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
- 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)
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
Do you love #kitchenoceanography, too?
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! :-)
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:
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…
Many of my kitchen oceanography experiments use dyed ice cubes, usually because it makes it easier to track the melt water (for example when looking at how quickly ice cubes melt in freshwater vs salt water, or for forcing overturning circulations).
But the dyed ice cubes tell interesting stories all by themselves, too!
Salt water doesn’t freeze
“Salt water doesn’t freeze”? Then how do we get sea ice in the Arctic, for example?
When freshwater freezes, the water molecules arrange in a hexagonal crystal structure. If there is salt (or anything else) in the water, however, the ions don’t fit into the regular structure. Ice freezes from the water molecules, and all the disturbances like salt get pushed in the last remaining bits of liquid water, which therefore gets higher and higher concentrations of whatever was dissolved in it. As those little pockets with high concentrations of salt get cooled further, more and more water molecules will freeze to the surrounding freshwater ice, leading to even higher concentrations of salt in the remaining liquid water. So the freshwater is freezing, while rejecting the salt.
Of course if you cool for long enough, also the last bit of remaining water will freeze eventually, but that takes surprisingly long (as you can try by freezing salt water in some of the cups ice cube trays and freshwater in others, for comparison. Also the structures of freshwater vs saltwater ice look very different and are interesting to look at, see how here).
When the ocean freezes, this rejection of high-salinity water leads to interesting phenomena: Even when you melt it again to include all the pockets of high salinity water, sea ice will have salinities way lower than the water it froze from. This is because of a process called brine release. Since you are cooling the ocean from above, sea ice also forms from the surface downwards. This means that it is easy for the salty water to be pushed, “released”, or “rejected”, downwards, into the liquid ocean below. That ocean will then of course get more salty right below the ice!
In the picture below you see something similar happening in the left pictures. Instead of salt, I have used blue food dye for visualization purposes. In the top left, you see an ice cube exactly as it looked when I took it out of the ice cube tray it froze in, and in the bottom left you see the same one after I let it melt a little bit so the surface got smoother and it got easier to look inside (a lot more difficult to hold on to, though!).
Do you see how the top part of the ice cube is pretty much clear, while the bottom part is blue? That’s because it froze top-to-bottom and the dye got pushed down during the initial freezing process!
Stuck in an ice cube tray
Something else that you see in the top left picture is the effect of the ice cube being stuck in the ice cube tray as it froze: Pores filled with blue dye that had nowhere to escape!
Had I taken out those ice cubes earlier, when they had just frozen half way through, we would have found a clear ice layer floating on a cold, blue ocean. Maybe I should do that next time!
Checking on the temperature distribution of your freezer
Something else fun we can observe from the right pictures: Here, the dye was concentrated towards the center of the ice cube rather than the bottom! How did that happen?
My theory is that those ice cubes were located in an area of the freezer that was cooling from all sides (more or less) equally, whereas the ones shown on the left must have been placed somewhere where cooling happened mainly from the top.
So if you ever want to know where the cooling in your freezer happens, just put lots of dyed little water containers everywhere and check from which side the dye gets rejected — that’s the cooling side! Actually, I might check that for the freezer below just for fun. Would you be interested in seeing that done?
Now it’s your turn!
Let’s look back at the ice cubes I froze yesterday in the picture above. I’ve now written about a lot of things I see when I look at them. What else do you see? Do you think it’s interesting to use with kids, for example? I’ve used those experiments with first year university students, too, I think there is plenty to observe and explain here!