Evaporation!

A few weeks ago, our new boss, Vice Chancellor Professor Max Lu, visited the physics department. He started last year and is touring the departments in his new domain. I think it went well, it was good to both tell him what we do, and listen to what he had to say. In a UK academic environment that often looks dominated by league tables and competition, it was good to hear him stress the importance of our teaching students great stuff, stuff that they will use and that will make them better people.

During the visit I gave him a five-minute spiel on what the soft matter research group of the physics department does. At the end he asked to name one big research challenge. I am never prepared for questions like that, they always blindside me. So I did a brief open-mouthed guppy impersonation, then blurted out ‘evaporation’.

Evaporation is one of those commonplace things that takes place all around us, and that we can just lazily assume just happens, and so should be easy to understand. But evaporation is really rather complex.

To get the water molecules out of the liquid and into the air above takes energy, which must come from somewhere – the energy of the sun in the case of a puddle drying on a sunny day. But evaporation is extremely slow if the water molecules just leave the liquid and hang around above the puddle, slowly diffusing away. This is why wind greatly accelerates evaporation, the wind blows the water molecules away from the surface of the puddle, preventing them from forming a very humid layer on top of the puddle. If there is a very humid layer on top of a puddle, then almost as many water molecules in vapour hit the water surface and are reabsorbed, as leave the water.

Understanding whether energy input, or water molecule movement, controls the rate of evaporation, is tricky. And if you look more closely you find other problems. For example, we know that evaporation tends to cool liquids, this is why we blow on hot coffee. So, if evaporation is fast, the top of the liquid may cool, and so working out what the temperature actually is in the liquid is may be very difficult.

So evaporation is a complex, messy process. It is telling that at the time of writing its Wikipedia page has two messages on it requesting help from experts. I guess this should teach us that just because a phenomenon occurs every day in our kitchens, does not mean it is trivial to understand.

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