I have been a bit quiet on here recently, because I had so many exciting projects going on that I did not manage to document them in real time (well, not on here anyway, but partly on my Insta).
One of those projects is on #KitchenOceanography with coffee, where I have compiled a lot of interesting experiments into a postcard. And the postcards arrived yesterday! (Thanks, Vanja, for picking them up!)
My super cool new postcards in front of the first slide of my presentation, because I had to take a picture when they arrived and that was the only background available :-D
Juuust in time for the “Hammer talk” presentation that I was scheduled to give at the Geo department of the University of Oslo. Where — you guessed it — I invited people to play with coffee!
Lots of things happening in coffees everywhere!
And I was so excited to see how well people played, and how beautiful the stratifications turned out!
Diffusive layers formed by double-diffusive mixing in a milk-coffee mixture
Here is even an internal wave on the stratification!
I thought I had posted the picture below some time in winter already, but when I recently searched for it, I couldn’t find it. So either I didn’t post it, or I didn’t post any sensible search terms with it, in any case: It’s useless. So here we go again.
Below you see two tea bags that were placed into cold (left) and hot (right) water at the same time. On the left, the tea is sinking down in streaks, while at the same time on the right everything is completely mixed through and through, showing how molecular diffusion depends on the temperature. Which is why we usually make hot tea.
Funnily enough, as I was about to write this blog post and had the picture already open on my laptop, I felt thirsty and decided to prepare a cold brew tea, which you see in the picture below. Here again you see the tea spreading from the tea bag, but it comes out in those plumes and only slowly diffuses throughout the whole carafe.
This would of course be easier to see had I chosen a white background, but since I am still so touched that my friends showed up at the train station with a flower and a flag on Friday, and also since this is literally the spot I put the tea down after I had prepared it, you get to enjoy a view of my flower and flag!
Also that fake flower on the left makes for really interesting reflections on the carafe. Especially the top two that are joint in the middle!
Day 17 of my 24 days of #KitchenOceanography is about double-diffusive layering, and the post is using “go have a nice latte” as instructions. However, in times of Covid-19 (and a hard lockdown in Germany since Wednesday) that’s unfortunately impossible. And since Lars Henrik said that he was especially curious about this experiment, and today is the day of my “inaugural” lecture back at GFI in Bergen, I thought that was reason enough for a little upgrade to my advent calendar.
So here we go: Looking at diffusive layering in a coffee-and-milk scenario!
The experiment is SUPER easy. 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
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)
That’s it! Awesome, isn’t it?
Here is a movie the full experiment:
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.
Out-takes: Can you guess what happened here? (It does look super awesome, but why is the stratification being eroded?)
I can tell you. The first time I ran the experiment, some sugar stuck to the condensation inside the glass just above water (coffee?) level. As there was more and more water vapor rising from the coffee and more and more condensation collecting on the side of the glass, sometimes some of that sugar sinks down in dense plumes that break through some of the layers (but isn’t it awesome to see how the layers still catch some of the dense plume?!)
Check out the movie of that experiment, it’s awesome!
Have you ever noticed how, if you stir your latte*, when you pull out the spoon it’s piping hot, yet there is no steam rising from the latte itself? That’s because the milk foam on top is such a good thermal insulator thanks to all the tiny air bubbles trapped in it. Cool, isn’t it?
*I never noticed before today, when my friend Sara pointed it out, because I have NEVER before put a spoon in my latte. Because I am always observing double-diffusive mixing in my latte and would never do anything that might destroy the stratification. But this once it might have been worth it. The things we do for science… :-D
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! :-)