Time to enjoy a Beethoven coffee

Portrait bust of Beethoven, Anna EG Hoffman, in the British Museum collection © Trustees of the British Museum

Portrait bust of Beethoven, Anna EG Hoffman, in the British Museum collection © Trustees of the British Museum

It is said that Beethoven prepared his coffee by counting, precisely, 60 beans per cup. Biographies of Beethoven certainly suggest that he had a significant coffee habit. Banned by his doctor from drinking coffee towards the end of his life, there are many references to him frequenting coffee houses in earlier years. Sadly, I have not found the source for the 60 beans story and so would not like to comment on its veracity. Nonetheless, it is a good story and it does link with coffee so, as today (17th December) is the 244th anniversary of his baptism (it is assumed that he was born the day before on 16th December 1770), it is “Beethoven day” on the Daily Grind.

To me, what lends some credibility to the 60 beans story is the fact that, as coffee lovers, we can be very particular about the way we prepare our brew. Some people, for example, weigh the amount of the coffee and the quantity of water and brew their coffee according to instructions from one of the various online brewing tutorials (see here for a good one from Hasbean). Personally, in the morning, I am far too bleary eyed to consider getting the kitchen scales out, nor would I count a certain number of beans. I do however count the number of seconds that I take to grind my coffee with my trusty burr grinder (always set to the same level of grind of course). Can counting the number of seconds for a quantity of grind possibly be a good way of measuring a specific quantity of coffee?

Did Galileo drop balls from the top of the tower?

Did Galileo drop balls from the top of the tower?

Galileo Galilei (1564-1642) died before coffee was properly introduced to Europe. He is relevant to this story though owing to his work on clocks and timing devices. One way that Galileo measured time was to collect water in a jug over the measurement period. It seems that this is almost the reverse of my morning coffee ritual. To check that he was measuring time correctly however, he needed a second, independent method. Of course, Galileo couldn’t use a watch or pendulum because watches hadn’t been developed at the time and Galileo himself was doing the work needed to understand pendulums and make them useful for clocks. So what else could he use to measure time? There is a clue to another method that Galileo used in his experiments on falling balls. Although there are questions as to whether Galileo really did drop balls from the top of the Tower of Pisa, we do know that he did experiments which involved rolling bronze balls down a groove. Along the groove were marks where strings made from gut had been pulled across the groove such that they made a sound as the ball passed, perhaps like the sound of a harp being plucked. By adjusting the position of these strings, the interval between the sounds from different gut strings could be made to match a known rhythm. The time it took for a ball to fall down the groove was being measured by matching its descent to a known tune. This suggests that Galileo sang while he was making his key measurements and that it was this that allowed him to start to understand how bodies fell under gravity. Singing was Galileo’s (surprisingly accurate) method of measuring time.

Which brings me full circle back to Beethoven. Beethoven certainly knew the “mechanician” Mälzel who invented the metronome as we now know it. There are also indications that Beethoven was aware of early versions of Mälzel’s invention. In 1813, the Wiener Vaterländische Blätter wrote “…Herr Beethoven looks upon this invention as a welcome means with which to secure the performance of his brilliant compositions in all places in the tempos conceived by him, which to his regret have so often been misunderstood“.  It seems that in the two hundred years between Galileo and Beethoven, there had been so many improvements to clocks and timing devices that singing, which had started off as a way to measure time, was now itself being regulated by the clocks that singing may have helped to develop.

How many beans go to make your morning coffee?

How many beans go to make your morning coffee?

So how is a Beethoven coffee, assuming that there is any veracity to the legend? Sixty beans works out as 8-10g which, depending on the amount of water in the cup could be weaker (or stronger) than modern brews. In my cup, it was slightly weaker than I am used to. I enjoyed my “Beethoven coffee” while listening to his String Quartet Op 74, “Harp”. As I sipped the coffee while listening to the first movement, I could almost hear the gut strings of Galileo’s experiment being plucked as the balls rolled by. The coffee itself (Costa Rica, Finca Arbar El Manatial, Yellow Honey, Caturra/Catual) was very smooth and rich, as you would expect from a coffee from Has Bean. Described in the tasting notes as “….An amazing caramel and milk chocolate sweetness partnered with delicate peach and apricot acidity…” It was the perfect coffee to enjoy with the Harp quartet piece. Sometimes it is important to take time to go slow and enjoy the coffee.

So why not raise a mug today to Beethoven and savour a Beethoven coffee? Please leave any comments using the form below, especially if you know a reliable reference to Beethoven’s coffee habit or have suggestions as to how to improve my morning brew.

Further reading:

Quotes taken from “Thayer’s life of Beethoven”, Revised and Edited by Elliot Forbes, Princeton University Press, 1967

Information on Galileo and time: “Styles of Knowing, A new history of science from ancient times to the present”, Chunglin Kwa, University of Pittsburgh Press, 2011

Copper latte

Brew&Bread, latte art Sun, KL latte art

Taken at Brew & Bread, One City Mall, Subang, KL, Malaysia

Pop into any cafe and order a latte and chances are you’re going to see some great latte art. With the number of good baristas around competing to produce the best and most consistent latte art, it is easy to see some good art while waking up of a morning. Brew & Bread is a cafe with a couple of outlets in Kuala Lumpur in Malaysia. One of their customers sent me these images of their latte art (via Bean thinking on Facebook), which I think are among the finest examples I have seen of latte art being served, as a matter of course, at cafés. Apparently the people at Brew & Bread take their latte art very seriously, so if you find yourself in One City Mall, Subang or Kota Kemuning in Kuala Lumpur, do take the opportunity to pop in.

Not being a barista I can only guess at the skill that it takes to produce such great images as those at Brew & Bread. As a scientist though I can see some connections between latte art and copper mining. Or rather, the link between good latte art and bad copper mining (and vice versa). How? It’s all about the bubbles.

The small bubbles in the foam on the left trap coffee between them. The larger bubbles in the foam on the right allow coffee particles (and water) to leak and don't trap them so well.

The small bubbles in the foam on the left trap coffee between them. The larger bubbles in the foam on the right allow coffee particles to drain by gravity and don’t trap them so well.

Now, I am on dangerous ground here because I have no experience in making latte art, nor really in steaming milk, so I hope that any baristas out there will leap in and leave comments if I have something awry in my description of how latte art is sustained. However, from various videos and how-to’s available online it seems that a key component for good latte art is making the milk into a micro-foam; a ‘velvety’ structure of tiny bubbles. From a physics perspective this makes sense. As the milk is first introduced into the espresso it picks up the crema on the espresso and captures the coffee-liquid mixture between the surfaces of the bubbles of the froth. A large number of very small bubbles will trap the coffee liquid and particles around the bubbles very well (see diagram). If the milk has too many large bubbles, not only will the mouth-feel get affected, the coffee itself is not held and trapped so well within the bubbles. When the art is about to be created, the barista slows the rate of pouring such that the coffee does not get pulled up with the milk and instead the milk foam is allowed to float on top of the espresso where it remains white. It is this contrast between the trapped coffee in the fast-poured milk and the pure milk of the more slowly poured milk that leads to the contrasts of what is known as latte art.

beer foam, bubble size

The bubbles get larger as they move higher up in a foam column. Shown here in a narrow glass of Corsendonk Agnus (beer)

Now consider copper mining. It is an unfortunate fact that we as a society are very reliant on mined products including copper. Copper is the backbone of our electricity network meaning that if you are reading this at all, you are relying on copper that has been mined somewhere in the world. Mining is a fact of our modern way of life. The question is how to reduce its environmental impact to a minimum. One way to minimise the environmental aspect of mining would be to ensure that it is as efficient as possible. Copper is often found in two forms, a relatively easy to extract oxide and the sulphides of copper which are harder to extract. The ‘froth flotation’ technique has been developed to maximise the extraction of these sulphides by using a foaming vat in a process that is the exact opposite of latte art. The copper sulphide rocks are ground until they are very small (around 0.05mm diameter) whereupon they are reacted with chemicals that make them hydrophobic (resistant to bonding with water). Other particles and rocks, that are mined together with the copper sulphides, do not react with the chemicals and so are less hydrophobic. The resulting ‘grind’ is mixed into a slurry and then introduced into a chamber which is aerated to form bubbles. As they are hydrophobic, the copper sulphide particles attach themselves to the newly formed bubbles to reduce their contact with water. The bubbles are then carried up through the chamber, taking the copper with them. The small bubbles at the bottom of the vat trap a lot of water and waste material between them. As the bubbles move upwards through the vat, they get larger (by combining with each other) and, whereas the copper sulphides, which are chemically attached to the bubbles remain with the larger bubbles, the liquid and waste material drains out towards the bottom of the tank. The copper products can then just be skimmed off the top of the vat. Unlike latte art, larger bubbles are useful in froth flotation in order that particles do not get trapped between the bubbles. What is good for the copper mining is bad for the latte art and vice versa. The more we know about the bubbles in foams (in both latte art and froth flotation) the more efficient and the more aesthetically beautiful our world can be.

Another from Brew & Bread

Art for Christmas, another piece of great latte art from Brew & Bread

I would be very interested to know your thoughts on why a microfoam is needed for good latte art or indeed, any aspect of latte making. Please do feel free to share any good photos of latte art (or cafe recommendations) either here in the comments section or on Bean thinking’s Facebook page. There will be another latte art article in the New Year so new photographs (or cafe recommendations) would be greatly appreciated.

With special thanks to Oh Ying Ying for the photographs from Brew&Bread.

Calming the waves at Brutti & Boni

Brutti And BoniBrutti & Boni is a fairly new Italian cafe in South Kensington. Located at the less busy end of Gloucester Road, it was quiet when we popped in to try it a couple of weeks ago. The bright interior has light coming from a roof window at the back of the shop, though it seems that many people opt to sit outside with their espresso in the morning, watching the traffic go past. They serve Caffe Molinari coffee together with a good selection of Italian food items. All in all, a good place to go if you are in the area visiting the Science, Natural History or Victoria and Albert museums and fancy a break and a relaxed coffee nearby.

Inside, the shelves are stacked with various Italian condiments, pasta and olive oil. It was this that prompted me to visit Clapham Common to retrace the steps of Benjamin Franklin. Franklin of course was one of the founding fathers of the USA. He was also a keen scientist, diplomat, printer, in fact the man in some ways defines the word “polymath”. His interests and importance span so many areas that it is difficult to write a two-sentence description of him. Fortunately, for the purposes of today’s Daily Grind, I do not need to. Today, all that is important is that Franklin did some experiments on Clapham Common with oil.

Shelves of olive oil at Brutti & Boni

Shelves of olive oil at Brutti & Boni

Franklin had been investigating the “old wives tales” that a small amount of oil placed onto water ‘calmed the waves’. In fact, the old wives tales can be traced back to Pliny (the Elder) in his Natural History written in around 77AD. Pliny had written of pearl divers and how they sprinkled oil on their faces so that the water above them became calm, allowing them to see the oysters that they were looking for on the sea bed. Franklin himself describes, in his letter to the Philosophical Transactions (1774), an event that he experienced in 1757 while sailing to the UK. Noticing that the wakes behind two of the boats in the fleet were calm, he describes how he asked his ship’s captain about this curiosity. Replying slightly dismissively, as if to someone who is quite ignorant of the workings of the world, the ship’s captain replied that “The cooks… have I suppose been just emptying their greasy water through the scuppers, which has greased the sides of these ships a little”. Obviously it was common knowledge that oil calmed the waves.

So, one day in the 1760s, Franklin took a walk to Clapham Common and to Mount Pond. Emptying about a tea-spoonful of oil (oleic acid) into the pond he watched as the oil produced an “instant calm [on the pond] over a space several yards square, which spread amazingly, and extended itself gradually till it reached the lee [opposite] side, making all that quarter of the pond, perhaps half an acre as smooth as a looking glass.” Oleic acid is the principal component of olive oil. Franklin had effectively calmed the waves on the pond with a mere tea-spoonful of olive oil.

A view over Mount Pond, Clapham Common

A single tea spoon of oil would calm the ripples on Mount Pond, Clapham Common

We can calculate how thin the layer of oil had become by dividing the volume of oil in a teaspoon (5cm³) by the area of half an acre (2023 m²) to get an oil layer that was 2.5 nm thick. To put this in perspective, a coffee bean of width 7 mm would fit nearly 3 million of such oil layers in itself width-wise. Later, more precise, measurements of the thickness of such an oil layer, by Lord Rayleigh and Agnes Pockels, gave 1.6 nm and 1.3 nm respectively. This is approximately the length of a single oil molecule. It seems that the waves on water can be stilled by a single molecular layer of oil. How does this work? Why not let me know what you think in the comment section below.

The attractive power of coffee

Just imagine, you are trying to fill 3 espresso cups at once but all you have is a portafilter with two spouts and a balloon? Ok, that sounds unlikely. The experiment that I’m going to describe however will allow you to bend a stream of coffee with a balloon. Moreover, in order to work it relies on sub-atomic particles. What a party trick; investigating sub-atomic physics while filling two cups with one stream of coffee. It could be mind bending, instead it is coffee bending.

What happens?

When you rub an inflated balloon on your (dry) hair, electrons are transferred from your hair to the rubber balloon. Electrons are, of course, sub-atomic particles and, together with protons and neutrons, they build up atoms. As these electrons carry an electric charge, the balloon becomes the source of a static electric field.

Thanks to Artemisdraws for the schematic

The electric field from the balloon aligns the water molecules such that the coffee gets attracted towards the balloon.

Water molecules are composed of two hydrogen atoms and an oxygen each. They are electrically neutral. However water is also a strongly polar molecule, meaning that when it is subjected to an electric field, the molecules will tend to align such that they are more positively charged closer to the negatively charged balloon and more negatively charged further away from the balloon. This charge distribution means that the stream of water gets attracted towards the balloon. The amount that the coffee stream bends is dependent on the strength of the electric field from the balloon and the mass of the stream which is still being pulled down by gravity.

The video suggests using cold brew coffee when you test this at home. There are two reasons for this. Firstly, if the balloon gets too close to the coffee stream, it can get splashed. There is a chance that this may burst the balloon. Secondly, and more fundamentally, the water molecules are more agitated at higher energies (temperatures). This means that thermal agitation weakens the average dipole moment of the water thereby weakening the attraction between the coffee stream and the balloon. In this effect as in its taste, cold brew is a stronger drink than your ordinary hot filter.

Let me know if you try this and how you get on. It would be particularly interesting to see any attempts made on bending coffee from an espresso machine. My thanks, as always, to artemisdraws for the helpful schematic shown here.

Arepa and Co, Haggerston

Haggerston Canal

Arepa and Co are on the right hand side of this canal

Edmond Halley (of comet fame) was born in the London district of Haggerston in 1656. More recently, Arepa and Co a Venezuelan cafe located alongside the canal that runs through the district, has just celebrated its first birthday there. This cafe serves a variety of Venezuelan foods including the arepas of the name which are, apparently, a traditional corn cake that can be filled with a variety of fillings (more info here). There are seats both inside the cafe or outside, overlooking the canal. As it was the early afternoon and we’d already had lunch, we decided upon a coffee, a sugar cane lemonade and, to accompany it a plate of Tequenos de Chocolate. These unusual little pancakes filled with chocolate were delightful to enjoy with a cup of coffee and a view over the canal. Sitting back and enjoying this relaxing view, I noticed a tree on the roof of a building on the opposite side of the canal. Hanging on the tree were a number of glass shapes. As the wind blew, the different faces of the shapes caught the Sun. Looking towards these glass shapes, they appeared to change colour as the sunlight was refracted through them. A glinting rainbow array of light fell onto our side of the canal.

The story of the investigation of colour is a great example of how our preconceived ideas can influence the results that we think we see. Up until the seventeenth century, colour was viewed as a property of the surfaces of an object as opposed to “light” which was that which rendered objects visible. Therefore trying to explain how rainbows formed or light scattered from ornaments was a difficult task. Indeed, medieval philosophers (the term ‘scientist’ is a nineteenth century invention), considered that there were only seven colours: Yellow, orange, red, purple, green and black and white.

Prism associated with Isaac Newton

A late C17th prism in the British Museum collection, © Trustees of the British Museum

Work understanding colour as a refracted component of white light started with Marci in his 1648 work Thaumantias (another name for Iris, the Greek goddess of the rainbow) and continued with Newton’s famous experiments with prisms. Newton showed that a glass prism refracted the different colours of light by different amounts (resulting in a spectrum). If two prisms were placed at right angles to each other, the rainbow of light from the first prism recombined into white light emerging from the second. With the change in mindset that this brought about, phenomena such as the rainbow could be more easily explained.

Grecian, Coffee House, London Coffee House

The Devereux pub now stands on the site of the Grecian coffee house, a former meeting place of the Royal Society

Which brings me back to coffee. Back in the eighteenth century cafes (or coffee houses) were not just places to have coffee but places to engage in the latest philosophical, political or scientific discussion and debate. Scientists of the day regularly gave public lectures and demonstrations in coffee houses both as a way of entertainment and of education. One scientist who participated in this was Stephen Demainbray (1710-1782). Demainbray demonstrated Newton’s experiments and theories on colour to a coffee drinking audience. The models that he used to explain the refraction of light are now on display in the Science Museum which is well worth a visit if you are in London. In the present day, there are still cafes and coffee houses that try to do a similar thing (of showing fun science to a coffee drinking audience), although perhaps sadly there are fewer now than there were then. Two movements that are trying to put the science back into coffee houses are Science Cafes and Cafe Scientifique. Although not always held in cafes, both movements have the aim of combining interesting science with a cup of coffee or glass of wine. Somewhat poetically the next Cafe Scientifique in London is to be held, on the 9th December, at the Royal Society. It is poetic because back in the time of Newton, discussions with the Royal Society president (Newton) and other society members took place at the Grecian Coffee House.

Both “Science Cafes” and “Cafe Scientifique” have events worldwide. It is worth taking a look at their websites to see if there is an event near you. Why not pop along and see what you can find out while having a cup of coffee?

 

Sources used:

The Rainbow Bridge, Raymond L Lee, Jr and Alistair B Fraser, Pennsylvania State University Press, 2002

The Nature of Light, Vasco Ronchi, Heinemann, 1970

London Coffee Houses, Bryant Lillywhite, George Allen & Unwin Ltd, 1963

The destructive power behind a spoon

Have you ever sat waiting for someone in a coffee shop, slightly bored? Resisting the urge to check your email or Twitter on the phone (perhaps the battery is dead), you have been stirring your coffee and playing with the vortices that form behind the spoon. Have you wondered why they form? Or played with detaching a vortex from the spoon and getting it to ‘bounce’ off of the side of the cup?

chimney, coffeecupscience, everydayphysics, coffee cup science, vortex

The spiral around this chimney helps to prevent vortex formation in high winds

Such vortices form behind objects in a flow of liquid when either the speed of the liquid, or the size of the object, reaches a critical value. The research about how and why these vortices form is a huge field. From improvements to plane design, through understanding insect flight and even into how wind instruments such as flutes work, understanding these vortices is a challenging topic. It is also useful to know about the behaviour of these vortices when designing chimneys in order to prevent their collapse.

Chimneys are of course stationary, but when they are in high winds, vortices form around the chimney just like the vortices behind the spoon (rather than the spoon moving through the coffee, the wind moves past the chimney). At relatively low speeds, the wind forms small whirlwinds as we see behind the spoon in the coffee. At higher wind speeds, the vortices forming behind the chimney can start to detach and form a pattern known as a Karman vortex sheet. As each vortex detaches from the chimney it subjects the chimney to a small force. Under some conditions and around some objects, this can result in the rather beautiful sounds of the Aeolian harp. Under more extreme conditions, it can result in the collapse of chimneys. The Ferrybridge C cooling towers collapsed in 1965 in high winds as a result of the turbulence around the cooling towers. To minimise the chances of such vortex sheets forming, chimneys are now designed with a spiral pattern (pictured) around them. Far from being an aesthetic feature, this spiral channels the wind so that vortex sheets cannot form behind the chimney.

Something to think about next time you’re waiting for someone in a cafe.

Helium in your coffee?

C-C bond, Esters N16

The sign board at Esters. Note the zig zag underline

As a website based on the physics inside a coffee cup, it was only a matter of time before I visited Esters in Stoke Newington. The name has significance to anyone interested in the physics (or chemistry) of coffee and the signboard outside the shop confirmed it. Under the name, there is a zig zag underline that represents part of a molecular structure. The end of each straight line signifies a carbon atom which is bonded to its neighbouring carbon atom by either a single (one line) or a double (two lines) bond. Inside you can enjoy (as I did) single estate coffees that can be prepared (if there are 2 or 3 of you) in a Chemex, appropriately enough. As I left Esters, I wandered through a local park where, near the entrance to the park, were two helium balloons caught in a tree. One had deflated, the other floated, dejectedly, just beneath the branches. Such a timely observation! The story of the discovery of helium connects the signboard at Esters, a cup of coffee and helium itself, how could that be? You’ll just have to keep reading to find out.

neon sign, light emission

The colours of “neon signs” depend on the particular gas (eg. neon) in the tubes

Helium is the second most plentiful element in the universe but on earth it is relatively rare. It was therefore not discovered on earth but, instead, by looking at the Sun. Its ‘discovery’ in the Sun was due to the way in which atoms interact with light. The atoms in each element emit (or absorb) light at specific frequencies. These frequencies correspond to different colours. It is this property of atoms that creates colours such as the distinctive hue of neon lights. In 1868 two astronomers were observing the same solar eclipse. Independently of each other, they noticed a distinct emission of light from the Sun at a wavelength of 587.49 nanometres (yellow-ish). This emission line corresponded to no element that had been found on earth and so one of them, Norman Lockyer suggested naming this new element helium, after ‘Helios’ the Greek god of the Sun. Helium was not found on earth for another 27 years when William Ramsay isolated it from a uranium based compound. The gas that Ramsay extracted, absorbed and emitted light at the same frequency as the two astronomers had observed for the element in the Sun. Helium had been found on earth.

Think of energy levels as rungs on a ladder. Image credit © www.artemisworks.co.uk

Think of energy levels as rungs on a ladder. Image credit © www.artemisworks.co.uk

Atoms absorb (and emit) light because of the way that the electrons in the atoms are arranged around the atomic nucleus. The electrons exist in discrete energy states that we can imagine as rungs on a ladder.  Electrons move between the states by absorbing, or emitting, light at specific energies (corresponding to a step up, or a step down on the ladder). As the energy of light depends on its frequency, the colour of an element depends on the spacing of the rungs of this atomic ladder, which is different for different elements. The energy ladder of helium atoms means that helium emits light at 587.49 nanometres. In organic molecules (ie. all the molecules that make up you and I and coffee), it is often the double carbon bonds that provide the energy ‘step’ in the visible range of light. Depending on the number of carbon atoms that are double bonded and the number in the molecule that are not, the energy step is tweaked slightly so that it will absorb in the red region in some materials and in the blue in others. We have our link between the sign at Esters and the observations of the astronomers.

However the explanation above depends on knowing some properties of electrons in atoms and some details of quantum mechanics. Neither electrons (discovered in 1897) nor quantum mechanics were known to the discoverers of helium. How did the astronomers recognise that their observation of a particular colour of light meant that they had identified a new element? Part of the answer must be based on experience. Experimentalists had already found out that different materials absorbed (and emitted) light at different but specific, frequencies. The other part of the answer brings us to our link with coffee.

A milk ring in water. Once it was thought that atoms might look like this.

A milk ring in water. Once it was thought that atoms might look like this (but a lot smaller).

In the video Coffee Smoke rings, we can make rings of milk travel through coffee or water. These rings are vortices which are closed up on themselves to form a doughnut shape. Mathematically, the vortex ring is a completely stable structure, it never decays. You could argue that the reason that it decays in the video is because we live in a non-ideal world with non-ideal liquids (milk and water). Returning to the mathematical world, each vortex ring will vibrate at specific, (resonance) frequencies dependent on its diameter, just like a bell rings with a note dependent on the size of the bell. So, even without knowing about electrons or quantum mechanics, it becomes conceivable that the atoms that go to make up a substance have specific resonance frequencies. If you imagine that atoms are in fact extremely small vortex rings (of the kind you find in a coffee cup), the model even has a predictive power. In 1867 William Thomson proposed such a “vortex atom” model and suggested that the distinct vibrations of the rings led to energy levels, like the ladders of later quantum mechanics and exactly of the sort that were observed by the astronomers. By considering that a sodium atom was made out of two inter-locked vortex rings, the light emission of sodium could even be accounted for. It was therefore entirely conceivable that elements would have distinct fingerprints as the astronomers had observed for this new element, helium.

We have therefore found the connection between the signboard at Esters, milk rings in a coffee cup and the discovery of helium. You would be forgiven for thinking that part of the connection is purely historical, after all, our current models of the atom do not rely on vortex rings at all. However, there is a relatively new theory called “string theory”. More fundamental than atoms, string theory proposes that there exist ‘strings’ that may be closed on themselves and that have specific vibrations that depend on their size and geometry. Sound familiar? Perhaps the connection with the milk rings lives on.

Does nature hate a vacuum?

The problem tea pot

The problem teapot

A few weeks ago, while having lunch with colleagues, one of them was complaining about his problems with his morning tea. So desperate he was to get his cup, he kept tipping the teapot to steeper and steeper angles in an attempt to increase the rate of pouring. Unfortunately, when he did so, the flow out of the spout became chaotic and, rather than having a nice cup of tea, he had a mess on the table. Another colleague suggested (sensibly) that it was a problem with the air-hole at the top of the teapot, not enough air was getting into the pot to enable the tea to flow smoothly out. In fact, my colleague’s tea pot problem turned out to have a different cause that will be featured in the Daily Grind in a few weeks. However, it did get me thinking about the purpose of the air hole in take-away coffee cups.

On the lid of a take-away cup are two holes. One, for drinking from while in a rush to get from A to B, the other, a very small air inlet hole that allows the coffee to flow nicely from the drinking hole. The requirement for such an air inlet has been known for millenia, however it was not understood why it was needed. Traditionally it was explained by saying that “nature abhors a vacuum”, the idea being that the coffee could not leave the cup because if it did so it would leave a vacuum which nature ‘does not allow’.

Take-away cup, plastic lid, equalisation of air pressure

The lid of a take-away cup has two holes. One for drinking from, the other to let air in.

An immediate problem with such an argument is that it implies that coffee has a will; nature ‘does not want’ a vacuum. Indeed for Rene Descartes (of “I think therefore I am” fame) this was a key problem with the traditional explanation. Descartes died in Stockholm in 1650, although for twenty years before that he had lived in Holland. For Europeans, the Dutch were fairly fast off the mark in terms of the introduction of coffee into their society. They had managed to get hold of a coffee plant in 1616 but only started properly growing coffee for themselves (in Ceylon!) in 1658, a few years after Descartes’ death. It is therefore unlikely that Descartes ever had the opportunity to try much coffee. Instead, when Descartes thought about the importance of air holes, the example that he used was a wine cask. In ‘The World‘, written in about 1632 he states “When the wine in a cask does not flow from the bottom opening because the top is completely closed, it is improper to say, as they ordinarily do, that this takes place through ‘fear of a vacuum’. We are well aware that the wine has no mind to fear anything; and even if it did, I do not know for what reason it could be apprehensive of this vacuum…”

Oranda, fish, Descartes water fish example, air pressure equalisation

The space behind a swimming fish is immediately filled with water as the fish moves forward.

For Descartes, the reason that an air hole was needed in the wine cask was not because nature hated a vacuum but because, on the contrary, nature was completely ‘full’ of matter. Whether that matter was wine, air or the material that made up the barrel, the world was full of ‘stuff’, meaning that if wine came out of the cask the air that it displaced had to go somewhere. Having nowhere else in the universe to go, this displaced air would have to go into the region of the cask that the wine had just vacated. Descartes compared this movement of air into the top of the cask to the displacement of water by fish as they swam through water. We may not notice the water in front of the fish moving to the back as the fish swims through the water but we know that the water must fill the empty space left by the moving fish. In the same way we do not perceive the air to flow from the outlet of the wine cask to the top of the barrel, but we know that it must (because, Descartes thought, it had nowhere else that it could go).

This explanation had far reaching consequences for Descartes world view. He could explain gravity and the motion of the planets as a consequence of the planets moving in a giant vortex of a substance around the Sun. The image of the solar system as a giant cup of coffee being stirred is one that the Daily Grind is sure to return to at some point. For the moment though, we need to step back and think. We know that the universe is not ‘full’ in the sense meant by Descartes and so this part of his explanation must be wrong, but why is it that blocking the air inlet hole stops the flow of water out of the cup?

coffee cup science, coffeecupscience, everydayphysics

Whether coffee leaves the cup or not depends on a balance of forces

Think about the schematic shown here. Gravity is pulling on the mass of coffee in the cup through the drinking hole. Air pressure is acting against this pull, pushing the coffee back into the cup (if you ever wanted a demonstration of how powerful air pressure can be, try sealing an empty water bottle before coming down a mountain or at the start of the descent in a plane). There is also air pressure inside the cup acting downwards on the coffee. With the air hole open, this air pressure is fairly equal to that outside of the cup. The inside air pressure cancels the outside air pressure, gravity wins and the coffee comes out. Imagine now closing the air hole. No air can get into the cup so, after a little coffee leaves, the air pressure inside the cup drops to less than the air pressure outside of the cup. This time, the air pressure outside the cup pushes the coffee back into the cup more than gravity pulls it out and the coffee stays in the cup. Can we test this explanation? One way to test the theory would be to somehow change the pressure inside the cup. Using two identical cups (which I got from the very friendly people with good coffee at Iris and June), the video below shows two experiments. In the first, both cups are filled with the same amount of cold ‘coffee’ (no coffee is ever wasted in these videos, dregs are recycled). The second experiment shows one cup holding cold coffee, one holding steaming coffee. Why might these experiments support the theory that it is air pressure that keeps the coffee in? Perhaps you can think of better experiments, or improvements to this one, let me know in the comments section below, but most of all, enjoy your coffee while you do so.

(note that the cups had got a bit water damaged through practise runs before filming. Note also that for this experiment to be meaningful, you would need to repeat the measurements many times so that you can build up a statistical picture, but that would make the video quite boring).

Brunswick House

Brunswick House, coffee, cortado

Coffee at the Brunswick House cafe

Last Thursday, I had the opportunity to try the coffee at Brunswick House. The old building which houses this cafe/restaurant sits on the corner of a major junction two minutes walk from Vauxhall tube station and feels somewhat out of place with the buildings around it. Inside, the incongruity continues with quirky decor and bookcases stacked with all manner of titles. Coffee beans are supplied by the roasters Coleman Coffee. As it was a lunchtime, I had a very enjoyable cortado (an espresso “cut” with steamed milk in a ratio of 1:1 – 1:2) which was full of flavour but not too bitter. With friendly staff and a spacious interior, this is definitely a place to return to whenever I am next in the Vauxhall area.

However, The Daily Grind is not so much interested purely in the coffee as in the connections between what we can observe in the coffee cup and the physics of the wider world. At Brunswick House, this came in the form of the link between one way in which we know that space is cold and a seemingly mundane observation, the condensation of water onto cold surfaces. Lifting my glass to appreciate the cortado, I noticed a number of water droplets on the (cold) saucer underneath the (hot) cup. As I kept the cup on the saucer, the saucer became warmer and the water droplets evaporated. By the time I finished my coffee, the saucer was dry. We can observe a similar phenomenon on the inside rim of a cup of steaming hot coffee. As we watch, water droplets form around the cold rim of the cup before starting to evaporate off again as the cup gets warmer. How is this related to the coldness of space? For that, we have to digress to an essay written two hundred years ago about dew.

cortado, Brunswick House, everyday physics, coffee cup science

The cortado on the saucer. 

William Charles Wells published his “Essay on Dew” in 1814 after two years of patient observation of the circumstances under which dew formed in the mornings. By carefully noting the weather conditions of the night preceding the dew fall and the surfaces onto which the dew formed, Wells came to some important conclusions. Firstly, the surfaces onto which dew formed suggested that the earth must be radiating heat into space; space must be cold. Secondly, the earth lost more heat on some nights than on others, it appeared that certain clouds kept the surface of the earth warm. If Wells was right it suggests that there is a natural greenhouse effect which is helpful for life on earth. This in turn suggests that the surface temperature of the earth is the result of a delicate balance between heat transfer to and away from our planet. Upsetting this balance (by introducing more greenhouse gases for example), could have serious consequences. Was Wells right? Perhaps we should start noticing when and where dew forms. So, over the next few weeks, make a note of dew laden mornings. Where did the dew form and under what circumstances? Do you agree with Wells? Let me know in the comments section (below). In a few weeks we will revisit Wells and his essay, in the meanwhile, enjoy your coffee!

A note of thanks

review, referee, thanksThe world of scientific research is supported by a vast volunteer network. Yes, scientists get paid to research their subject but in order to let the wider community know about their results, they have to publish it. To filter out the good papers from the bad or mediocre, working scientists volunteer to review papers submitted for publication. Each ‘reviewing’ scientist is anonymous to the authors of the paper, though the authors are not (yet?) anonymous to the reviewers.

At its best, the scientists reviewing the paper help the authors of the submitted manuscript to refine their papers, suggesting thoughts, other experiments or methods of phrasing that may better capture what the authors had intended. The review process is not just essential for filtering the good from the bad, it is extremely helpful for working scientists to have the input of someone, not involved with their research, commenting on it and trying to improve it. Although it is far from perfect, the review process is indispensable.

Which brings me onto Bean thinking. As a working scientist, I have got very used to the benefits that ‘peer’ review can bring. Therefore, over the past month, I asked 8 physicists and 8 non-physicists to review Bean thinking. These sixteen people were asked to critically read Bean thinking and let me know what they think. Unlike the scientific review process, it wasn’t anonymous, I knew who they were. But I also chose them because I respected each of their opinions and thought that each of these sixteen people would truly tell me what they thought. In this way, I hoped that Bean thinking could be refined to make it more engaging and entertaining while remaining scientifically rigorous. These sixteen got nothing tangible in return for their help which is why they are getting a mention on this page.

You know who you are, I know who you are. I am very grateful to all of you who took the time to read over and comment on Bean thinking. I have tried to take on board as many of your comments as possible (note the number of new photos on the pages!). Obviously, there are some comments that I haven’t been able to incorporate and any errors in the science (of which I hope there aren’t many) remain mine. Nonetheless, I am very grateful to you sixteen (and a few others who came in along the way). I hope you continue to visit Bean thinking, let me know what you think and join the discussion. And to all visitors, please leave any thoughts about the new look Bean thinking including any ideas for experiments that could be included in “Coffee cup science” in  the comments section below.

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