For some reason my workflow regarding all things #KitchenOceanography and #WaveWatching changed at the beginning of this year. I started editing frames on the pictures I’m posting on Instagram, and, since I was most likely doing this on my computer anyway, scheduling the posts through a program on my computer, which meant that I was typing captions on the computer, too, writing a little more. But somehow that meant that I had already written everything I wanted to write about the pics and didn’t feel the urge to blog later, so … I didn’t. Until now, that is!
Here is a collection of my Instagram posts on coffee in #KitchenOceanography!
Enjoying your lazy first morning coffee of the year (or already back from your New Year’s morning walk, but forgot to take pictures — what’s wrong with you, 2021?)? Then it’s a perfect opportunity to look at wind-induced currents in your coffee! Gently blow across the cup and observe how two counter-rotating eddies develop. This becomes especially clear if you take milk in your coffee (or something else that creates a surface film). Enjoy!
Maybe not The Best Thing about morning coffee, but definitely very important: Observing what happens when you pour milk or cream in! Here the cold milk is denser than the coffee, so it sinks down to the bottom of the glass (it would probably even shoot to the bottom of the glass if it was the same density as the coffee since it’s coming in with a lot of momentum). Hitting the bottom, it shoots along the curved rim of the glass and up in these cute little turbulent billows. But eventually, it will settle on the bottom of the glass, forming a denser layer under the less dense coffee — that’s what we call a stratification, both in density and in coffee&milk. And it’ll stay like that for a little bit, until other processes come into play. So stay tuned for those :-D
Actually, not only internal waves, but also current shear! When you pour milk into coffee, the milk will form a layer at the bottom of the coffee. Similarly to when you poured the coffee in and it surface leveled out, the surface of the milk wants to level out. And similarly to the waves that you probably observed initially on the coffee when you poured it in, waves appear on the interface between milk and coffee. Except that these waves have larger amplitudes, move more slowly and persist for longer. That is because the density difference between milk and coffee is orders of magnitude smaller than that between coffee and air. Those waves are called “internal waves”. And what we see in the pic, too, is that the milk layer is moving relative to the coffee layer, therefore the wave crests are being pulled into these sweeping strands. Pretty awesome!
My sister & nieces made this mug for me for Christmas. Isn’t it just perfect together with the swirl in the last bit of my coffee? I’m considering making this my logo and profile pic and EVERYTHING because I think it is Just. Perfect.
I’ve been playing around with different glasses and different ways of lighting them in order to get clearer pictures of the things I want to point out: The behavior of the fluids, not reflections on the side walls of the tanks I am using. At least here there are only two stripes where the light is reflected? And the internal waves on the interface between milk at the bottom and coffee on top come out quite clearly. Even from this photo you can see how dynamic the system is!
Again, there is a milk layer at the bottom of the coffee. And those mushroom-y milk fingers appear when the milk is warming up and its density is thus decreasing. As it gets less dense than the coffee, the stratification becomes unstable and milk starts rising until it reaches a level of its own density.
Today there is some interesting surfactant on my coffee. It might be due to oils in last night’s tea that I didn’t clean off, or maybe it’s the cream (but I would think that that’s the little blobs of oil you see). In any case, the surface film behaves in very interesting ways: It is showing us a front in the coffee, with lots of small instabilities on the front! The front must be related to me drinking from the mug somehow, but I’m not sure how. Thanks to the surface film, we also see convection occuring in the top left, where we get all those small-scale structures in the color, lighter areas indicating convergence zones where the surface film gets pushed together, darker areas where it is pulled apart.
A little while ago I posted a picture of the front you see in my coffee here. And what I did then was twist the mug a little bit: I wanted the front to be in the picture more nicely together with the little boat. BUT: exciting things happened (predictably): As I was twisting the mug, it did not behave as a solid body together with the coffee. Rather, it twisted while the coffee was not! And this created shear between the sidewalls of the mug and the coffee, which is what we see all around the edge: shear instabilities breaking into eddies! And all that due to inertia of the coffee.
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
Sometimes sitting in a café for a work meeting with #lieblingskollegin Julia can lead to unexpected discoveries of oceanographic processes — in my latte! It’s those little things that inspire blog posts…
“Kitchen oceanography” brings the ocean to your house or class room!
Oceanography is often taught in a highly theoretical way without much reference to students’ real life experience. Of course a sound theoretical basis is needed to understand the complexity of the climate system, but sometimes a little “kitchen oceanography” — doing experiments on oceanographic topics with household items — goes a long way to raise interest in the kind of processes that are not easily observed in the real world. I’ve previously written a lot about simple experiments you can perform just using plastic cups, water, ice cubes, and a little salt. But sometimes it’s even easier: Sometimes your oceanography is being served to you in a cafe!
Oceanic processes can be observed in your coffee!
Have you ever looked at your latte and been fascinated by what is going on in there? Many times you don’t just see a homogenous color, but sometimes you see convection cells and sometimes even layers, like in the picture below.
Layers in a latte.
But do you have any ideas why sometimes your latte looks like this and other times it doesn’t?
When you prepare latte in the right way, many layers form
Layers forming in latte (and in the ocean or in engineering applications) are an active research field! In the article “laboratory layered latte” by Xue et al. (2017), the authors describe that the “injection velocity” of espresso into the warm milk has to be above a critical value in order for these pretty structures to form in a latte. They even provide a movie where you can watch the layers develop over a period of several minutes.
The homogeneous layers with sharp boundaries are caused by double-diffusive mixing
Double-diffusive mixing, which is causing the formation of these layers, is the coolest process in oceanography. In a nutshell, double diffusive mixing is caused by two properties influencing density having different rates of molecular diffusion. These different rates can change density in unexpected ways and an initially stable stratification (high density at the bottom, low density on top) can, over time, become statically unstable. And static instability leads to adjustment processes, where water parcels move in order to reach the position in the fluid where they are statically stable — the fluid mixes.
Layers in half a glass of latte.
But there are more fascinating things going on with the latte. Would you expect this stratification to remain as clearly visible as it is in the picture above even though the glass is now half empty? I did not! And then check out what happens when you move the glass: Internal waves can travel on the boundaries between layers!
You can use this in class to teach about mixing!
Mixing in the ocean is mostly observed by properties changing over time or in space, and even though (dye) tracer release experiments exist, they are typically happening on scales that provide information on the large-scale effects of mixing and not so much on the mixing itself. And they are difficult to bring inside the classroom! But this is where kitchen oceanography and experiments on double-diffusive mixing come in. If you need inspiration on how to do that, I’ve recently published an article on this (unfortunately only in German), but there are plenty of resources on this blog, too. Or shoot me an email and we’ll talk!
P.S.: Even though the coffee company is displayed prominently in the pictures above, they did not pay for my coffee (or anything else). But if they’d be interested and make me a good offer, I’d definitely write up some fun stuff on learning oceanography with coffee for them ;-)
My friends know me well. Especially A&I, which was proven again when they sent me the link to an article about two things that I am mildly obsessed with: Latte and double-diffusive mixing.
My obsession with latte is a fairly recent thing, but I have been known to blog about interesting convection pattern in it (for example here). The obsession with double-diffusive mixing, however, is well documented for more than the last 12 years (for example when I am writing experimental instructions, poems or scientific articles about it).
The double-diffusive process that I have been most concerned with is salt fingering, because it is oh-so-pretty, and also fool-proof to create for teaching purposes (when you know how to do it).
Diffusive layering I seem have to be a little frustrated with, at least in teaching (but reading back this post now, it turns out that that was entirely my own fault and not my students’. Oh well, you live and learn! Isn’t this exactly the kind of stuff that makes for great teaching portfolios? ;-)).
And it also turns out that I did the experiments themselves all wrong. According to the article “laboratory layered latte” by Xue et al. (2017). I should not have been trying to carefully stratify a tank in order to see diffusive layering. Instead, I should just have quickly poured the lower density fluid into the higher density one, and layers would have formed by themselves!
So there is one thing that you won’t see any time soon:
Yep. Me drinking latte from any kind of vessel that doesn’t let me look at the stratification! I don’t know how I could ever have fallen into the trap of missing out on observing fluid dynamics while having my early morning coffee in the office. Now I urgently need a nice glass mug!
And you should go check out the article, it’s a really nice read. My new ambition in life: Write a fluid dynamics research article that applies the FD to some really cool, yet mundane, every day thing. Are you in, Elin? :-)
Xue, Nan and Khodaparast, Sepideh and Zhu, Lailai and Nunes, Janine K. and Kim, Hyoungsoo and Stone, Howard A., Laboratory layered latte.Nature Communications 8(1), 2017