Tag Archives: kitchen oceanography: food related

Molecular diffusion at different temperatures (involving tea bags and some convection)

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!

Diffusive layering — a super easy experiment!

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

  1. Make coffee
  2. Pour coffee into glass
  3. Drop a teaspoon or two of sugar into the glass (NO STIRRING RIGHT NOW!)
  4. Pour some milk into the coffee (don’t stress if it looks very turbulent, it’ll settle…)
  5. Observe layers forming!
  6. (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
  7. 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!

Destroying the diffusive layers in a latte in order to observe the insulating properties of milk foam

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

Do you love #kitchenoceanography, too?

Ostfriesentee — Double diffusion in a tea cup

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! :-)

#KitchenOceanography with Judith and her hot chocolate

Let’s talk about zonal jets! They keep popping into my life all the time right now, and that has got to mean something, right?

Zonal jets, for all that are not quite familiar with the term, are fast-flowing currents (i.e. “jets”) that move along lines of constant latitude (therefore “zonal”). The occur in the ocean (e.g. the Antarctic Circumpolar Current, or the Gulf Stream after separating from the coast) and in the atmosphere (e.g. the subtropical jets stream). And you might be familiar of pictures of Saturn with all the belts around it? Yep, zonal jets!

In December I went to the Science and Industry Museum in Manchester (a.ma.zing place!) and they had one exhibit there that shows zonal jets: A sphere sitting inside a transparent sphere with some sort of fluid between the two. You can put the outer sphere in rotation and, through friction, this puts the fluid in motion. But instead of all the fluid moving with the outer sphere, there is of course also friction with the inner sphere, so a shear flow develops, which breaks up into those zonal jets (which then break up into all the eddies when the outer sphere slows down again).

Please excuse the crappy video. You see the largest part of the upper half of the sphere, but I was filming with one hand and turning the thing with the other… And I didn’t plan on writing anything about it, but then this happened: My friend Judith (check out her Instagram!) and I went on a mini cruise (all the way across Kiel canal!) in freeeeezing temperatures, and therefore obviously ended up with this:

Picture by Judith Schidlo (check out her Instagram!)

And this is where kitchen oceanography comes in. What do you think happens when you drop in that yummy chocolate and start stirring? This!

Do you see how the fluid doesn’t move solid body-ish, but how there are jets and then more stagnant areas? Doesn’t this make you want to have a hot chocolate, and Right Now? For scientific purposes, of course…