Kelvin

Making coffee work

Cogs, Wimbledon Common, Windmill, Contact S2b, instant coffee and washing soda developer
Cogs on Wimbledon Common, taken using a film based camera and developed using (instant) coffee. A way to make coffee ‘work’ but not the one linked to Helmholtz.

Search online and you can find videos of machines that lift cogs or turn wheels that are powered by the steam rising off a coffee. You can see an example here and find instructions for how to build your own here (a “stay at home” hobby project perhaps?).

Although such machines are very much for the hobbyist, the principle of the steam engine drove the industrial revolution. And, even now, much of our power network relies on somehow heating water which then drives a turbine that generates electricity. Underlying this is the principle that energy can be transformed from one type to another but is, ultimately, always conserved.

The concept is easiest to visualise with a pendulum or a swing. At its height, the pendulum has a maximum of potential energy but is not moving (so its kinetic energy is zero). As it passes through its minimum height point, the speed of the swing (or pendulum) is maximum and the potential energy at a minimum. Indeed, the amount of potential energy lost by the pendulum is equal* to the amount of kinetic energy gained. The same can be extended to “work” which, in the language of physics is always energy. If a certain amount of energy is put in, a certain amount of work can be produced. In a closed system and without loss, the amount of energy you put in is the amount of energy you get out. In any real system, some of that energy is lost to heat or other methods of loss.

Press Room coffee Twickenham
Whether you make your coffee as a pour over or an espresso, the principle of conservation of energy is always the same. The energy you put in will equal the energy you get out (with some lost as heat as the coffee cools). Pour over at the Press Room, Twickenham.

To use a more coffee related example, in an espresso machine, work is done to put the water under high pressure (and separately to heat it). This pressurised water is then allowed to escape through the coffee puck where the work originally done pressurising it gets transformed into the kinetic energy (speed) of the water going through the group head. Changing the pressure changes the speed at which the water goes through the puck in an analogous way to how changing the height of the pendulum drop affects the speed as it goes through its central point. Of course you won’t always see this because changing things like the grind size in the espresso puck will also affect the route that the water takes as it travels through the puck and so the actual speed at which you see the coffee-infused water leaving the espresso basket will be affected by that too. The real world is never quite so easy as the ideal.

A pour over works in a different way. Here, the energy is stored in the water ‘bath’ in the filter as gravitational potential energy. As the water falls, it gains kinetic energy at the expense of this gravitational energy (or height). As the espresso machine also works with gravity, the conclusion would be that the water will move much faster through the espresso puck than the pour over bed. That this often doesn’t seem to be so is again because of the effects of the resistance of the coffee bed or espresso puck, on the espresso and the pour over.

This concept of the conservation of energy has been engrained into us from an early age. And so it may be surprising that it is a fairly recent principle in physics. For although versions of this principle had been considered for many years, it had not been recognised until the 1840s (by James Joule and Robert Mayer in 1843 and 1842 respectively) that work and heat were interchangeable. And it wasn’t until 1847 that Helmholtz recognised that all energy was conserved. Although at that time he was using the word ‘force’ for what we now call ‘energy’, and what we now call kinetic energy was thought of as a ‘living’ energy. He wrote:

“… the loss in the quantity of potential force is always equal to the gain in living force, and the gain of the first is the loss of the second. Thus, the sum of the existing living and potential forces is always constant.”**

So, among the many contributions to physics that he made, Helmholtz also has a claim to being among those who developed the field of thermodynamics which remains crucial both for physics and for our industrial and technological progress.

Rag&Bone, Rag & bone, coffee Victoria, coffee Westminster
Rag & Bone Coffee in front of St Matthew’s Church. Much of our understanding is based on our assumptions about how the world works. The challenge for us is to identify those assumptions that underlie our thinking.

There is perhaps a cautionary note here for any who are tempted to think that science and religion are always somehow in opposition. For the British scientists who contributed to the development of the idea of conservation of energy (such as Joule and William Thomson (Lord Kelvin)), the concept was founded on the idea of a Creator God: as only God could create or destroy, so it followed that energy of itself, could never be either created nor destroyed, it could only be transformed from one form to another. The idea of a God was, for them, implicit in the idea of the conservation of energy**.

Helmholtz had a philosophical disagreement here. For him, the principle was founded on a Kantian understanding of philosophy***. Certainly certain things had to be assumed at the beginning (such as the principle of causality and the existence of matter outside of our perception of it). But once these assumptions had been made, the principle of conservation of energy followed in a deterministic manner.

Does this matter? In our everyday experience of engines and the way things work, conservation of energy certainly seems to be crucial. We no longer question the principle but assume that one form of energy is transformed into another and is continuously conserved even as it is dissipated into the universe as heat as our coffee cools. But nonetheless, Helmholtz’s understanding was founded on certain assumptions, beliefs, just as Joule and Kelvin’s. It helps to be aware of the philosophical underpinning of our science so as to ensure we don’t have over confidence in what we can, and cannot, know.

So Helmholtz can teach us something else as we gaze into our coffee. Our world is multifaceted, and what we believe about what the world is, influences and informs our understanding of how the world works. Our challenge is to look into ourselves as we sip our coffee and to start to see what we believe we know and what we can actually know. And if we were to really do that, what conflicts would we find?

*With the usual caveats of no energy being lost to friction etc.

**”Helmholtz and the British Scientific Elite: from force conservation to energy conservation”, David Cahan, Notes Rec. R. Soc (2012), 66, 55-68

***”Helmholtz: from enlightenment to neuroscience”, M Meulders, MIT press, 2010

Drip coffee

The universe is in a cup of coffee. But how many connections to different bits of physics can you find in the time it takes you to prepare a V60? We explore some of those links below while considering brewing a pour-over, what more do you see in your brew?

1. The Coffee Grinder:

coffee at VCR Bangsar

Preparing a V60 pour over coffee. How many connections can you find?

The beans pile on top of each other in the hopper. As the beans are ground, the bean pile shrinks along slipping layers. Immediately reminiscent of avalanches and landslides, understanding how granular materials (rocks & coffee beans) flow over each other is important for geology and safety. Meanwhile, the grinding itself produces a mound of coffee of slightly varying grain size. Shaking it would produce the brazil nut effect, which you can see on you breakfast table but is also important to understand the dynamics of earthquakes.

Staying at the grinding stage, if you weigh your coffee according to a brew guide, it is interesting to note that the kilogram is the one remaining fundamental unit that is measured with reference to a physical object.

2. Rinsing the filter paper:

V60 chromatography chemistry kitchen

A few hours after brewing pour over, a dark rim of dissolved coffee can be seen at the top of the filter paper. Chromatography in action.

While rinsing the filter we see the process of chromatography starting. Now critical for analytical chemistry (such as establishing each of the components of a medicine), this technique started with watching solutes ascend a filter paper in a solvent.

Filtration also has its connections. The recent discovery of a Roman-era stone sarcophagus in the Borough area of London involved filtering the excavated soil found within the sarcophagus to ensure that nothing was lost during excavation. On the other hand, using the filtered product enabled a recent study to concentrate coffee dissolved in chloroform in order to detect small amounts of rogue robusta in coffee products sold as 100% arabica.

3. Bloom:

bloom on a v60

From coffee to the atmosphere. There’s physics in that filter coffee.

A drop falling on a granular bed (rain on sand, water on ground coffee) causes different shaped craters depending on the speed of the drop and the compactness of the granular bed. A lovely piece of physics and of relevance to impact craters and the pharmaceuticals industry. But it is the bloom that we watch for when starting to brew the coffee. That point where the grinds seem to expand and bubble with a fantastic release of aroma. It is thought that the earth’s early atmosphere (and the atmosphere around other worlds) could have been helped to form by similar processes of outgassing from rocks in the interior of the earth. The carbon cycle also involves the outgassing of carbon dioxide from mid-ocean ridges and the volcanoes on the earth.

As the water falls and the aroma rises, we’re reminded too of petrichor, the smell of rain. How we detect smell is a whole other section of physics. Petrichor is composed of aerosols released when the rain droplet hits the ground. Similar aerosols are produced when rain impacts seawater and produces a splash. These aerosols have been linked to cloud formation. Without aerosols we would have significantly fewer clouds.

4. Percolation:

A close up of some milk rings formed when dripping milk into water. Similar vortex rings will be produced every time you make a pour over coffee.

Percolation is (almost) everywhere. From the way that water filters through coffee grounds to make our coffee to the way electricity is conducted and even to how diseases are transmitted. A mathematically very interesting phenomenon with links to areas we’d never first consider such as modelling the movements of the stock exchange and understanding the beauty of a fractal such as a romanesco broccoli.

But then there’s more. The way water filters through coffee is similar to the way that rain flows through the soil or we obtain water through aquifers. Known as Darcy’s law, there are extensive links to geology.

Nor is it just geology and earth based science that is linked to this part of our coffee making. The drips falling into the pot of coffee are forming vortex rings behind them. Much like smoke rings, they can be found all around us, from volcanic eruptions, through to supernovae explosions and even in dolphin play.

5. In the mug:

Rayleigh Benard cells in clouds

Convection cells in the clouds. Found on a somewhat smaller scale in your coffee.
Image shows clouds above the Pacific. Image NASA image by Jeff Schmaltz, LANCE/EOSDIS Rapid Response

Yet it is when it gets to the mug that we can really spend time contemplating our coffee. The turbulence produced by the hot coffee in a cool mug prompts the question: why does stirring your coffee cool it down but stirring the solar wind heats it up?

The convection cells in the cooling coffee are seen in the clouds of “mackerel” skies and in the rock structure of other planets. The steam informs us of cloud formation while the condensation on the side of the cup is suggestive of the formation of dew and therefore, through a scientific observation over 200 years ago, to the greenhouse effect. The coffee cools according to the same physics as any other cooling body, including the universe itself. Which is one reason that Lord Kelvin could not believe that the earth was old enough for Darwin’s theory of evolution to have occurred. (Kelvin was working before it was known that the Sun was heated by nuclear fusion. Working on the basis of the physics he knew, he calculated how long the Sun would take to cool down for alternative mechanisms of heating the Sun. Eventually he concluded that the Sun was too young for the millions of years required for Darwin’s theory to be correct. It was the basis of a public spat between these two prominent scientists and a major challenge to Darwin’s theory at the time).

 

Of course there is much more. Many other links that take your coffee to the fundamental physics describing our world and our universe. Which ones have you pondered while you have dwelt on your brew?

Coffee and the world

Welcome to the first post of 2018, Happy New Year! But before embracing 2018, perhaps let’s take a moment to remember those things that we discovered in 2017 that connect your coffee cup (or brewing device) with the physics of what occurs in the wider universe. Here are some of the highlights for me this year, if you want to share your highlight, please comment in the section below.

latte art, flat white art

A properly made latte. But what if you add hot espresso to the milk instead of the other way around?

1) Latte layering

In mid-December a study was published in Nature Communications that explored the complex, but elegant, physics involved in making lattes (ok, not quite by the technique that you would hopefully find in your neighbourhood café but keep with this…). When a hot, low density, liquid (espresso) was poured into a hot higher density liquid (milk) contained within a cold mug, the competition between the density gradients of the liquid (vertical) and the temperature gradient from the cup wall to the liquids (horizontal) produced multiple layers of varying coffee/milk concentration in the cup. Too late for a 2017 Daily Grind article, this looks to be too good an experiment to pass by, hopefully it will appear on the Daily Grind in early 2018.

 

science in a V60

Could this V60 mystery now be solved?

2) Bouncing drops

November 2017 saw research published about what happens when a cold droplet falls onto a hot liquid (think milk and coffee). The temperature difference causes currents to be established within the droplet (and in the main liquid) that in turn create air flows between the droplet and the liquid bath that prevent the droplet from merging with the bath. The research can explain why it is that you can sometimes see raindrops staying as spheres of water on the top of puddles. It may also explain a puzzling phenomenon that I have seen while brewing coffee in a V60.

 

Vortex rings get everywhere.

3) Vortex rings in coffee

June 2017 and it is again about adding milk to coffee (why do I drink coffee black?). When one liquid (such as milk) is dripped into another (such as coffee), it is very likely that you will observe the milk to form “vortex rings”. These rings are related to smoke rings and have, in the past, been proposed as an atomic model. This year however it was suggested that these vortex rings could form as a type of magnetic nanostructure. Mathematically impressive, beautiful, perhaps quite useful and mathematically similar to something you can find in your coffee.

 

bloom on a v60

How do craters form?

4) Crater shapes

April 2017. What happens while brewing a pour over? As you drip water onto a granular bed (or, in coffee terms, ground coffee in a V60 filter), each drop will create a crater. The size and shape of the crater will depend on the density of the granular bed (espresso puck or loose grounds in a filter) and the velocity of the falling drop. Fast frame photography revealed how the shape of the crater changed with time for different scenarios.

 

Coffee bag genuinely home compostable

How it started.
The Roasting House bag before it went into the worm composter.

5) A home experiment

Perhaps not quite in the theme of the other four stories but this is an experiment that you can do at home. Some have proposed compostable coffee cups as a more environmentally conscious alternative to ordinary, disposable, coffee cups. But how “compostable” are compostable cups and compostable packaging? Between May and September 2017, #howlongtocompost looked at how long it took the Natureflex packaging (used by the coffee roasting company Roasting House for their ground coffee) to compost in a worm composting bin. This one worked quite well. Within 17 weeks, it had been eaten by the worms. In comparison, the “completely compostable” take away coffee cup is still in the worm bin (although considerably degraded) 37 weeks after the start of the experiment. If you are interested, you can follow #willitcompost on twitter. Will it finally compost? I’ll leave you to place your bets but you may decide that a link to Brian’s coffee spot guide to re-usable cups will be helpful.

 

What will 2018 bring? Certainly there will be more composting experiments as I have a coffee bean bag from Amoret coffee, 3 different compostable cups and a compostable “glass” to try with the worms. But in terms of the science? We’ll have to wait. Meanwhile, if you have a coffee-science highlight from 2017, please do share it either here in the comments section, on Twitter or on Facebook. Happy New Year to you all.

 

 

 

 

 

On rings, knots, myths and coffee

vortices in coffee

Vortices behind a spoon dragged through coffee.

Dragging a spoon through coffee (or tea) has got to remain one of the easiest ways to see, and play with, vortices. Changing the way that you pull the spoon through the coffee, you can make the vortices travel at different speeds and watch as they bounce off the sides of the cup. This type of vortex can be seen whenever one object (such as the spoon) pulls through a fluid (such as the coffee). Examples could be the whirlwinds behind buses (and trains), the whirlpools around the pillars of bridges in rivers and the high winds around chimneys that has led some chimneys to collapse.

Yet there is another type of vortex that you can make, and play with, in coffee. A type of vortex that has been associated with the legends of sailors, supernovae and atomic theory. If you add milk to your coffee, you may have been making these vortices each time you prepare your brew and yet, perhaps you’ve never noticed them. They are the vortex rings. Unlike the vortices behind a spoon, to see these vortex rings we do not pull one object through another one. Instead we push one fluid (such as milk) through another fluid (the coffee).

It is said that there used to be a sailor’s legend: If it was slightly choppy out at sea, the waves could be calmed by a rain shower. One person who heard this legend and decided to investigate whether there was any substance to it was Osborne Reynolds (1842-1912). Loading a tank with water and then floating a layer of dyed water on top of that, he dripped water into the tank and watched as the coloured fluid curled up in on itself forming doughnut shapes that then sank through the tank. The dripping water was creating vortex rings as it entered the tank. You can replicate his experiment in your cup of coffee, though it is easier to see it in a glass of water, (see the video below for a how-to).

Reynolds reasoned that the vortices took energy out of the waves on the surface of the water and so in that way calmed the choppy waves. As with Benjamin Franklin’s oil on water experiment, it’s another instance where a sailor’s myth led to an experimental discovery.

chimney, coffeecupscience, everydayphysics, coffee cup science, vortex

In high winds, vortices around chimneys can cause them to collapse. The spiral around the chimney helps to reduce these problem vortices.

Another physicist was interested in these vortex rings for an entirely different reason. William Thomson, better known as Lord Kelvin, proposed an early model of atoms that explained certain aspects of the developing field of atomic spectroscopy. Different elements were known to absorb (or emit) light at different frequencies (or equivalently energies). These energies acted as a ‘fingerprint’ that could be used to identify the elements. Indeed, helium, which was until that point unknown on Earth, was discovered by measuring the light emission from the Sun (Helios) and noting an unusual set of emission frequencies. Kelvin proposed that the elements behaved this way as each element was formed of atoms which were actually vortex rings in the ether. Different elements were made by different arrangements of vortex ring, perhaps two tied together or even three interlocking rings. The simplest atom may be merely a ring, a different element may have atoms made of figure of eights or of linked vortex rings. For more about Kelvin’s vortex atom theory click here.

Kelvin’s atomic theory fell by the way side but not before it contributed to ideas on the mathematics (and physics) of knots. And lest it be thought that this is just an interesting bit of physics history, the idea has had a bit of a resurgence recently. It has been proposed that peculiar magnetic structures that can be found in some materials (and which show potential as data storage devices), may work through being knotted in the same sort of vortex rings that Kelvin proposed and that Reynolds saw.

And that you can find in a cup of coffee, if you just add milk.