especially true in a system where flow or mixing is involved. Some diverse examples include the mixing/reaction process that provides us with many of the common plastics we use, and in the production of crude oil containing water droplets and sand. Under some conditions, the particles contribute to droplet break-up, i.e. the droplets will be smaller than they would be under solids-free conditions. This can make it much more difficult to separate the droplets from the liquid that carries them. Also, the presence of the droplets can make it more difficult to transport the particles, both in terms of the energy required and the ability to keep them from settling. Particle accumulation can lead to pipeline corrosion and failure, since the settled particles trap water near the pipe wall. Currently, the ability to model - or even to understand - the interactions among droplets and larger particles is extremely limited. One problem is that almost no fundamental research has been done on these systems. Also, despite the fact that the behaviour of droplets alone or settling particles alone is reasonably well understood, the combination of the two does not seem to follow the same rules. This research program is designed to address some key knowledge gaps. In the short term, the goal is to determine (i) how the presence of the droplets affects the settling characteristics of the particles and (ii) how the presence of the particles produces changes in the droplet sizes. In one series of experiments, identical systems of droplets and particles will be tested in a pipe flow, a custom-built shearing cell (two concentric cylinders, with flow between them produced by rotating the inner cylinder) and in a mixing tank. In each case the power needed to suspend the particles with and without droplets will be measured. A second series of tests will be conducted where the effects of energy added to the mixture (think mixing speed in a tank or velocity in a pipe), particle size and concentration on droplet size will be measured. The ultimate goal is to produce models of particle settling and droplet size, which would apply to any type of mixing or transportation process. The results are needed in a wide range of industries: examples include cleaner, more energy efficient processes for oil production and refining, and to produce biofuels from plants.