coffeecupscience

The hot chocolate effect

hot chocolate effect, Raphas

A ready prepared hot chocolate

This is an effect that reveals how sound travels in liquids. It enables us to understand the milk steaming process involved in making lattes and yet, it can be studied in your kitchen. It has an alternative name, “The instant coffee effect”, but we won’t mention that on this website any further. To study it you will need,

1) a mug (cylindrical is preferable),
2) some hot chocolate powder (no, instant coffee really will not do even if it does work)
3) a teaspoon
4) a wooden chopstick (optional, you can use your knuckle)

Make the hot chocolate as you usually would and stir. Then, remove the spoon and repeatedly tap on the bottom of the mug with the wooden chopstick (you could instead use your knuckle). Over the course of about a minute, you will hear the note made by the chopstick rise (not having a musical ear, I will have to trust that this can be by as much as three octaves).

resonator, mouth organ

The length of the pipes in this mouth organ determine the note heard. Photo © The Trustees of the British Museum

What is happening? Well, just like an organ pipe, the hot chocolate mug acts as a resonator. As the bottom surface of the hot chocolate is fixed in the mug and the top surface is open to the air, the lowest frequency of sound wave that the hot chocolate resonator sustains is a quarter wavelength. The note that you hear depends not just on the wavelength, but also on the speed of sound in the hot chocolate, and it is this last bit that is changing. When you put in the water and stir, you introduce air bubbles into the drink. With time (and with tapping the bottom surface), the air bubbles leave the hot chocolate. The speed of sound in a hot chocolate/air bubble mixture is lower than the speed of sound in hot chocolate without air bubbles. Consequently, the frequency of the note you hear is higher in the hot chocolate without bubbles than in the former case.

Let’s use this to make a prediction about what happens when a barista steams milk ready for a latte. At first, the steam wand introduces air and bubbles into the mixture but it is not yet warming the milk considerably. From above, we expect that the speed of sound will decrease as the bubbles are introduced. This will have the effect of making the ‘note’ that you hear on steaming the milk, lower. At the same time the resonator size is increasing (as the new bubbles push the liquid up the sides of the pitcher). This too will act to decrease the note that is heard as you steam (though the froth will also act to damp the vibration, we’ll neglect this effect for the first approximation). At a certain point, the steam wand will start to heat the milk. The speed of sound increases with the temperature of the milk and so the note will get higher as the milk gets warmer.

So this is my prediction, musically inclined baristas can tell me if there is any truth in this:

1) On initially putting the steam wand into the cold milk, the tone of the note heard as the milk is steamed, will decrease.
2) This decrease will continue for some time until the milk starts to get warm when the note increases again.
3) Towards the end of the process, the note heard on steaming the milk will continue to increase until you stop frothing.
4) It should be possible, by listening to the milk being steamed, to know when the milk is ready for your latte just by listening to it (if you are experienced and always use similar amounts of milk per latte drink).

So, let me know if this is right and, if it is wrong, why not let me know what you think is happening instead. I’d be interested to know your insights into the hot chocolate effect in a milk pitcher.

Levitating water

V60 from Leyas

Time to look more closely at the surface of your black coffee.

Have you ever sat watching the steam that forms above a hot Americano? Beneath the swirling steam clouds you can occasionally see patterns of a white mist that seem to hover just above the dark brew. Bean Thinking is about taking time to notice what occurs in a coffee cup and yet I admit, I had seen these mists and thought that it was something that was just associated with the evaporation of the water and that “someone”, “somewhere” had probably explained it. So it was entirely right that I was recently taken to task (collectively with others who have observed this phenomenon and taken the same attitude) for this assumption by the authors of this paper who wrote “The phenomenon that we studied here can be observed everyday and should have been noticed by many scientists, yet very few people appear to have imagined such fascinating phenomena happening in a teacup.

ineedcoffee.com, espresso grind

The water particles in the white mist are a similar size to the smallest particles in an espresso grind. Photo courtesy of ineedcoffee.com, (CC Attribution, No Derivs). The coin shown is a US nickel of diameter 21.21 mm

The authors of the study show that the white mists (these “fascinating phenomena”) are, in fact, layers of water drops that have a typical diameter of around 10 μm (which is roughly the size of the smallest particles in an espresso grind). Although the white mists exist above tea and even hot water as well as coffee, they are probably easiest to see against the black surface of the Americano.

More surprising than the fairly uniform distribution of water droplet size though is the fact that the authors of this study showed that the droplets were levitating above the coffee. Each water droplet was somehow literally hovering above the surface of the coffee at a height of between 10 – 100 μm (which is, coincidentally, roughly the particle size distribution in an espresso grind).

white mists, slow science

You can (just about) see the white mists over the surface of this cup of tea (which is a still from the video below)

One of the questions that the authors of the paper have not yet managed to answer is what is causing this levitation? Could it be the pressure of the hot coffee evaporating that keeps these particles held aloft? This would explain the observation that the mists form patterns similar to those caused by (heat) convection currents. Alternatively perhaps the droplets are charged and are kept away from the coffee by electrostatic repulsion, an explanation that is suggested by the behaviour of the droplets when near a statically charged object (eg. hair comb, balloon, try it). Perhaps the levitation is caused by the droplets spinning and inducing an air cushion under them? Why not design some experiments and try to find out. It would be great if we can drink hot black coffee in the name of science. Let me know the results of your observations in the comments section below. In the meanwhile, here is a video of the white mists in tea, enjoy your coffee:

You can read the study at: Umeki et al., Scientific Reports, 5, 8046, (2015)

 

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.

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

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).

From a Caravan to the Grecian

It is a Saturday morning as I write this while sitting in Granary Square in Kings Cross, London. I’ve just enjoyed an Ethiopean filter coffee at Caravan. If only more cafes offered the possibility of sampling single estate coffees rather than the espressos that are otherwise so popular in London.

Caravan, Granary Square, coffee, single estate, good cafes in London

The fountains in front of Caravan

In the square outside, people are laughing (and dancing!) in front of the old warehouses that accomodate Caravan. Amongst them all, four sets of ground-level fountains push jets of foaming water 50cm into the air, in patterns that change as you watch. There is so much physics here to observe: The white colour of the water foam, the dance of the water droplets as they emerge from the main jet of the fountain and then fall back to earth, the fact that the wet concrete around the fountains is darker than the dry concrete nearby.

Consider though one more observation. As the water shoots upwards, it is pushed by occasional gusts of wind from west to east making the fountains appear as loops rather than columns of dancing liquid. Although the direction of the wind is determined by local weather patterns, over the UK the prevailing wind direction is Westerly, that is flowing from west to east.

People have wondered about the origin of the winds from ancient times. The Greeks had four wind Gods who had authority over the winds from each direction: Boreas, god of the North wind, Notus of the South, Euros of the East and Zephryos of the West. Pliny the Elder speculated at length on the causes of the winds and yet the start of the modern conversation regarding the origin of the winds had to wait until 1686 with the publication of a work by Edmund Halley.

Grecian, Coffee House, London Coffee House

The Devereux now stands where the Grecian once was

Halley (1656-1742) is now more famous for the comet that is named after him rather than his meteorological work but, as with many scientists of the time, he had his finger in many pies. He also seems to have been a keen coffee drinker, or at least, he regularly spent time in one of London’s coffee houses, the Grecian, discussing science with Isaac Newton, Hans Sloane and others. A pub, the Devereux, now stands on the site of the old Grecian in a little side street off of Fleet Street.

Did Halley ponder cloud formation, rain and the origin of the winds while contemplating his steaming coffee cup on cold days in 17th Century London? Regardless, Halley did recognise that the heat from the Sun was the driving force for the wind system. Halley surmised that as a parcel of air was heated by the Sun and rose upwards, the cold air surrounding it would have to flow in to its place so as to replace the risen air so “..by a kind of Circulation N.E. Trade Winds below will be attended by a S.W. above, and the S.E. with a N.W. Wind above”* The problem for Halley was that his explanation of the wind system could account for a North-South wind direction owing to the Sun’s heating the air at the equator, but not the Easterly direction of the Trade Winds near the equator nor the Westerly direction of the winds over the UK.

A few years later, George Hadley (1685-1768) suggested that it was the rotation of the earth that was responsible for the east-west component; the mass of air, being detached from the earth, would appear to flow in a particular direction as a consequence of the earth spinning below it. The idea was not new, Galileo had proposed it some years earlier while similar arguments were made later by the philosopher (and scientist) Immanuel Kant (1724-1804). At first sight, such an argument looks appealing but there are problems, as John Herschel (1792-1871) pointed out. If this were the explanation for the wind direction, the effect would be “so great as to produce not merely a wind, but a tempest of the most destructive violence”.

Herschel suggested, as had Hadley before him, that friction could slow the wind to the speeds that we normally observe, but while this may explain the wind speed at ground level, what about the upper circulatory patterns noted by Halley: What friction could slow these down?

Grecian, Devereux, Coffee house London

A plaque outside the Devereux pub

It turns out that this is not the reason for the discrepancy in the wind speed. Hadley’s theory was wrong on a number of issues (if you are interested, I suggest reading this article). The real driving force for the Trade Winds is the Coriolis effect which deflects the warm air rising at the equator towards the right as it travels to the North pole. The majority of this air then cools and descends at about 30 degrees latitude, circling back on itself (as per Halley) as the Easterly trade winds. However the air that continues in the westerly direction north (or south) of 30 degrees latitude becomes those prevailing westerlies of the sort that batter the shores of the UK (see here for more information).

Even if Hadley’s simple model was wrong, its contemplation did lead to an important discovery that is still relevant for us today. The question was: What was it in the upper atmosphere that could cause a friction effect that could slow the winds? The person contemplating this question was taking a walking holiday in the Alps in the first half of September in 1886. Hermann von Helmholtz (1821-1894) observed a layer of clouds which showed “whirls formed by perturbation and rolling up” of the surfaces of two neighbouring layers of air. Helmholtz had observed what became known as “Kelvin Helmholtz clouds”, a beautiful but very rare cloud type, for an example click here. Helmholtz realised that the formation of these clouds required that two layers of air rubbed against each other. In the region between the two layers, the air became unstable, wavy and finally showed the whirls which are actually a series of vortices. As these vortices developed, the two layers of air would get more thoroughly mixed and it was in this way that friction could develop in the upper atmosphere.

Such vortices and “surfaces of discontinuity” are now an important concept in many places including the coffee cup. The video “Coffee Rings” presents another manifestation of the effects of surfaces of discontinuity. So we have returned from contemplation of the wind in a late summer square in London, through a famous Coffee House and back to the coffee.

I have not yet had the opportunity for myself to see a Kelvin Helmholtz cloud. If any reader has been so fortunate please share photos with @thinking_bean. Let me know what you think and what you see around you in the comments section below and most importantly, enjoy your coffee!

*from E. Halley, An Historical Account of the Trade Winds, Transactions of the Royal Society, 1686, p. 133, via “From Watt to Clausius”, DSL Cardwell, Cornell University Press, 1971

†Quotes taken from Anders O Persson, “Hadley’s Principle: Understanding and Misunderstanding the Trade Winds”, History of Meteorology, 3, (2006) p. 17 (linked in article)

Introducing Bean thinking

As this is the first true blog post, let’s do the introductions. What is Bean thinking and who is @thinking_bean?

Breakfast coffee, introductionThe human bean behind @thinking_bean has worked for a fair few years in university research centres, researching obscure but fascinating areas of physics where magnetism meets superconductivity. Such research fields can be very beautiful but perhaps not of immediate technological relevance. Understandably, this can cause some in our society to question the utility of investigating these phenomena. Part of the motivation behind Bean thinking is to explore this question, why do we do science?

A second motivation is to share the wonder of the world that today’s understanding of physics gives us. Some of these beautiful areas have not yet been fully understood even though they occur in something as apparently simple as a coffee cup. Through teaching, outreach, talking to friends and even in conversations with some colleagues, I became aware of the way that science, perhaps particularly physics, can be perceived as a very interesting, but perhaps very difficult subject, far removed from people’s everyday lives.

Yet this is not true! Slow down, put down your smart phone, e-book or tablet, observe the world. Physics is all around you. Warming your hands around a mug of hot coffee, you may not realise how it is related to the Big Bang. Looking at a glass of milk can illustrate the reasons that the sky is blue. Even the mere act of stirring coffee can be related the Heathrow minute (link, link2).

Hence Bean thinking, which hopefully will become a space where curious individuals can come and discuss interesting phenomena that they notice in the day to day. If this can be done with a cup of coffee, all the better. The point is to slow down and start noticing. Each Wednesday I will update the Bean thinking blog, the “Daily Grind” with things that I have noticed or that I find interesting. Who knows, if anyone starts to read this and shares their observations perhaps the Daily Grind can also include these. As this website develops, I may add a forum, but for the moment, please let me know what you think about the concept and what you observe around you in the comments section below.