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.