Categories
Coffee cup science Coffee review General Science history

Is it a third?…. Treelogy, Paddington

Outside Treelogy on Eastbourne Terrace, Paddington. The topiary could suggest a series of strontium atoms lining up on top of each other.

Good coffee near a mainline train station? It’s often difficult to find a good spot to take time to enjoy a coffee if you only have about 30 minutes (or less) before your train. Fortunately for coffee lovers in London, both Kings Cross (/St Pancras) and Paddington have several very good speciality coffee places nearby. There’s the cafe in the Pilgrm hotel just across the road from Paddington on London St, but Treelogy is perhaps even closer, directly opposite the buildings that house the new Elizabeth Line on Eastbourne Terrace.

Treelogy appears to have opened in April 2023. There does not seem to be much information online about it apart from Trip Advisor reviews so, having approximately 50 minutes before we needed to catch a train, we decided to stop at this new cafe. The interior is very modern and open. The counter is in front of you on the left as you enter with plenty of seats in the window and along the wall, as befits a cafe that is also close to a station. The style of some of the seats in the cafe and the fact that it is going to attract people who are about to embark on journeys (or have just come off a journey) means that there are elements here that could remind you of the scene in Edward Hopper’s Nighthawks. The coffee appears to be roasted by Treelogy themselves. There was a wide selection of pastries and breakfast bagels arranged on the counter and so we ordered two coffees to stay, and a bagel for the train.

The “Real Time” clock by Maarten Baas in Paddington. How much do they pay that man to be there all day?

We intended to sit on the bench just outside the cafe with our coffees but nonetheless we were offered our coffee in ceramic cups which was a nice touch. Inside, there was plenty to notice: circular lights on the wall leading to the back of the cafe which resembled ship lighting. A coffee dictionary book (and a book by Martin Wolf) that could offer a good read or a thought train on the physics of finance and the (Brownian-motion) links to coffee. The travellers with their roller bags going in and out of the cafe, who are they and where are they going? Yet, moving outside and settling down, the oat milk flat white and long black were both a very enjoyable way to spend time with a coffee.

As we were ‘spending the time’ with the coffees, the hands of the “REAL TIME, Paddington” Maarten Baas clock were being re-drawn every minute. Installed back in 2021, this clock appears as if the time is being painted onto the clock face by a man who seems trapped inside the clock. Each minute he erases the minute hand before redrawing it into its new correct position. Literally marking the minutes before our train is due.

For a physicist waiting for a train, an immediate thought may occur: what does ‘Real time’ mean? Admittedly, this question fades into the background again as the man wanders around, points at something on the clock, adjusts his position and then gets ready to move the clock hand again. The art is distracting from the question. But the question keeps surfacing: what is a minute, what is a second, is time absolute? There is perhaps a diversion that could be made here to a more philosophical question about the nature of time and our perception of it but we only had one long black and one flat white, the physics may take longer than that anyway!

A closer view of the man in the clock as he is erasing the minute hand of the clock. The colour bands on the clock face are not really there but are the result of the projected video onto the clock face and the way that the camera images that.

The physics bit remains because you may remember hearing about Einstein’s twin paradox, a thought experiment arising out of an aspect of his theory of Special Relativity. Relativity in general in physics refers to moving ‘frames of reference’, a classic case is that of a person on a train relative to a person on a station platform. For the person on the train, they are stationary, with respect to the train carriage. If they bounce a ball on the floor of a carriage, the ball bounces straight back up at them. They do not experience themselves moving (apart from when the train is accelerating or braking) and instead to them it appears that the person standing on the station platform is moving, backwards at the speed of the train.

Ordinarily our brains will process this and recognise that it is we who are on the train that are moving and we identify the ‘rest frame’ (the frame that is not moving) with the station platform. However we may all have experienced the sensation when on a train in a station next to another train. As the guard whistle blows the train moves but we cannot immediately tell whether it is our train that moves or the train next to us. This is the essence of relativity: all reference frames move relative to each other. The frame that is genuinely at rest is the one we define so (even the station platform is moving relative to the Sun, we just don’t notice this movement of the Earth at all).

Einstein’s theory of special relativity arises out of the special case when one of the moving frames is travelling at close to the speed of light, c. As the speed of light in a vacuum is constant, what would happen if someone travelling in a car at a speed just less than c looked at themselves in the rear view mirror? Einstein’s answer was that they would see themselves as anyone would because, relative to the reference frame of the car, the speed of light is still constant, it is still c. However an observer outside the car looking at the car and the person looking at themselves in the mirror also measures the speed of light as c, not nearly 2c (the speed of light plus the speed of the car). The speed of light in a vacuum is constant!

The explanation for this apparent problem is that our perception of time (and of distance) is not the same at different speeds. A person moving at a fast speed (relative to a person defined at rest) would have a wrist watch that was slow, relative to the person at rest – moving clocks go slow. This is the origin of the twin paradox which is that if one of a pair of twins travels away from Earth at close to the speed of light and returns, they will return younger than their twin who remained ‘at rest’ on Earth (but not relative to the twin who travelled who considered themselves at rest too so their earth bound twin should, to them, be younger).

Topiary at the entrance to Treelogy. The atomic clocks used in the study described in the text used super cold strontium ions positioned just above each other.

The solution of the twin paradox comes with Einstein’s second theory of relativity: General relativity.  Special relativity only concerns the case when different frames of reference move at constant relative velocity to each other. General relativity extends the case to accelerating frames and gravity. In order to meet again, the twin in the space ship had to turn around (decelerate and accelerate again). This changes the situation from the case expected purely from special relativity. There is a lot of experimental evidence for both special and general relativity, but recently one test of general relativity tested the idea on a very small scale.

The theory of general relativity postulates that it is not just moving clocks that go slower. Clocks in strong gravitational fields will also run more slowly. The extreme example of this would be the event horizon of a black hole, but even on Earth, a clock closer to the centre of the Earth will tick more slowly than one that is further away. Remarkably this prediction has recently been verified using extremely accurate clocks by measuring time using atoms spaced just 1 mm apart. The ‘clocks’ of the atoms 1 mm lower moved slower than the clocks of the atoms 1 mm above. Absolutely astonishing! And yet absolutely expected because one remarkable and weird feature about physics is that it seems to be universally applicable: what happens at the event horizon of a black hole shares the same physics as what happens in conditions far less extreme, conditions found in a coffee cup.

The Real Time clock is 7.8m above the pavement where I was enjoying my coffee. These experiments mean that I can be confident that the clock is going very slightly faster than the time I experience sitting on the bench. However, I shouldn’t use this thought to justify enjoying my coffee much longer and thereby miss the train! It seems that our trains aren’t quite so precise as the deviations implied by the theory of General Relativity. It is still necessary to get through the barriers with several minutes to spare. Treelogy, and the clock man, will have to wait for a return visit.

Treelogy is at 48 Eastbourne Terrace, W2 6LG

More about Einstein’s theories of relativity can be found here or in a good book in a library.

Categories
Coffee cup science Home experiments Observations

Cracking Magnets

Rare earth magnets are very strong despite their size. These magnets are several times stronger than an ordinary fridge magnet.

Can you hear it? The first, second and then third and fourth cracks as a magnet is brought near a magnetic (but not magnetised) material, such as a piece of cutlery? Unlike the first and second cracks during coffee roasting, which are clearly audible, it is unlikely that you would have actually heard the cracks of a magnet. To hear them you would need to amplify the effect and connect it to a loudspeaker (there’s a link to how you can do this experiment here). Nonetheless, if you were to do so, you would hear the cutlery cracking. And while these sounds are not connected to the first and second cracks in coffee roasting, they are connected, via physics, to coffee. To see why we need to think a bit more about what is causing these magnetic creaking noises.

The effect is known as the Barkhausen effect after Heinrich Barkhausen who discovered it in 1919. It turns out the the effect reveals quite a lot about how magnets work because it reveals what is going on at an atomic level in the kitchen fork. Some metals are attracted to magnets but not others. So a fridge magnet would stick onto materials containing iron but would not stick to a sheet of aluminium; we can pick up pins, paper clips and some cutlery with a strong magnet but we could not pick up a piece of kitchen foil. These iron containing metals are magnetic but not magnetised, they will be attracted to a magnet but they will not ordinarily attract other items to themselves. We may remember from school that we can make them magnetised by continuously stroking a strong magnet along the length of the pin (or fork, or paper clip) until the pin itself is able to attract other pins to it. We may even remember the explanation for this which was that for something to be magnetised, it had to have a clear magnetic orientation of North-South throughout its structure. Within the pin (or fork or paper clip) there are many small regions, called domains, which within themselves have a north-south orientation but they do not all point in the same way throughout the fork. Each little region points in a different direction to the others and so the net effect is that there is no overall North-South magnetism in the fork as a whole. As the strong magnet is used to stroke the fork, so the small regions move to align to the direction of the stroke of the magnet. The regions stop cancelling each other out and align so that the fork itself becomes a magnet with its own North-South.

inverted Aeropress and coffee stain
The link between coffee and the Barkhausen effect in magnets can be seen in this photo: a coffee spillage. It is the way that coffee evaporates and that coffee stains form that forms this physics connection between coffee and magnetism.

To return to our un-magnetised fork, you can imagine that where all these domains meet, there will be an area of confusion where the direction changes from one orientation to that of the neighbouring domain. This is called a ‘domain wall’ and it is these domain walls that are responsible for the Barkhausen effect. You can feel the effects of domains and domain walls in this experiment taken from the Institute of Physics Spark series: take two flat fridge magnets and turn them over so that the magnetic side of each faces the other. Move one of the magnets along the length of the other one. Think about how it feels to move it. Now move the same magnet perpendicular to the direction that you initially moved it in. Try it again. You will find that in one direction the movement feels smooth whereas in the other the magnets judder against each other, the movement is not smooth at all. You are feeling the effects of moving across a series of domains and domain walls, you can read more about the experiment here.

What actually happens as you bring a strong magnet towards an object such as a fork is that those domains in the fork that are aligned in the same direction as the magnet will tend to grow slightly at the expense of the ones that are not aligned with the magnet. The initial growth happens as the aligned domains get a bit bigger, a bit rounder and fatter. The domain walls bend a bit and the domains of the non-aligned regions get a bit thinner, a bit more squished. As the magnet is brought closer still, the aligned domains will actually start to grow at the expense of the non-aligned: the domain walls of the aligned domains will start to move outwards ‘eating’ into the neighbouring regions. It is at this point that you can pick up the Barkhausen effect because as the domain walls move, they can get stuck on defects in the metal rather like an elastic band would get stuck on an obstacle. The defect could be just one or two atoms that are out of place but the effect is that, just like the elastic band, the wall around the obstacle continues growing and the domain wall stretches more like an elastic band until pop – crack – the wall moves releasing a bit of energy that you pick up on the loudspeakers. This is what you hear as the Barkhausen effect. As the walls continue to grow so they will repeatedly get snagged on different defects in the metal and repeatedly ping – crack – into growth. Eventually, as the fork itself becomes magnetic* the last few non-aligned domains also start to align with the approaching strong magnet and the whole fork acts as if it is one magnet.

coffee ring, ink jet printing, organic electronics
A coffee stain. There are many experiments you can do at home with these.

The pinging domain walls have a direct link with an effect you can see in coffee, or more specifically spilled coffee. When you spill a few drops of coffee on a movable surface, you may have noticed that you can angle the surface a surprising amount before the drop starts to run down the side. You could try it now on a coaster if you have one available to you. The drop does not move because the edge is stuck, ‘pinned’, on defects on the surface of the coaster. These defects could be a crack in the material or a bit of dust or even a slight irregularity on the surface. Whatever it is, this defect acts to keep the edges of the drop in place. The first effect you would notice is that you can move the drop to a near vertical without it moving, the drop shape gets distorted but the drop itself does not move. The second effect is more subtle and is what happens if you leave the coffee drop there to dry.

Once spilled, the water in the droplet starts evaporating and eventually the droplet will dry leaving a coffee stain. The consequence of the pinning that you have just noticed is that the edges of the drop are quite stuck: the drop can’t just shrink. Instead, as the water evaporates, the drop will get flatter and because the water evaporates more quickly from the droplet edge (to see why click here), there will be a flow of water inside the drop from the centre to the edges. As the water flows outwards so it takes the coffee sediment with it which means that the dried coffee becomes a ring of sediment at the edge of the dried droplet.

Although it is on a different scale, it is the same sort of pinning that is happening in the coffee ring and in the Barkhausen effect. There are connections between physics and coffee to be found in many surprising places. Where will you find one today?

*This is an instance in which scientific English is not the same as English-English. In scientific-English, the fork is always a magnetic material it is just not fully magnetised. In English-English we tend to use the word ‘magnetic’ only for those materials that attract iron etc. to them. For ease of reading I have kept with the English-English usage here but if you are interested, you can read more in these links about magnetism and magnetic materials.

Categories
Coffee cup science General Observations Science history Sustainability/environmental

Pure Percolation

Pure over boxed
The Pure Over in its box. The glass base is designed with an inbuilt filter, avoiding the need for disposable paper filters but making the physics of percolation unavoidable.

It was entirely appropriate that the first coffee I tried in the Pure Over coffee brewer was the directly traded La Lomita Colombian from Ricardo Canal via Amoret Coffee. Ricardo was a special guest at one of the Coffee and Science evenings we held at Amoret Coffee in Notting Hill (pre-pandemic) where, among other things, he spoke about how he is using Biochar on his coffee farm. Biochar is a porous, charcoal based material that can help to provide the coffee plants with nutrients as well as water, thereby reducing the amount of fertiliser the plants need. To understand how it works, we need to understand a bit about percolation, which of course we also need to understand in order to brew better coffee in the Pure Over. Indeed, there are enough similarities, and an extension to a quirk of how espressos are brewed, that it is worth spending a little more time thinking about this process and the connections revealed as we brew our coffee.

Percolation recurs in many of the brew methods we use for making coffee. The V60, Chemex, Kalita wave, percolators and the espresso itself, all rely at some point on water flowing through a bed of ground coffee. The flavour of the resultant cup is dependent on the amount of coffee surface that the flowing water is exposed to together with the time that it is in contact with the coffee. What this means is that grain size, or the degree to which you grind your coffee, is critical.

Playing with brewing coffee, we know some things by experience. Firstly, frequently, the flow through a coarse grind of coffee will be quite fast (probably too fast to make a good cup). Secondly, we know that for any particular brew method, the more water we pour into the brewer, the faster the water initially comes through. We also know that we can affect the flow rate of water through the coffee if we increase the area of the coffee bed, or decrease its thickness. These observations were quantified into an equation by Henri Darcy in 1856. Darcy’s work had been as an engineer, designing and building public works such as the aqueduct that brought drinking water into the city of Dijon in the 1840s. Darcy received significant recognition at the time for his work including the Légion d’honneur, but it is more for a later set of experiments and particularly for his equation that we remember him today. In the 1850s Darcy was working on the problem of water purification. Passing water through a bed of sand is still used as a method of purifying the water today. Darcy used a series of cylinders filled with sand to investigate how quickly water trickled through the sand bed in order to come up with a proper quantification of those things that we too know by experience with our coffee filters. You can read about the mathematics of Darcy’s equation here.

espresso puck
An espresso puck. The compact structure nonetheless allows water to percolate through it at high pressure.

Darcy found that the flow rate of water through the sand bed increased when the porosity of the bed was higher (fine, dense sand would delay the flow of the water more than coarse, loosely packed sand). If there was a greater pressure on the water at the top of the bed (ie. more water is on top of the sand), the flow rate through the bed would increase too. Conversely the flow would get slower as the water was made more viscous. This is something we too know from experience: try to pour honey through the coffee grounds and it just won’t work.

For us to apply Darcy’s insights into making better coffee, it means that we need to think about the grind size. Too coarse and there will be lots of empty space through the bed of grounds: the porosity is high, and the water will flow straight through. Too fine and the flow rate will decrease so much that rather than just the sweet and slightly acidic solubles that first come out of any coffee extraction*, there could be too much of the bitter organic compounds that come out later, changing the character of the cup. With coffee we have an additional concern. Unlike sand, coffee grinds will swell, and splinter, as water is added to them, closing up any narrower paths and lengthening the brew time. This also means that, unless we properly wet the grounds prior to filtering our coffee, the extraction will be non-uniform and not reproducible. Another reason to bloom coffee thoroughly before brewing.

There is one more factor in brewing our coffee however that Darcy’s equation, which is valid for more stable systems, overlooks. Darcy assumed a constant flow rate of water through the sand bed, but coffee is different. In his book about espresso*, Illy showed that the flow of the water through an espresso puck was not constant over time. Something really interesting was happening when you looked carefully at an espresso puck. Ground coffee can come in a large distribution of sizes. In addition to the grind that we are aiming for, we also get a whole load of smaller particles called ‘fines’. Sometimes this is desirable, but with espresso, and by extension with our filter coffees, these fines add a twist to the physics of the percolation. As the espresso water is pushed through the coffee puck, the fines get pushed down through the puck between the ‘grains’ of the coffee grinds. This reduces the flow rate of the water until the point at which they get stuck. This will have the effect of increasing the contact time between the coffee and the water and so allowing more flavour solubles to be extracted. But crucially, these fines remain somewhat mobile. If you were to turn the whole espresso puck upside down (and Illy had a machine that allowed him to do this in-situ), the fines would again go on the move. Migrating from the new top of the puck to the new bottom. Filling the voids between the slightly too coarse grains. Complicating the simplifications in Darcy’s equation, but adding flavour to our brew.

Watch House coffee Bermondsey
There is a fountain on the wall (right hand side) of the Watch House cafe in Bermondsey. Many public fountains in London date from the 1850s emphasising just what a problem access to drinkable water once was.

Which leaves the connection between the farming method and the coffee. Biochar is formed by burning carbon containing waste (such as plant matter) in a low oxygen environment. Burying the resultant charcoal is therefore a way of storing carbon, and preventing its release into the atmosphere, for many years. But it is not just good for carbon storage. The buried charcoal is highly porous and traps nutrients within its structure so that the plants growing near it can be fertilised more efficiently. Moreover, the fact that it is porous, just like the coffee or sand beds, means that it traps water for a long time. Consider how long it takes a used filter full of coffee grounds to completely dry out! The water gets trapped within the porous structure and does not evaporate easily. This aspect of the biochar means that, as well as nutrients, the plants that grow nearby get a good source of reliable water. The ancient civilisations of the Amazon region used something similar to biochar in their farming techniques resulting in soil now known as “Terra Preta”, an extremely rich form of soil that improves plant growth. On his farm, Ricardo is going fully circular and making his biochar out of old coffee trees. The old trees thereby giving new opportunities to the fresh growth. It is a carbon capture scheme that reduces the need for fertilisers and that relies on percolation physics to work to best effect for the plants.

It seemed a moment of perfect coffee-physics poetry to use coffee grown on a farm using these techniques while initially experimenting with my own, percolation sensitive, Pure Over brewer. Percolation physics and interconnectedness all in one cup.

*Illy and Viani (Eds), “Espresso Coffee”, 2nd Edition, (2005)

Categories
Coffee cup science General Observations Science history Uncategorized

Coffee quakes

ripples on coffee at Rosslyn, the City
From ripples on the surface, to listening to the sound your coffee makes. What links a coffee to an earthquake?

What do you hear when you listen to your coffee? Or a related question, what links your coffee to earthquakes and seismology?

In recent weeks I have been making coffee with milk, not often, but enough to notice something slightly strange. While heating the milk in a small saucepan, I have accidentally tapped the side of the pan while the milk was in it. The tap, perhaps unsurprisingly, produced a ripple on the surface of the milk propagating away from the point of tapping. But what was surprising was that a very short time later, a second ripple was generated, this time from the other side of the pan propagating back towards the original wave.

The first ripple had not yet travelled across the milk surface before the second ripple had been generated and travelled back towards it. Something was causing a vibration on the other side of the pan before the first ripple had had a chance to get there. Was the pan acting like a type of bell which, as I tapped it, started to resonate all around its circumference?

Assuming that the vibration of the tap travels at the speed of sound through the metal of the pan, it would take about 50 μs for the vibration to travel half way around the circumference of the pan (diameter 14cm, with a speed of sound in steel ~ 4500 m/s). But then, if the pan were resonating, the resonance frequency would depend on the speed of sound in the milk filling the pan, which would increase as the milk was warmed. Would we see evidence for this if we video’d tapping the pan as we heated the milk?

coronal hole, Sun
Observing periodic changes to the luminosity of stars can indicate the elements within them. Image credit and copyright NASA/AIA

Rather than watching the liquid within, we could also learn about the interior of a cup of coffee by listening to it. The “hot chocolate effect” is the classic example of this. The effect occurs when hot chocolate powder is added to warm water or milk and stirred. Think about the pitch of a sound made by tapping gently on the base of your mug while you make a cup of hot chocolate. Initially, adding the powder and stirring it will introduce air bubbles into the liquid. As you stop stirring the hot chocolate but continue to tap the base of the cup the air bubbles leave the drink. The cup is acting as a resonator, so the sound that you hear (the resonance of the cup) is proportional to the speed of sound in the liquid in the cup. As the speed of sound in hot water containing lots of air bubbles is lower than the speed of sound in hot water without the air bubbles, the note that you hear increases in pitch as the bubbles leave the drink. You can read more about the hot chocolate effect in an (instant) coffee here.

It is here that we find the first connection between coffee and earthquakes. Seismologists have been listening to the vibrations of the Earth for years in order to learn more about its interior. By observing how, and how fast, waves travel through the earth, we can start to understand not only whether the inside is solid or liquid, but also what the earth is made from. This is similar to learning about the air bubbles in our hot chocolate by listening to the sound of the mug. More recently, the seismologists have shown the effect of the Covid-19 related “lockdowns” on reducing seismic noise. Something that does not have an obvious coffee cup analogy.

But seismology is not just confined to the Earth. Vibrations of a different kind have also been used recently to learn more about the interior of stars, although here it is a mix of seeing and ‘listening’. Generally, when the surface of an object vibrates, it leads to compressions and expansions of the medium within the object. This is the essence of what sound is. But in a star, these compressions and expansions also result in changes to the luminosity of the star. So, by looking carefully at the frequency of the variation in brightness of different stars, it should be possible to work out what is going on inside them. It is a branch of physics now known as “Astroseismology”. Recent astroseismology results from NASA’s Kepler satellite have been used to challenge theories about how stars form and evolve. It had been thought that as a star develops, the outer layers expand while the core gets smaller. The theories proposed that this would result in a certain change to the rotation speed of the core of the star. The astroseismology observations have revealed that, while the gist of the theory seems right, the core rotates between 10 and 100 times slower than the theories would predict. As one astroseismologist said “We hadn’t anticipated that our theory could be so wrong…. For me, finding that problem was the biggest achievement of the field in the last ten years.”.

We now use strain gauges in electronic measuring scales. They were originally invented for an entirely different purpose.

Seismology and astroseismology offer clear links between listening to your coffee cup and earthquakes (or star quakes). But there is one more earthquake related connection to the coffee cup and it could be noticed by any of us who want to improve our home brewing technique.

To brew better coffee, we need to measure the mass of the coffee beans that we are using. Typically we will use a set of electric scales for this. Inside the scales is a device, called a strain gauge, that shows a change in its electrical resistance as a result of the pressure on it (from a mass of coffee for example). The scales translate this change in the electrical resistance to a mass that is shown on the display. One of the inventors of the strain gauge however was not thinking about measuring the mass of coffee at all. His interest was in earthquakes and specifically, how to measure the effect of the stresses induced by earthquakes on elevated water tanks. In order to do that he needed a strain gauge which led to the devices that you can now find in your measuring scales.

Two links between your coffee cup and earthquakes or seismology. Are there more? Do let me know of the connections that you find, either in the comments below or on Twitter or Facebook.

Categories
Coffee cup science General Science history

A demon in your coffee

Americano, Maxwell's demon
So innocent looking. But could an imaginary demon lurking within help us to understand more fully a major theory in physics?

Is there a way of preparing an Americano that can reveal a particularly knotty problem in physics with implications for information theory?

The question arises out of a field of physics, developed through the nineteenth century, that deals with energy and temperature: thermodynamics. It is the theory that describes how a hot coffee, left in a cold room, will eventually cool to the temperature of the (ever so slightly warmer) room. And though this may seem a trivial example, the theory is immensely powerful with applications from steam engines to superconductors. But it is back with the cooling coffee that we may find a demon, and it is worth finding out a bit more about him.

There are four laws of thermodynamics (the original three and then what is known as the ‘zeroth’ law). But it is the second that concerns us here. It can be phrased in a number of different ways but essentially says that there is no process for which the only result is the transfer of heat from a cold object to a hot one. To think about our coffee, the coffee will cool down to the same temperature as the room, but as the law describes, the room cannot get colder by giving its heat to the coffee cup (so the coffee gets hotter)!

It is in fact, one of the few places in physics where there is a ‘direction’ to time. For most of the laws of physics, time could run in the opposite direction without changing the effect, but not so for this one. The second law of thermodynamics is a definite provider for an arrow of time.

coffee clock, Rosslyn coffee
The clock at Rosslyn Coffee in the City of London. But the image alludes to a fundamental truth: the way that coffee cools is one of the few areas of physics for which it matters which way time ‘flows’.

But that is a digression. We ought to return to the demon in the coffee. The second law of thermodynamics seems to be based on our common sense (though perhaps that is because our common sense is formed within the laws of physics that determine the second law of thermodynamics). But with confidence in our common sense to understand the second law of thermodynamics, let’s do a thought experiment in which we make a strange type of Americano. Imagine a cup of coffee with an impermeable partition cutting through it. Into one half of the cup we pull a lovely, single origin, espresso. The crema rising onto the surface with some brilliant tiger striping on show. Into the other half of the cup we pour some water, initially at the same temperature as the coffee. We drill a small hole in the partition and watch what happens. Of course we know what happens. Ever so slowly, the coffee starts to get into the water and the water into the coffee until we are left with a balanced Americano on both sides with both sides at the same temperature.

Great, but now let us introduce the demon. Actually, he’s called “Maxwell’s Demon” because it was Maxwell who first proposed him (in ~1871), but we can call him anything we like. Perhaps he’s not a he at all. Our demon sits next to the small hole we have made in the partition and watches as the molecules travel towards the hole from the water’s side and the side holding the coffee. This demon is a bit of a trouble maker and so any fast moving molecules (hot) from the water he allows to get into the coffee and any slow moving molecules (cold) from the coffee he allows to get into the water. He does not allow slow molecules from the water into the coffee or fast molecules from the coffee into the water. Just to add to the mix, any coffee solubles he returns to the coffee allowing only water molecules through the hole in the partition.

If our demon exists, we would end up with a lot of very fast molecules on the coffee side (which will therefore be hotter) while the water would hold slower molecules (and be colder). We’d have a very hot espresso on one side of the partition and some luke warm water on the other. It’s not only a terrible Americano but a violation of the second law of thermodynamics! Which is worse?

Although he was proposed as a thought experiment, it is a problem with serious implications for the second law of thermodynamics (which otherwise seems to be a very good model of how things work). Because while we may not seriously consider an actual demon in the coffee, what stops some mechanical tool that we make from violating the second law, if the demon, in principle, could exist? Could the second law be wrong? Could there be a way of getting heat into our coffee from a cold room?

3D hot chocolate art on an iced chocolate, Mace, Mace KL, dogs in a chocolate
Art on a hot chocolate at Mace in KL. Well, what is your mental image of Maxwell’s demon?

The consensus has been that even were the demon to exist, ultimately he is powerless against the second law which does not get overturned by his presence. Because even if we could end up with a super hot espresso on one side of the barrier and cold water on the other side, this is not the whole system; the whole system includes the demon. And the second law applies to the whole system not the system minus the demon. So when we consider the energy (and entropy) of the demon in doing the work necessary to decide which molecules to let through and which to filter out, we find that work is done on the system (by the demon) and the entropy, the disorder if you like, of the whole system has increased (which is another way of phrasing the second law). Calm is restored, we get our Americano back, the laws of physics as we understand them are retained.

But Maxwell’s demon has not been completely exorcised yet, or at least, he is proving to be quite helpful. Because it turns out that there are methods for which the energy cost for the demon is minimal and the argument above no longer works. It seems we are back to square one. But even in that situation, it was realised that the demon has to record, make a note of, which molecules are fast and which are slow, which are coffee and which are water. It has led to an understanding that information has to be part of our consideration of thermodynamics. And as our ability to manipulate nanostructures and individual atoms improves, so experiments are able to explore how information ties into thermodynamics and why Maxwell’s demon still has not undone the second law yet. But it is here that we encounter another demon, the one that is found in the details, so if you are interested you can read more about it here.

Categories
Coffee cup science Coffee review Science history Tea

Schrodinger’s Katsute (100), Angel

Katsute 100, tea in Islington
It was a sunny day when we visited Katsute100 in Angel, Islington

When Bean Thinking started, it was always going to be about coffee and yet, Katsute 100 is definitely a tea place. Not only that, but the idea was to see how the physics that we use to describe our universe is mirrored by the physics of the coffee and in a cafe, the physics of the every-day. On the other hand, the whole point of Schrodinger’s cat is to demonstrate how aspects of quantum mechanics are absolutely unlike our everyday experience: a cat both (and neither) dead and alive? And yet, without giving too much away, today’s cafe-physics review is absolutely this – a review of a tea house that features the famous thought experiment. How far Bean Thinking has moved!

Katsute 100 is a welcoming, and peaceful, Japanese tea place in Angel. With a full tea menu and some really great desserts, it is definitely a good place to spend half an hour, maybe more, watching the coming and going and exploring the tea. And there is certainly a lot of tea to explore, different tasting notes revealing themselves as the tea cools, the carefully placed tea pot and tray adding to the experience.

The shop itself is fairly narrow, decorated in sympathy with the Georgian age of the shop itself and with a view into a garden at the back. Japanese tea making equipment is displayed (and for sale) on the various wooden cabinets around the shop. My tea had been buttery (exactly as it had been described in the tasting notes) and the Ichigo Daifuku I had had with it was a fascinating exploration of texture. There were some Japanese art works on the wall and it was then that I saw my first one: a cat. Not a real one of course but one of several decorative cats that are, almost hiding, around the shop. The word “Katsute” has nothing to do with cats apparently meaning “once”, but nonetheless, a few cats do pop up here and there. And even where cats don’t pop up, there are drawers in the wooden cupboards that seem much like boxes, is there a cat there in the box? Is it dead, alive, both, neither? What does this even mean? And is it connected to Katsute, “once”, after all?

note the pouring slits on the teapot
Tea pot, tea cup and ichigo daifuku at Katsute 100

Looking carefully at my teapot, three grooves were carved into the spout allowing the tea to flow out. Each stream of complex flow interferes with the neighbouring stream to present an aesthetic of flowing liquid to match the sound and flavour of the tea. And of course it is reminiscent of an experiment that is key to the unfamiliarity of quantum physics: the double slit experiment.

When light (of a single wavelength, such as from a laser) is shone at a sheet with two holes in it, the light that has travelled through shows interference fringes and patterns. Indeed, it is one of the experiments that went to establishing the theory that light was a wave (and not, as Newton among others had thought, a stream of particles). The situation is quite different if you tried to pass particles through two slits, imagine a sieve with two holes and a stream of coffee beans travelling towards it, we’d expect each bean to go through one hole or the other, not both. In classical physics that’s what we would expect too and yet, when sub-atomic particles (such as electrons) were aimed at two slits and made to travel through them they interfered with each other, as if they were not particles but waves. But other experiments had shown conclusively that they were also particles and indeed, when each individually hit the detector it did so as a single spot, as a particle. Particles and waves? What was going on?

cupboards in Katsute 100
A lot of sake and a fair number of drawers. But what is behind each drawer and why is one missing?

In fact it was a result that had been predicted: Louis de Broglie had shown, theoretically in 1923, that all particles should have wave-like properties and simultaneously, that all waves should have particle-like properties. We should expect that under certain circumstances, light, electrons, neutrons etc, even atoms, should behave as particles and under certain other circumstances (such as the double slit experiment) as waves. But there was an important catch. The electron travelling through a double slit will behave as if it is a wave, passing through both slits and interfering with itself to produce the characteristic “diffraction pattern” of a wave but only if we do not try to look at it to see which slit it really passed through. If we try to detect which slit the particle has travelled through, we can indeed find that some of the electrons travel through one slit and some through the other but when we look at the resulting interference pattern it is gone! What we are left with is the (classically expected) pattern of two particles going through two slits exactly as if they had been very small coffee beans. (You can see a video of Jim Al-Khalili explaining this peculiar result here).

What is going on? To a certain extent, this question is part of the reason that quantum mechanics can seem so strange. We can’t really ask what is going on, or rather, if we ask, we cannot expect to get an answer! We can describe what happens and we can make predictions based on the mathematics that we use to describe the processes. Our technology and our understanding of physics has developed hugely because we can describe how things will behave. But we will stumble if we try to understand what is really going on behind these processes. As Feynman said in lectures he gave to physics undergraduates:

“We cannot make the mystery go away by ‘explaining’ how it works. We will just tell you how it works. In telling you how it works we will have told you about the basic peculiarities of all quantum mechanics.”§

And so things remain enigmatic. Questions that appear to show paradoxes such as the problem of Schrodinger’s cat* continue to puzzle and intrigue us. Is the cat dead or alive? Can the cat be both? Is the cat an observer and what role does the observer have in physical measurements? What does this imply for the fabric of reality? And is there a connection back to the name of this cafe, “once”?

You perhaps should not expect to find any answers in Katsute 100, but pondering these things with a good cup of tea may help advance your understanding. It will certainly help advance your mood if you are in need of some peaceful, thoughtful, time out.

Katsute 100 is at 100 Islington High St, N1 8EG

§ Feynman Lectures on Physics Volume III, 1965

*The story of Schrodinger’s cat is that a cat is placed in a box together with a small amount of radioactive source material. The box is then closed and we cannot see inside. The amount of radioactive material is such that in one hour it has a 50:50 chance of decay. If the material decays radioactively, it triggers the release of a vial of poisonous gas which would kill the cat. Our mathematical models of quantum mechanics suggest that, until it is measured, the radioactive material is in a ‘superposition of states’: it has both decayed and not decayed; the cat is both dead and alive. Only when we open the box after an hour and thereby measure the state of the radioactive material does the cat, at that point, ‘collapse’ into a state that is either dead or alive.

Categories
Coffee cup science General slow Sustainability/environmental

Coffee and science: a problem shared?

coffee and Caffeine at Sharps
What is the future of coffee? Science? Our society? Are these things held together more closely than we imagine?

There is a lot of science in coffee (and a lot of coffee in science). And there are also many scientists who are keen coffee drinkers and vice versa. But is there more in common between these two fields than even this? Could a shared problem be hiding a different (shared) problem?

One issue for coffee drinkers is reliability and reproducibility. How can we ensure that we get a good cup each time we visit a café or brew our own? In a similar way, how do we ensure that our experimental results in science are correct and reproducible? It is a fairly fundamental tenet of science that an experiment should be able to be reproduced in another lab with similar equipment. The suggestion is that this is not always happening, we have a ‘reliability’ problem in science (and sometimes in coffee).

A possible solution, in both fields, is some form of automation. In the world of coffee this is quite obviously just by making coffee via a machine. There have been several attempts to make reproducibly good coffee using an automated pour over machine. In the world of experimental science, it is not quite so clearly an automation process but is described instead as “data sharing”. The results of all experiments, or at least those that are published, should be shared (uploaded with the published paper) so that others can examine the data in more detail and form their own conclusions†.

For both coffee and science, it is suggested that this opening up of our process so that it is more transparent or reliable, will increase the reproducibility of good results and, crucially, mean that we get those results faster. We will get reproducibly good coffee without having to queue so long in the morning; we will make discoveries more quickly and have faster progress in science.

It seems that the problem that we thought we had, reproducibility, is perhaps not the one that we are actually aiming to solve. The problem we seem to be interested in is ensuring faster progress.

coffee under the microscope
We can look at coffee under the microscope (here are two different coffees ground to the same degree). But do we need to look more closely at the process of making good coffee or of doing science?

But an emphasis on faster progress can undermine our initial ideal of a reproducibly good result. For scientific research the emphasis on getting results quickly (at least within the time frame of the science funding cycle) has led to predictable problems. There are cases that I know about where results that were contrary to those that were wanted were suppressed. Not permanently. No, that would be demonstrably scientific fraud. No, suppressed just long enough for the first ‘ground breaking’ paper to be published. Then, after a suitable length of time, the second paper showing the problems with the first can follow up. Those involved get two papers (at least), and rapid progress is shown to be happening in the field. Would data transparency help here? Clearly not, because the initial set of data would still be suppressed until it was wanted those few months later.

What we need is a change in the structure of how science is done. We need to value scientific integrity and so trust that other scientists do too. We know that the current situation whereby promotions, funding etc are determined by the number of papers in ‘good’ journals, can act to undermine scientific integrity. This needs to change if the reliability issue is to be addressed. In the type of case described above, there would be no consequences for the people who kept their names on the paper(s) published. The only consequences would be if anyone refused to have their name on the paper as they knew it was misleading. And even then, the consequences would be to that person/those people in terms of their CV, and publication list, not those who published the paper and of course, shared the data.

coffee at Watch House
A good pour over takes time.
What are we looking for in coffee, in science? Is progress an aim of itself?

For the coffee, there are already discussions about whether the increase in throughput offered by automated coffee brewing techniques really contributes to the coffee experience that the cafés are trying to encourage. Can we really expect someone to slow down, take in the aroma, the mouthfeel, the taste and flavour if we rush the cup through to them on a production line? Isn’t part of the enjoyment of something to have to wait for it (hence lent before Easter; fasting before a feast)?

It is not that automation necessarily is bad. We can get a genuinely all round good coffee in a café that utilises a machine based pour over (perhaps). We can also get genuinely reproducible data in a situation where data is routinely shared. It’s just that data sharing does not solve the reproducibility problem, nor does automation give us continually good coffee. What makes the difference is a café that cares about the product that they are serving; scientists that care about the integrity of the research that they are doing. Automation processes give us faster results, they do not, automatically, give us better science (coffee).

Moreover, our desire for faster progress obscures questions that we should be asking if we slowed down a little. What is a good coffee experience? Who (if anyone) should own the scientific data shared? Is our desire for good coffee, quickly and (relatively) cheaply obtained, an aspect of that consumerism that is damaging for the planet’s ecological health? How much do we need to trust each other (and take responsibility for our own integrity) for our society, including our scientific society, to function? Is faster progress in and of itself, a “good” to aim for?

And perhaps, there is a final, more fundamental question. Have we become so accustomed to seeing ourselves and our work as merely a cog in a machine that we have become inured to the dehumanisation of society which seems to us almost natural and itself progress? Is this what we want for society?

The process of making a good cup of coffee indeed shares many things with the process of doing science. Perhaps this should not be surprising, both are practises embedded in our society. Certainly our view of the society that we live in can be informed by slowing down with our coffee as we enjoy a little science (or should that be the other way round)?

 

†It is not quite an automation process in the sense that the data is taken by a machine and then uploaded. However, it is still a dehumanisation process. At the root of the concept is the idea that the human experimenter can be taken out of the process. I would be happy to expand on this in the comments but for the sake of readability haven’t done so here.

Categories
Coffee cup science General Home experiments Observations Science history

Telling the time with an Aeropress?

Aeropress bloom, coffee in an Aeropress
The first stage of making coffee with an Aeropress is to immerse the coffee grind in the water. Here, the plunger is at the bottom of the coffee.

On occasion, it takes a change in our routine for us to re-see our world in a slightly different way. And so it was that when there was an opportunity to borrow an Aeropress together with a hand grinder, I jumped at it. Each morning presented a meditative time for grinding the beans before the ritual of preparing the coffee by a different brew method. Each day became an opportunity to think about something new.

Perhaps it is not as immediately eye catching as the method of a slow pour of water from a swan necked kettle of a V60, and yet making coffee using the Aeropress offers a tremendously rich set of connections that we could ponder and contemplate if we would but notice them. And it starts with the seal. For those who may not be familiar with the Aeropress, a cylindrical ‘plunger’ with a seal tightly fits into a plastic cylinder (brew guide here). The first stage of making a coffee with the Aeropress is to use the cylinder to brew an ‘immersion’ type coffee, exactly as with the French Press (but here, the plunger is on the floor of the coffee maker). Then, after screwing a filter paper and plastic colander to the top of the cylinder and leaving the coffee to brew for a certain amount of time, the whole system is ‘inverted’ onto a mug where some coffee drips through the filter before the rest is forced out using the plunger to push the liquid through the coffee grind.

clepsydra creative commons license British Museum
A 4th century BC Ptolemaic clepsydra in the British Museum collection. Image © Trustees of the British Museum

Immediately perhaps your mind could jump to water clocks where water was allowed to drip out of two holes at the bottom of a device at a rate that allowed people to time certain intervals. It is even suggested that Galileo used such a “clepsydra” to time falling bodies (though I prefer the idea that he sang in order to time his pendulums). With many holes in the bottom of the device and an uneven coffee grind through which the water (coffee) flows, the Aeropress is perhaps not the best clock available to us now. However there is another connection between the Aeropress and the clepsydra that would take us to a whole new area of physics and speculation.

When the medieval thinker Adelard of Bath was considering the issue of whether nature could sustain a vacuum, he thought about the issue of the clepsydra¹. With two holes at the bottom and holes at the top for air, the clepsydra would drip the water through the clock at an even rate. Unless of course the holes at the top were blocked, in which case the water stopped dripping, (a similar thing can be observed when sealing the top of a straw). What kept the water in the jar when the top hole was blocked? What kept it from following its natural path of flowing downwards? (gravity was not understood at that point either). Adelard argued that it was not ‘magic’ that kept the water in when no air could go through, something else was at work.

What could be the explanation? Adelard argued that the universe was full of the four elements (air, water, fire, earth) which are “so closely bound together by natural affection, that just as none of them would exist without the other, so no place is empty of them. Hence it happens, that as soon as one of them leaves its position, another immediately takes its place… When, therefore, the entrance is closed to that which is to come in, it will be all in vain that you open an exit for the water, unless you give an entrance to the air….”²

inverted Aeropress and coffee stain
The Aeropress inverted onto a coffee cup before the plunger is pushed down. Complete with coffee stain behind the cup where the inversion process went awry.

Now, we would argue that whether the water flows down and out of the Aeropress, or not, depends on the balance of forces pushing the water down and those pushing it up. The forces pushing the water down and out of the clepsydra, or Aeropress, are gravity and the air pressure above the water in the cylinder. Pushing it up, it is only the air pressure from below. Ordinarily, the air pressure above and that below the water in the Aeropress are quite similar, gravity wins the tug of war and the water flows out. In an enclosed system however (if the holes at the top are blocked), were the water to flow out of the bottom, the air pressure above the coffee space would reduce. This makes sense because, if no new air gets in, the same amount of air that we had before now occupies a larger volume as the water has left it, the pressure exerted by that air will have to be less than before. A reduced air pressure means a reduced force on the water pushing it down through the filter and so the force pushing the water down can now be perfectly balanced by the force (from the surrounding air) pushing the water up: the water remains in the Aeropress. The only way we get the coffee out is to change the balance of forces on the water which means pushing down the plunger.

But perhaps it is worth stepping back and imagining what the consequences could be of having the idea that the universe was just full of something that had to be continuous. You may find it quite reasonable for example to consider that vortices would form behind and around the planets as they travelled in their circular orbits through this ‘something’*. Such vortices could explain some of the effects of gravity that we observe and so there would perhaps be no urgency to develop a gravitational theory such as the one we have. There would be other consequences, the world of vacuum physics and consequently of electronics would be significantly set back. In his lecture for the Carl Sagan Prize for Excellence in Public Communication in Planetary Science, The Director of the Vatican Observatory, Br Guy Consolmagno SJ explored previous scientific ideas that were almost right, which “is to say wrong” (You can see his lecture “Discarded Worlds: Astronomical Worlds that were almost correct” here) If it is true that so many scientific theories lasted so long (because they were almost correct) but were in fact wrong, how many of our scientific ideas today are ‘almost correct’ too?

It makes you wonder how our preconceptions of the world affect our ability to investigate it. And for that matter, how our ability to contemplate the world is affected by our practise of doing so. They say that beauty is in the eye of the beholder. For that to be true, the beholder has to open their eyes, look, contemplate and be prepared to be shown wrong in their preconceptions.

What connections do you make to your coffee brew each morning? I’d love to know, here in the comments, on Twitter or over on Facebook.

 

* Does a connection between this and stirring your freshly brewed Aeropress coffee with a teaspoon trailing vortices stretch the connectivity a bit too far?

¹ “Much Ado about Nothing: Theories of space and vacuum from the Middle Ages to the Scientific Revolution”, Edward Grant, Cambridge University Press, (1981)

² Quoted from Adelard of Bath’s “Quaestiones Naturales” taken from Much Ado about nothing, page 67.

Categories
Coffee cup science Observations

A shocking coffee connection

There have been some fantastic thunderstorms in London lately. Perhaps nothing to rival thunderstorms in the tropics but for this region of the world they were quite impressive. One lightning storm in particular came very close. Thank goodness for lightning conductors! Perhaps the connection between lightning storms and coffee is not obvious. But maybe this is because you mop up your coffee spillages too quickly.

Reynolds, rain, waves, pond, raining
There are so many coffee-physics connections with rain and weather. It’s worth looking out for more.

The link is in the mess and the maths. It turns out that the maths describing water evaporating out of a drying coffee droplet is the same, in one crucial detail, as the maths describing the electric fields around a lightning conductor. If we want to see why this may be, we need to get a little bit messy and spill some coffee.

The question is how do coffee rings form? We know that to start with the solids in the coffee are distributed fairly evenly throughout the drink. It is the same when you spill it, initially a spilled drop of coffee looks like, well, coffee. But if you wait as this spilled coffee dries, you will find that a ring starts to form around the edge of the drop. How? How does a uniform coffee distribution when the drop is first spilled become a ring of coffee solids around the edge of the dried drop?

coffee ring, ink jet printing, organic electronics
Why does it form a ring?

A number of different aspects of physics feed into this problem but the one that is relevant to the lightning conductors concerns how the water in the drop evaporates. If you think about how a water molecule escapes (evaporates from) the droplet, it is not going to go shooting off like a rocket blasted out from the drop. Instead it will take a step out the drop then encounter a molecule in the air and get deflected to a slightly different path and again, and again, and so on. It follows the same sort of “random walk” that we know that the bits of dust on a coffee surface follow (and the same sort of random walk that provides a link between coffee and the movements of the financial stock exchange but that is a whole other topic).

Now think about the shape of that spilled coffee drop. If a water molecule were to evaporate from the top of the dome of the drop, it has a certain probability of escaping but it also, because its path is random, has a certain probability of re-entering the droplet. A water molecule at the edge of the droplet however will have a lower probability of re-entering the droplet purely on the basis that there isn’t so much of the droplet around it. Over many molecules and many ‘escape attempts’, this lower probability of re-absorption will translate to a higher flux of water molecules evaporating from the droplet at the edges. The water will evaporate ‘more quickly’ from the edge of the droplet than from the top of it.

artemisdraws, evaporating droplet
As the water molecules leave the droplet, they are more likely to escape if they are at the edge than if they are at the top. Image © @artemisworks

When this is written mathematically, the rate of evaporating water is related to the contact angle between the drop and the surface. The shallower the angle, the higher the rate of evaporation or equivalently, the greater the ‘flux’. It is this mathematical expression that is the same as for the lightning conductor if, rather than refer to an evaporating water flux we refer to an electric field. So the more pointy the conductor, the greater the field concentration around it. A shocking example of the idea that everything is connected.

Of course, there is much more to the coffee ring than this with physics that relates coffee rings to bacterial colonies, burning cigarette papers and soap boats. If you are interested, you can read more about how coffee rings form (including why a higher evaporation rate helps lead to a coffee ring effect) here. If on the other hand you want some well justified thinking time, go spill some coffee and watch as the coffee dries.

Categories
Coffee cup science General Observations Science history slow

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?