Showing double-diffusive mixing in tank experiments is a pain if you try to do it the proper way with carefully measured temperatures and salinities. It is, however, super simple, if you go for the quick and dirty route: Cream in tea! Even easier than the “forget the salt, just add food dye” salt fingering experiment I’ve been recommending until now.
The result of double-diffusive mixing of cream in tea is probably familiar to most (see above), but have you ever looked closely at the process?
Below, we pour cold cream into hot tea. The cream initially sinks to the bottom of the tea cup, but then quickly heats up and fingers start raising to the surface of the cup. They are visible as fingers because while the heat has quickly diffused into the cream, the actual mixing of substances takes longer and the opaque milk stays visible in the clear tea. Only when the fingers have risen to the surface the substances begin to mix due to shear and diffusion of substances. Hence the name “double diffusion”: First diffusion of heat, then of particles afterwards.
Pretty cool, isn’t it?
If you happened to stir the tea before pouring the cream, it looks even more awesome. Home-made galaxies :-)
And isn’t it fascinating how the blob of cream in the middle of the cup stays intact for quite some time?
So now you know the only reason why I am drinking black tea: So I can do salt fingering experiments with it! :-)
Yes! You should add marshmallows to prevent heat transfer both by evaporation and conduction.
Elsa, I’m pretty sure it was you I had that hot chocolate with back in 2011. Recognize your hands?
Actually, no matter what temperature you like your chocolate best at – you should always add marshmallows! :-)
For those of you who want to read more about marshmallows and ocean mixing, check out a very nice post here. For those others getting worried that I’ll only talk about tea until the end of time – nope! Tea week is now officially over and we’ll be back with “real oceanography content” pretty soon!
After the frustrations of taking pictures of steam in my last post, I decided that I could use the very same cute mug to show other stuff, too. I know it has been done over and over again, but we have new students every year, don’t we, so someone has to keep telling the old stories, right?
So. When should you pour the milk into your tea? Right away or a little later?
Milk and tea
The answer, as you know, is “it depends”.
Do you want your tea as hot as possible? Then put the milk in right away and it won’t cool the tea down as much. Want the milk to cool down the tea as much as possible? Then wait for as long as you can before pouring it in.
The explanation behind this is of course that the cooling due to evaporation is happening faster the larger the temperature difference between the tea and the surrounding air. If you let it sit without milk, due to the larger temperature difference it cools down faster than if you poured in the cold milk, thus cooling it closer to room temperature, and then waited.
And there are even occasions when you would you put milk into the cup before adding the tea: If you have delicate china and don’t want to risk ruining it by pouring in almost boiling tea. Plus allegedly that way the milk doesn’t scald and form those weird flakes?
Why would it be interesting to talk about this in a science class?
As a kid I used to wonder why blowing on a hot soup or beverage should be a good idea. Wouldn’t my breath be warmer than room temperature, and hence shouldn’t the soup get warmer instead of colder?
Then I didn’t think about this question for 25 or so years (scary, I know), and then today, when I was blowing on my tea, I realized that by now I knew why I was doing it, even though I had never related my science knowledge to the everyday act of blowing on hot tea.
So why do we blow on hot tea?
The main reason is that at the tea’s surface, evaporation takes place. We can oftentimes see the steam coming off. The molecules that left the cup condense in a fog over the cup. If they stay in place, evaporation will slow. If we blow them away, the air is replaced with colder surrounding air, and evaporation continues.
Another reason is that as we blow on the surface, we create ripples. Hence the surface area is larger than before and more exchange can happen over a larger area. But I would guess that that effect is much smaller than the first one.
The main reason I wanted to write this blog post was because I could see the picture I wanted to show before my eyes: This sweet cup with the rabbit on the handle and the steam rising from it. Turns out it is really difficult to take pictures of that! At least with my camera and my lack of patience. And believe me – I tried for a full 15 minutes with different light sources at different angles and everything! So for now all you get to see is the video below where it is slightly better visible than in a still picture – and please try to imagine the steam! And I will be back once I’ve figured out how to document it properly!