24 days of kitchen oceanography
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Diffusive layering
Why are there layers in that coffee?
Eksperiment 17:
Latte actually consists of three layers: warm milk, espresso and milk foam. In the picture you can see at least 5 distinct layers. This is due to the process of double- diffusive mixing. Since temperature equilibrates faster than salinity, warming from below leads to a rising motion above the interface, while simultaneously cooling from above leads to a sinking motion at the same interface. The interface is thus sharpened.
You might have seen these layers when you have ordered a latte at a café. However, due to the covid-19 situation, this is not easy for everyone to do now. The experiment is on the other hand easy to do at home. The only reasons you might not be aware of this happening are
- You don’t drink coffee in a glas
- You don’t add milk
- You stirr too much and/or too soon
- You drink the coffee too early
So. If you avoid all that, this is what you will see: A stratification with nice layers forming!
All you need to do is
- Make coffee
- Pour coffee into glass
- Drop a teaspoon or two of sugar into the glass (no stirring right now!)
- Pour some milk into the coffee (don’t stress if it looks very turbulent, it’ll settle…)
- Observe layers forming!
- (Optional: When you feel like you’ve seen enough of those layers, stirr carefully so a little of the sugar gets dissolved into the lowest layers of the coffee-milk-mixture
- Observe a different set of layers forming)
What’s happening here is that cold milk is denser than hot coffee, therefore it sinks to the bottom. But at the interface, there is a fast transfer of heat and a much slower transfer of matter, so the milk gets warmed up and raises until it reaches a level of its own density (the new interface). Within that layer, properties are pretty much homogeneous, but at the interfaces above and below, there are gradients both in temperature and coffee/milk content (salinity in the ocean). So at each interface, a new diffusive layer will form. Over time, many layers develop.
When we stirr in the sugar after some time, we add a new dissolved substance that influences density, and we re-start the diffusive layering process.