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Published on September 3rd, 2013 | by Daphane Ng

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# Droplet evaporation: More than meets the eye

By Joel Alroe, Queensland University of Technology

This student took part in the 2012/13 AMSI Vacation Research Scholarship program. For more information on this year’s program please click here.

Droplet evaporation is a familiar process that we encounter in many forms. It cools us down when we sweat and converts fuel to a flammable vapour in the engine of our cars. But why exactly do droplets evaporate, and how can we control it?

Of course we expect that a droplet in a hot environment will evaporate, so temperature is definitely an important factor.

Vapour pressure, or humidity in the case of water, also plays a part in this process. The saturation vapour pressure refers to the maximum amount of vapour that can be present at a particular temperature and liquids typically evaporate to increase the surrounding vapour pressure to this level. This is why wet clothes take far longer to dry in very humid climates – the high humidity leaves little incentive for the water to evaporate further and increase the vapour content in the air.

But these are only the tip of the proverbial iceberg. The complexity can be increased even further if we consider the effect of turbulent air currents which can deform the droplets and carry away the surrounding vapour. Electrically charged droplets have been observed to periodically eject smaller droplets as they evaporate and, in some nanofabrication processes, the droplets of chemical reactants form solid shells which slow any further evaporation.

So while you might expect that, after almost 40 years of extensive research, we might have a universal mathematical model for simulating droplet evaporation, in practice it is often necessary to develop models on a case by case basis to address the specific needs of a particular scenario.
How do we do this? It is often useful to begin with a straightforward case containing only the most fundamental characteristics of the desired evaporation scenario. For example, one of the simplest cases might model a single water droplet surrounded purely by water vapour.

With no other factors interfering with the scenario, we can be sure that mass, momentum and heat can only be transferred between the droplet and the vapour. Also as mentioned above, we know that evaporation will occur until the vapour reaches its saturation point. So by imposing well understood conservation equations which govern these processes, we can fully track the evaporation process.

With this groundwork in place, the complexity of the model can now be increased to address additional factors such as impure droplets or realistic atmospheric conditions. Ultimately the model can then be used to perform simulations exploring its response to varying conditions in great detail.
These droplet evaporation models are absolutely vital for a broad range of applications including optimising fuel injected engines, effective dispersal for agricultural spraying and even in fabricating novel new nanomaterials for the storage of liquid radioactive waste.

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