I remember when I first came across a latte, and it was life changing. I had never been a fan of coffee, and I still don’t like freeze dried coffee, but then I met the real stuff and I’ve been hooked since. One day at Uni we wandered into Sally’s, part of the food court at the University of Sheffield, and the course of my life was forever changed.
It wasn’t just the lovely coffee I liked; it was the display of physics in front of me. The foam floating on the layer of coffee, floating on the layer of milk. The reason that you can create layers in your coffee is all down to density, there is a slight difference in the density of the coffee and of the milk, and again to the milk foam. This means if they are properly prepared the layers will float on top of each other.
If you just pour the coffee into the milk with no precautions, then they will automatically mix from the turbulence caused. The difference in densities is not as much as say water and oil, so once mixed the coffee and milk will not separate out again. However, if you disrupt the funnel like pouring of the coffee and slow the speed down, e.g. by pouring it over a spoon, this will allow the mixing of the two liquids to be minimised and the difference in densities becomes apparent so the laws of physics can separate them out.
What happens to these layers if you leave them alone and don’t mix the contents and drink them? Well, firsthand I didn’t know as I have always chosen to drink the coffee. But this has been done in the name of science. What forms, due to double-diffusion convection is layers of different concentration of coffee and milk mixtures. The outsides of the glass cools faster than the inside, therefore the density increases in the material at the edge and it starts to fall. As the material falls it meets the vertical point where the density is the same, and then it moves horizontally inwards to continue its convection journey. Basically, the coffee has been split into horizontal layers of different densities, all with their own convection cell.
In the name of science, I decided to allow a cup of coffee to go cold, well I had to do a couple to get it right. You can see some of the layering in the pictures, you will have to trust me that it is more evident in real life!
Check out the nature article for more information on how your separated latte cools: https://www.nature.com/articles/s41467-017-01852-2
Density is also the reason that semi-skimmed and skimmed milk foam better than full fat milk, and why oat milk is the best foaming non-diary milk. Steaming milk forces air into the milk, increasing the temperature and making bubbles. The bubbles are supported by the protein in the milk, meaning the air can stay trapped in the mixture. However, if the milk has too much fat in it, it becomes too heavy and the protein can no longer support the bubbles and you are left with a sad flat froth.
The other great display of liquids of different densities in actions I really enjoy is in a lava lamp. I finally got my own a couple of years ago, I was bought a genuine Mathmos Astro lava lamp in polished silver, and it has been used every night since.
To create a perfect lava lamp you have a very simple set up of some coloured wax in a clear liquid all in a sealed glass container. The outside of the glass container is coloured to produce the amazing psychedelic effects. The density of the wax and liquid are carefully tuned so they are very similar, the density of the wax at room temperature is slightly higher than that of the water and so sinks to the bottom. There is a heat source, typically an incandescent light bulb, and this supplied heat reduces the density of the wax and the surface tension of the water. This means that the wax expands, becomes less dense and floats upwards. As it rises, it cools and the process is reversed so the wax contracts, becomes more dense and sinks. As it approaches the bottom it heats again, and the process continues.
This is all thanks to the Rayleigh-Taylor instability, the instability at the point where two fluids of different densities meet. It occurs when the lighter fluid is pressing the heavier fluid. You can also see this effect in supernovae explosions and the mushroom cloud in volcanic eruptions.
The other really exciting thing about a lava lamp is the stalagmite stage, it doesn’t go from solid to molten in the snap of your fingers. After about 20 minutes of heating the most amazing structures are formed in the wax. This is caused by the wax at the bottom of the glass, next to the bulb, getting hotter faster and this less dense wax pushes on the more solid wax. It eventually breaks out into the first wax column but being further away from the heat source now it quickly cools again leaving a crazy waxy finger. Cooler material then flows into the space created and it begins to heat up. The lava lamp stays in this state for about 1 hour.
After about an hour to an hour and twenty from switch on the lava lamp has heated up enough to get some currents forming, with single blobs, then at about 2 hours the lava lamp is fully functioning and ready to entertain for hours. The lava lamps can get into an overheated stage, where the wax just stays at the top of the lamp.
Have a look at the following video for a super speeded up progression of a lava lamp being switched on and heating up.
Hope you enjoy thinking about the many ways that density shapes the world around us as you have a look around for more examples!