Each year, the American Physical Society hosts the Annual Meeting of the Division of Fluid Dynamics. A highlight of this is the Gallery of Fluid Motion, a competition of videos showcasing fascinating science into all aspects of fluid dynamics. You can find a link to all of the videos, including this year’s prize winners here. Listed below however are a few videos with links to coffee, cafes or just generally beautiful physics that you may be able to replicate in your kitchen.
How do fish know how to swim together in a school? An illuminating study that helps us to find out:
The strange and wonderful patterns formed by dropping a small amount of dyed water onto glycerin:
Can you bounce a liquid drop on a liquid? Yes, it even explains something we may have noticed while brewing pour overs, but what about bouncing liquids on a solid:
Finally, although it refers to something we may not want to think too much about, there is some beautiful physics going on as people exhale, with and without masks:
You may recognise the sound of something that is deep frying. But what fluid physics is causing it? You need to “listen to your tempura”
It stretches it a bit to call this “coffee” or “cafe” physics but many people have tattoos and surprisingly, no one has really ever investigated how the ink gets under the skin. Until now of course:
The Leidenfrost effect is something that you will have seen often while frying eggs. This takes a closer look at the Leidenfrost drops:
Experiments you can do at home. The first is to look at the patterns formed as a drop of food colouring spreads on a mixture of water and xanthan gum (available in many supermarkets for gluten free cooking).
Secondly, how does water flow out of a bottle?
Do take a look at the full gallery (here) and even have a go at one or two of the experiments. It would be great if you would share your photos of fractal patterns formed by food dye or even if you’ve been inspired by any of the other videos. Whatever you do, enjoy your coffee.
Summer this year has so far been quite hot and dry. Perhaps you have seen the grass dying back. Or maybe you have noticed the cracks forming in the soil in your local parks and fields. Such cracking is the result of the very dry weather and hopefully you won’t find it in your coffee, but there is another effect concerning soil compaction that connects to brewing a morning coffee as well as farming it.
It’s about the rain. As each raindrop falls to the Earth, it makes an impact with the soil underneath. While a light drizzle is not going to have that much of an impact, a larger raindrop of diameter say, 5mm, is going to hit the earth at about 9m/s – and that could cause quite a stir. Each impact will shake off smaller sized particles of soil which dislodge and get stuck in the pores between the larger soil particles. So the smaller particles start to ‘clog’ the pores between the soil particles and reduce the ability of water to penetrate into the soil. And although it seems a small effect, the result of this clogging of the pores by the smaller soil particles is to reduce the water permeability of the soil by 200-2000 times*: a soil crust is formed.
This crust not only reduces the amount of water that gets through to the roots (by reducing the soil’s permeability), it also acts as a barrier for seedlings coming up: while many seeds can get through quite strong layers, even Sorghum struggles to push through if it needs pressures of 13-18 Bar to break through this crust*. So even without any farming machinery causing further soil compaction, just the rain is going to affect how additional water goes through the soil and how plants can grow out of it.
We are getting to the coffee bit.
The crust strength is influenced by the number of small (clay-type) particles in the soil. Clay particles are less than 2 microns in diameter which is smaller than the grind size you would find in even a Turkish coffee grind. But if we were to grind very brittle coffee beans (that shattered into many smaller particles as well as the grind size we want), or we were to use a blade grinder leading to a large distribution of grind size in our freshly ground coffee, we may expect to see some effects like this while brewing.
If we think about a pour over brew (as opposed to an espresso or an immersion type), the initial pouring of water over the grind bed will dislodge any smaller particles in the grind and clog the grind in the same way as the rain on the soil. So if we were grinding way too fine, or using a blade grinder, or had a preference for darkly roasted (more brittle) coffee beans, it is possible that our pour-overs would tend to ‘clog’ more than if we were using a uniform medium grind of more lightly roasted beans. Has anyone experimented with this?
But the second soil connection we may notice as we prepare our pour over is that after our initial pour, as we let the coffee ‘bloom’ and the CO2 bubbles out, we receive a lovely aroma. A wonderful coffee smell as the grind bed continues to out-gas. This may remind us of petrichor, which is that great, and distinctive, smell of rain. And it turns out that petrichor is formed by the rain hitting the soil surface and producing air bubbles as it falls. The air bubbles then burst releasing aerosols from the soil which are so familiar to us as the scent of rain. A similar process to the blooming of the coffee grind. But just as with the coffee grind, as the water continues to fall and particularly if the pour over clogs to leave us with a water layer on the surface of the coffee (or soil), the aroma will reduce (or at least change) as the mechanism producing the smell changes.
On a farm or in a garden, the effect of this soil compaction can be reduced by practises such as mulching. In addition to reducing the impact of individual rain drops on the soil surface, the mulch reduces evaporation of the water from the surface and changes the albedo of the soil. All things that may help coffee farmers to grow healthier coffee plants. In our pour overs, it is probably not a good idea to add any form of mulch! But this does mean that we can experiment more with the grind!
There are many more connections between your coffee and the earth around us, what will you notice?
*Soil Physics, WA Jury and R Horton, Wiley and Sons Publishing, 2004