Coffee whispering

coffee and cassette tape in Batch and Co
What does listening to your coffee tell you? Would a long black sound different to an Americano?

How does your coffee sound? Does an espresso sound different to a latte? Could you deduce how the milk had been frothed, or what milk had been used, by listening to your coffee before you drank it?

To see why there may be an effect, it’s worth thinking about your coffee for a moment. The tiny bubbles in the crema of an espresso are different from the larger bubbles of a milk froth made of semi-skimmed milk in a cappuccino. Bubbles of non-dairy milks may be different too, particularly if the initial small bubbles have combined to form larger bubbles as the froth ages. Indeed, sound is used as a characteristic of coffee: think about the sounds made by a steaming wand in milk. Somehow the environment of a café would not be the same without the constant hiss and whistle of a cappuccino being made. But can we use it to experience our coffee more fully? Not just the aroma, taste, sight and feel but also, can we start to listen to our coffee?

Take the example of the sound of a dripping tap: each drop of water falling into a bowl of water left under the tap ready for washing up later. Each “plink” is telling you something about the size of the drop coming from the tap. Intuitively, or perhaps from experience, we know that small drops produce a higher pitch, a higher frequency, than large drops: small drops ‘plink’, large drops ‘plonk’. But there is something wrong with this example, because, despite what we may think, we are not hearing the drops at all, only a consequence of the drops.

Drops on a coffee can reveal a lot, but this time we’re interested in the sound that they make.

As the drop falls, it creates a hole in the surface of the water, a dent that grows and then closes in on itself, so that the drop of water has formed a bubble of air under the water surface. As this bubble is unstable, it pulsates under the water just before it collapses and it is this pulsation that we hear. As the frequency of the pulsation will depend on the radius of the bubble, air cavities of different sizes will produce different sounds. And because a larger drop will generally produce a larger hole under the water, the larger drop will generate deeper sounds: plonks rather thank plinks.

How does this relate to the sounds made by your coffee? Well, it turns out that the sound of a bubble bursting reveals a lot about the surface tension and the size of the bubble. A recent study published in Physical Review Letters measured the sounds made by bursting soap bubbles through 24 microphones placed around each bubble. Analysing the sounds, the group found that not only could they ‘hear’ how the air escaped the bubble, by analysing the sounds recorded in the microphones they could determine, quantitatively, the movement and forces of the bubble ‘skin’ as it retreated back and the bubble burst. They suggested that listening to bubbles and liquid surfaces could be a complementary tool to high speed photography for understanding the forces on a liquid. This may prove useful for example when thinking about how a pond skater moves on the surface of the water.

To think about what this may mean for coffee, take the Aeropresses I’ve been making recently. First, I wet the grounds and allow a first stage of de-gassing to start. The sound here is of an almost continuous hiss, not entirely dissimilar to the sound you hear when you put an empty seashell to your ear.

latte art, hot chocolate art, soya art
Could we detect a difference between a semi-skimmed milk latte and an oat milk hot chocolate by the sound that they make? Some people listen to their bread in order to know when it is cooked. What does listening to coffee reveal?

At this point it was hard to know whether what I was hearing were the grinds or the ‘sound’ of the Aeropress ‘shell’. Topping up the chamber with water, the bubbles on the surface of the coffee became larger, and of a different form. And they sounded different too! A few pops, and a hiss.

Did I learn anything (apart from that putting one’s ear to the top of an Aeropress does get quite hot and a steamed ear is a strange experience)? I learned that there was much more to my coffee than I had appreciated, that there is always more to discover. It was almost as the author of the 1933 paper about determining the size of bubbles in water by the sound said:

“As a matter of fact we know very little about the murmur of the brook, the roar of the cataract, or the humming of the sea.”

What will you hear in your coffee? Do let me know, in the comments below, on Twitter or over on Facebook.

A key ingredient at Second Shot

Coffee, hot chocolate and cake at Second Shot in Marylebone,
Second Shot
Coffee, hot chocolate and cake at Second Shot in Marylebone

First impressions count, and the first impression we got of Second Shot‘s second branch north of Marylebone Station, was of a very friendly, local spot type of cafe. A small crowd were sitting around a table discussing a topic in an animated way. Various others were popping in or out, chatting with the barista, one person was sitting at a table with a laptop. And despite the fact that the weather was turning and the sky was becoming an ominous grey, the cafe itself seemed bright and open, with plenty of light coloured wood to complement the large windows.

We decided on a long black, hot chocolate and nut free brownie. Not wanting to spoil the initial taste of the coffee, I waited for the long black to arrive so that I could try that first before trying a bit of the brownie. The coffee was very well made, from Square Mile, and a perfect complement to the squidgy but moreish brownie.

On the walls, plants were hanging in pots with their leaves trailing down, while the light was reflected off a series of drawings sketched using coffee as an ink (they were for sale). It is surprisingly easy to make ink out of many different household (and not quite so household) items. Coffee is one base ingredient and, not surprisingly, makes a brown ink. But to bind the ink and to make it more viscous, gum arabic is frequently added to home-made ink recipes. The gum arabic is needed particularly if you are going to write with the ink with a fountain pen as it makes the ink viscous enough that it can flow through the nib.

coffee art, pictures using coffee ink, sketch, Second Shot
Hanging plants and coffee art, the walls at Second Shot, Marylebone.

Just as with the gum arabic, often ingredients are added to a product that are crucial to it, but that we do not realise they are there. Another example is the seaweed extract that is added to some plant based milks in order that they produce a better milk froth. But what if you don’t add all the ingredients at the same time, what happens if you add only one ingredient of the ink rather than all of those that are necessary?

One type of black ink that has been used for centuries is made from oak galls. It is even thought that ink based on galls was used to write the 4th century Codex Sinaiticus (the oldest complete copy of the Christian New Testament that we know of). The galls form on oak trees when the larvae of different types of wasp secrete a chemical that produces abnormal growth in the tree. The gall is the protective casing around the wasp larva, a type of home for the wasp larvae as they mature. Once the wasp has matured, it burrows through the growth which has turned from green to brown and it leaves a tiny little hole where it exited the gall. Galls can be seen on many oak trees during the summer and autumn and I’ve even seen them on the oak trees of various central London parks.

After the galls have been crushed and soaked, iron sulphate is added to the gall solution to produce a deep black ink*. Again, gum arabic is added to bind the ink to the paper and to increase its viscosity. But it is this ink that was also a popular ‘invisible ink’ first described over 2000 years ago. Philo of Byzantium** (who lived from 280-220 BCE) describes making an ink with the oak galls only. Forget about the iron sulphate for the moment. When it dries, the ink is nearly invisible on the paper, it can easily be missed and so can be sent to a collaborator as an invisible message. When the collaborator washes the paper with a solution of iron sulphate, the black ink appears on the paper and the message is revealed. Although the recipe has been known for two millennia, it has been used as an invisible ink even as recently as Mary, Queen of Scots and the American Revolution**.

coffe ink example
Coffee ink made for writing with a fountain pen (including recipe). Perhaps a yellow-ish paper was not the best medium to use to showcase the ink. An inadvertently (almost) invisible ink.

This idea, of making the invisible visible, by adding a key ingredient seems to form a nice metaphor for the societal aspect of the work of Second Shot. On their website they describe this aspect as:

We’re changing perceptions on homelessness by being a destination that serves some of London’s best coffee, alongside a unique community atmosphere, amazing food, and just so happens to be changing lives.

We employ people who have been affected by homelessness, train them up and transition them on to long term employment elsewhere, helping them on their individual journey taking them from where they are, to where they deserve to be.

Key ingredients of training, and accompaniment on individual journeys that combine to change our perceptions but that are not realised by us as we consume great coffee at this friendly cafe: Making the invisible visible, but doing it without us even realising that we have received a hidden message.

Second Shot’s second branch is at 49 Church St. NW8 8ES

*”Make Ink: A forager’s guide to natural ink making”, J Logan, Abrams New York, 2018

** “Prisoners, Lovers, & Spies: The story of Invisible Ink from Herodotus to al-Qaeda”, K Macrakis, Yale University Press, 2014

Seeing the unseen at Scarlett, Angel

Coffee Angel, Scarlett, roasters, coffee in Islington
Coffee at Scarlett, Angel

Although first alerted to Scarlett coffee in Angel by Double Skinny Macchiato last summer, we managed to visit during the one week of their summer holiday (and so we revisited Katsute100 around the corner instead). Nonetheless, it remained on the list and a few weeks ago we turned up for a mid-afternoon coffee at this inconspicuous looking venue on a side street just around the corner from Angel tube.

The roaster at the back of the cafe forms an immediate impression. With the large, communal table at the front of the cafe, backed by stairs leading up to the roaster, this is a place where coffee is taken seriously. The counter (on the left as you enter) offered a range of cakes and edibles but having recently come from lunch in Chapel Market, we passed on this on this occasion. Above the counter there were about 5 lights hanging down forming what looked like a giant Newton’s Cradle. Just too high for me to reach unfortunately.

I enjoyed my long black as I started to take in the surroundings of this cafe. Various people and regulars came and went, suggesting that this is a friendly local haunt for many. Noticing the number of different roasted coffee beans for retail, it was clear that this is a venue that you could return to for a different coffee experience each time. Each time exploring an aspect of the flavour of the coffee and building on the experience of coffee tasting that you have enjoyed before. It is definitely on the list for a repeat visit.

Interior Scarlett
One of the light fittings at Scarlett in Angel. Cube outlines drawn on paper can form an optical illusion where you can’t work out if the cube is coming out at you or going into the paper.

Above our heads, the lights were framed by the outline of a cube. Fantastic for optical illusions, these cubes offer us an opportunity to think about how we perceive depth and direction; how our eyes work and perhaps, more fundamentally, what it even means to see an object (as with Berkeley’s “New Theory of Vision”). Then, while looking through the menu, it became clear that here too there was an optical illusion of sorts. For the price list was not written on the board so much as cut out of it (see the photo below). The price you could read off the menu was, in some sense, precisely the information that was not actually on the board. Our brain makes patterns of that which we don’t see and, together with our assumptions about what should be there, we form an idea of the price we have to pay.

It is a similar thing with many algorithms in use around us now. Such tools can be immensely helpful, offering us suggestions for coffees we may like to try (based on our buying habit) or routes that we may like to take to get us to our destination. And yet, are there problems hidden in the assumptions that some of these algorithms make? What information are we getting based on elements in the programme that we do not see?

In her excellent book “Weapons of Math Destruction”, Cathy O’Neil explores some of the more dangerous ways that our biases and assumptions (particularly those that we don’t see in ourselves) can impact the results of algorithms that have been written to optimise processes from the sorting of job applications to determining the length of time a given convicted criminal will serve for an offence. In an example relevant for cafes, O’Neil related an example of how Starbucks had used an algorithm to determine which baristas and managers should work which hours, including who should close the shop at night and who should open it in the morning.

Scarlett menu
The menu at Scarlett. Apart from the filter coffee, the prices and information for each coffee is revealed by what is absent from the board rather than what is printed onto it.

The algorithm was programmed to calculate the most efficient use of the cafe’s time and money, specifically prioritising the profit that the company made. One measure of this was “revenue per employee hour”. This had the consequence that staff members were frequently in a position where they were told that they had to do both the (late night) closing and (early morning) opening of the shop and were given very few days notice of this expectation. Clearly this impacted the lives of their staff and affected their ability to arrange child care, support themselves through further education and other consequences. Eventually Starbucks was forced to amend this algorithm but change comes hard: how do you ask a computer to measure “fairness” to an employee (a subjective term) when you can use revenue per employee hour which is measurable, quantifiable and therefore ‘accurate’?

Perhaps you think that the link back to Scarlett here is obvious: That if you choose to drink your coffee in friendly neighbourhood cafes where cafe owners and baristas work to patterns formed by encounter rather than algorithm it would be better than a place which is run assuming all workers are cogs in a profit machine? Perhaps. But the link back to Scarlett in my mind is not that at all.

If you look at the front of Scarlett, or its webpage, and assume that the pink bird is a funny looking flamingo, you may make a series of assumptions about what you think the cafe will be like and why the owners have a bird on their front door. If you found out that the bird was actually a Scarlet Ibis and associated with the coffee growing regions of South America, your ideas about the cafe and the owners may be different. For a general customer, looking for somewhere to enjoy a great coffee, perhaps these assumptions and ideas do not matter so much. But if we are ever in a position to feed our assumptions into an algorithm, these hidden (to our own conscious) assumptions could matter a great deal.

Scarlett is at 30 Duncan Street, N1 8BW

“Weapons of Math Destruction – how big data increases inequality and threatens democracy” by Cathy O’Neil, Penguin Books, 2016

On dew, greenhouses & IR thermometers: Coffee & Science at Amoret

starting with a coffee
Always good to start with a coffee. The evening started with two coffees (from Ethiopia and El Salvador). What will be the effects of climate change on the coffee industry?

January 2020 was the 6th warmest on record in the UK, with a mean temperature 2C higher than the 1981-2010 average. Early in February it was announced that Antartica had recorded the highest temperature ever recorded there of 18.3C, beating the previous record of 17.5C in March 2015. The atmospheric concentration of CO2 in January 2020 was measured to be 413 ppm following the trend that has seen the atmospheric CO2 concentration increase more than 10% from just the year 2000. That the polar regions would warm faster than other parts of the planet had long been a prediction of global warming based on increased CO2 emissions. Nonetheless, to see the figures reported quite so starkly was startling.

Each month brings new headlines and more concerns about whether we are responding fast enough to limit global warming to 1.5 or 2C. And yet, the greenhouse effect was proposed back in 1824; the idea that carbon dioxide (and water vapour) were greenhouse gases suggested during the 1850s (1,2) and it was back in 1895 that Arrhenius predicted that doubling the atmospheric levels of CO2 (relative to 1890s levels) would result in a global temperature increase of 5-8C.

So given that it is such an established theory, why are we still arguing about it? And, more importantly perhaps, what has this to do with coffee?

It is, in many ways, an ideal connection for the theme for one of the Coffee & Science evenings that we’ve been holding at Amoret Coffee in Notting Hill. And so it was that a group of us got together over coffee to discuss the greenhouse effect and its links to coffee.

coffee bowl pour over
The first connections can be seen with the condensation. How does dew form, and why does it suggest that space is cold?

The first coffee-greenhouse connection is in the condensation. When you make a pour over, or even if you pour your coffee into a cold mug, you will notice the condensation forming on the colder glass (or ceramic) surfaces as the steam evaporates. We know that the droplets form because the temperature of the surface is below that at which water vapour will re-condense into liquid. Technically, this temperature is known as the dew point. And it is partly to dew that we owe our understanding of the greenhouse effect.

Back in 1814, William Charles Wells made a series of detailed observations about how, where and when dew formed. He was able to show that more dew formed on clear (or not terribly cloudy) nights and on surfaces that were exposed to the sky; they were space facing. Which brings us to a second coffee connection: just as your coffee cup warms you by radiating its heat (in the infra red) to your hands, so all objects with heat radiate their energy out. Wells realised that this meant that space was cold because, just as a coffee cup if it is not being heated and not surrounded by reflecting material (think about the inside of a thermos flask) will radiate its heat and get cold* so the surfaces of the earth, if there is no energy coming in from space and no surfaces above them to reflect their heat back at them, will also get cold.

If space is cold, you can calculate what the temperature of the Earth should be if the energy it is losing is balanced by the energy it gains from the Sun and when you do this, it turns out that the mean temperature of the Earth should be -18C or about 30C lower than that observed**.

Earth from space, South America, coffee
One common home.
The Blue Marble, Credit, NASA: Image created by Reto Stockli with the help of Alan Nelson, under the leadership of Fritz Hasler

This leads to the idea that there is a natural greenhouse effect whereby gases in the Earth’s atmosphere form a layer which lets through a large amount of the energy from the Sun but lets a lot less energy escape back through it from the Earth (owing to the lower frequency of the radiation being emitted by the Earth compared with that coming in from the Sun). This ‘natural’ greenhouse effect results in a warming of the Earth to a delicate balance and to the temperatures that we experience on Earth***. Fairly clearly, if this delicate balance is disturbed by adding extra greenhouse gases to the atmosphere it will lead to a warming effect (as Arrhenius predicted back in 1895), the question is how much and how fast?

We were very fortunate to be joined for the evening by Dr Robin Lamboll of the Grantham Institute of Imperial College London. Robin explained the latest science and understanding of the effects of climate change and of adding increased CO2 into the atmosphere. Particularly highlighting how an increase in CO2 leads to an increase in water vapour (owing to the initial temperature increase produced by the CO2) which is itself a greenhouse gas, and so the warming effects of a small amount of CO2 can be amplified by this mechanism.

At this point the conversation diverged away from coffee, not just because Robin is a tea drinker (!) but we moved onto the effects of sulphur dioxide in the atmosphere, local vs global temperature effects and the science of Eunice Newton Foote. We discussed what we know, and what we are just starting to understand, such as how what happens in one part of the world may lead to consequences in other parts of the world (weather wise). We also got to a discussion of albedo and the reflection of heat by ice via playing with a couple of infra red thermometers that we had to hand and the different ways that human eyes and shrimp eyes detect colour. How is this connected to climate change and coffee? I’m afraid that there is a connection but the path to it is a little circuitous for a write up. It’s the sort of thing that pops up when you have a number of people of different backgrounds all contributing to the discussion. This is what, from my point of view, makes these evenings so interesting (and on a personal level induces such pre-event nerves): the fact that the conversation can go in so many directions, with such different contributions from the attendees, that each evening takes on a different character, with a different set of connections and a new set of things to think about. I hope that others feel the same way!

“An Essay on Dew”, Wells book of 1815 summarising his observations on dew. An excellent piece of observational science.

Our next Coffee & Science evening is scheduled for March 2020. Please do sign up to the events list or keep an eye on the Facebook events page to learn details as they are announced. Thanks again to Dr Robin Lamboll for coming along in January. I look forward to seeing both familiar faces and some new people in March.

Bean Thinking’s Evenings of Coffee & Science @ Amoret Coffee are held approximately every 2 months from 5.30 until about 8pm at Amoret Coffee in Notting Hill. More details can be found here.

*Two caveats here: firstly the coffee will also get cold through convection and conduction, the connection is illustrative rather than precise – though were you to put your coffee into a vacuum it would cool via radiative cooling only. Secondly, Wells himself never made the coffee connection but instead considered the latest physics theories about heat.

**In “Introduction to Atmospheric Physics”, David Andrews, (2000)

***For details about how we can know what the temperatures have been over such a time period and the effects of other cyclical temperature variations on the climate, it’s worth reading “The Ice Chronicles” P Mayewski & F White, (2002)

Carbon Kopi

Carbon Kopi, coffee Hammersmith, coffee Fulham
Carbon Kopi, the sign in the window above giving a clue to the name without being a direct reference.

The name of this relatively new cafe in Hammersmith/Fulham was intriguing on several levels. Kopi means coffee in both Malay and Indonesian and, having recently travelled back from SE Asia, it was interesting to see what the link to this cafe may be. Then there was the pun in the name. The website explains it as representative of a desire to make a consistently good coffee, each being a carbon copy of the other. So both the name, and the cafe’s symbol have appeals for a coffee-science website. And so it was that we wandered down Fulham Palace Road to finally arrive at Carbon Kopi a few Saturdays ago.

The cafe occupies a corner building and is much larger than you may expect it to be. It is also friendly, airy and light with large windows giving plenty of illumination to the space. Allergen information was clearly labelled on the cakes and edibles (with extra information in a folder), which is always great to see.

Coffee was by Square Mile with guest roasters on batch brew and so we had a long black and oat milk hot chocolate which came served in huskee cups. Huskee cups are produced by re-using the husks otherwise discarded during the coffee milling stage. A re-usable cup that reduces waste certainly, but does it reduce the carbon footprint? An answer that depends on how many times you use it. Continuing the environmental theme, near the door, there was a separate bin for compostable cups. This was excellent to see because if compostable cups do not get to an industrial composting facility, they can take an absolute age to break down in a conventional compost heap.

Hat and huskee
Coffee in a huskee cup at Carbon Kopi. A protrusion on the saucer fits into the base of the huskee cup and stops it slipping across the saucer. Unlike graphite where the regular hexagons of carbon form layers that slip over each other to form a solid lubricant.

Across the road, the St Alban’s Church was made of brick. One row upon another, the set of bricks formed a layered structure. Where they met at corners or against the pavement they formed abrupt discontinuities, a sort of dislocation. Together with the small protrusion on the middle of the huskee cup saucer (to stop the cup slipping?), and the speaker above the door entertaining us with 80s music, the natural connection here was graphene.

Graphene is a form of carbon that is a single atomic layer thick. Each carbon atom is arranged into a flat hexagonal structure exactly like graphite but, unlike graphite, there is only one layer of these atoms in graphene. The strength and strange electrical properties of this material, together with its lightweight form, have made this material an intense subject of research for the past 15 years or so. A recent Physics World podcast tested a set of headphones with the vibrating membrane made of graphene. The idea being that the strength of the material combined with its relatively low mass, would enhance the way that we heard the sound coming through the speakers. You can listen to the review (though not the speakers) here.

coffee Hammersmith
Each layer of bricks forms a regular set of layers. But where they come up against each other discontinuities are formed. These can cause special problems in sheets of graphene.

But are there aspects of graphene that may be more applicable to the cafe and coffee industry? Various teams around the world have been working to make membranes of graphene work as single molecule detectors. The idea is that molecules adsorbed onto the surface of a graphene membrane change the electrical properties of the membrane to an extent that can be measured even in the case of single molecule adsorption. The sensitivity of the electrical properties of the graphene to different molecules could mean that graphene based devices would make very sensitive contamination detectors, including allergen detectors. Such sensors are the subject of a research collaboration at the National Physical Laboratory in Teddington and could mean that, rather than be in any doubt as to whether a substance contains an allergen, it could be quickly tested by passing it through a graphene sensor.

All this is quite far from the coffee and cakes at Carbon Kopi. But if you are in the area, it is a lovely place to stop, enjoy a coffee and contemplate the bricks of the church opposite.

Carbon Kopi is at 11 Margravine Road, W6 8LS

A little fog

canali Curators Coffee
Clouds viewed from a London cafe. How do clouds form?

Clouds form because water droplets condense. But there is a hidden difficulty in this seemingly simple statement because, while true, it is not, at first sight, clear how they condense.

To see the problem imagine spilling coffee on two consecutive days, a really damp one and then a really dry one. We know from our experience that when the weather is dry the coffee will evaporate and dry more quickly (leaving a coffee-ring). On damp days the drop is more stable against evaporation, that is, it stays as a droplet. We can continue our thought experiment by thinking what happens when we have a large coffee spillage versus a drop. The drop will dry quickly whereas the large spill will take ages to dry out (unless we mop it up).

Where does this leave clouds? Consider two water molecules coming together and condensing into a very small drop. It is quite clear that, just like our small coffee drop spill, this droplet will be unstable against evaporation, meaning that it evaporates almost as soon as it has formed. Perhaps this seems an extreme example, we do not often consider a water droplet as being formed of two molecules (nor is it clear that the term ‘evaporation’ is strictly appropriate in this case). So how many water molecules would have to, spontaneously, come together to form a droplet that does not evaporate almost as soon as it is formed? Take a guess. What seems reasonable?

coffee bowl pour over
Frequently water droplets will condense onto something as opposed to merely forming on their own. Consequently it is useful that our planet is fairly dusty.

At a relative humidity of 101%, a stable droplet has to be larger than about 0.1 µm diameter* (larger than about 1/50000 of a coffee bean). Which means that to form a stable droplet, about 140 million water molecules would all have to combine in the same spot and condense together simultaneously. It seems not terribly likely and yet this is what our statement at the start of this post implied “clouds form because water droplets condense”.

Which aspect of our understanding is wrong, or incomplete?

It is in our assumption about how the water molecules are condensing. Rather than spontaneously combine, each molecule condenses onto something, just as a dew forms on the ground, so these droplets start to condense onto dust particles and other, smaller, aerosols in the atmosphere. This means that at the heart of most cloud droplets is a bit of dust and that dusty places may be expected to be, all else being equal, more cloudy. Or at least that last part was my understanding of the situation: would a take-away coffee taken away in a polluted London street appear to steam more than in the by-ways of a village?

Unable to determine how to design an experiment to test this idea (while keeping all other things equal), it was interesting to come across an almost flippantly made statement in a book while I was preparing for the next evening of Coffee & Science at Amoret.

“Urban emissions of sulphate aerosols from fossil fuel burning and dust production caused by industrial activity and human occupation result in poorer visibility and increased frequency of fogs [in dense urban areas such as cities]”.

Does our coffee steam more in the cities? The answer to this may well be ‘yes’!

And there was more, apparently the increasing use of clean air legislation has resulted in fewer foggy days and more days of sunshine in urban areas**. Is it possible therefore that you could start to use your take-away coffee cup as some form of pollution detector? Watching for the steam to appear as you wander from street to street.

Sadly for the coffee cup pollution detector, whether the water condenses onto the dust droplet or not is also a function of the temperature and humidity, parameters that will make it tricky to develop the pollution detecting re-usable coffee cup. But, the physics is sound and if you do one day come across such a device, please remember where you read it first!

*”Introduction to Atmospheric Physics”, DG Andrews, (2008)

**”The ice chronicles: the quest to understand global climate change”, PA Mayewski & F White, University Press of New England (2002)

Which direction? At Jacob the Angel, Neal’s Yard

Jacob the Angel, coffee Covent Garden
It is easy to miss Jacob the Angel as you enter Neal’s Yard, but an angel above the hoarding gives it away.

Jacob the Angel is tucked into Neal’s Yard in London’s Covent Garden. Named after Thomas Neale (1641-1699), Neal’s Yard is part of his development which is now known as Seven Dials. And rather like the larger 17th century development, this cafe-physics review of Jacob the Angel has a similar dilemma: so many avenues to explore, each wrestling for attention, which one to pursue?

But first the coffee. Roasted by Square Mile, coffee is available as the usual espresso based drinks or on V60 pour over. I had a Rwandan V60 that was full bodied and full of treacle like flavours. Owing to the geometry of the cafe, I didn’t get to check my ‘flavour notes’ against what the tasting notes thought I should perceive. The cafe space itself is fairly small but with a surprising amount of seating. Given this, it can feel a bit close as you squeeze past some of the people sitting down in order to place your order, (hence not double checking the tasting notes) however that is quickly set aside as you can gaze at the large selection of cakes (all with allergens clearly marked) arrayed on the counter.

On the walls of the cafe were paragraphs about the history of coffee and how a man named Jacob opened Britain’s first Coffee House in 1651 at the Angel Inn (in Oxford). The coffee itself came presented in a manner that was reminiscent of solar eclipses, while the sink next to our table was strikingly similar to those in my A-level chemistry lab. Unable to dissociate my memories of the sink with the reality of the environment at Jacob the Angel, it was a bit shocking when someone came to fill their glass of tap water there – don’t they realise what could have been in that sink?! Each thought train surfacing as a potential direction for the review, but then, above me, something moved. Looking up it was clear that a plant that was hooked to the ceiling was moving in a draught, but where was the breeze coming from? A small air-conditioning/heater unit was on the other side of the coffee house, circulating the air that was moving the plants.

plants, Jacob the Angel, Coffee Covent Garden
Plants above the tables at Jacob the Angel. How do they move in the breeze?

The moving plant had appeared to my peripheral vision as if it was floating in the breeze or perhaps flying. Now clearly there cannot be a flying plant, but in some ways the swaying leaves do illustrate the fluidity of air, which is a necessity for flight. The moving air demonstrates how the air imparts a force to the leaves (and the pot) causing them to sway. For things that genuinely fly this would be experienced as ‘drag’ – something that we have probably all experienced, even when not flying. Drag is increased if the object moving through the fluid (air) has a larger surface area perpendicular to the direction of movement: all being equal, bigger objects experience more drag. Imagine moving a spoon through coffee, it is easier to move a stirring stick rather than a tablespoon. But then, drag also depends in a non-trivial way on the shape of the object and how that changes the vortex wake behind it (look again at the spoon and how the vortices form behind it as it is dragged through coffee, you do not see those so easily with a stirring stick).

It is partly this sort of shape effect that seems to be behind Orsted’s recent restatement of the calculation of the amount of energy that their off shore wind farms can generate. By actually going out and measuring the air flow around the off shore wind farms, Orsted discovered that the air flow (which would be used to generate power) is affected not just by the individual windmill (as had been known and calculated), but its neighbours and the way these combine into the shape of the wind farm. There is still a lot we don’t understand about exactly how spoons move through coffee.

vortices in coffee
Vortices behind a spoon dragged through coffee.

But there is also a connection to a different type of “flying machine” if only through the name of the coffee house. For it was from the Angel Inn in (what is now) Aldwych that, more than 250 years ago, that an unusual ‘flight’ took place*. It was described in an advertisement in the paper:

“On Monday, the 5th of April 1762, will set out from the Angel Inn behind St Clements Church in the Strand…. a neat flying machine, carrying four passengers, on steel springs and sets out at four o’clock in the morning, and goes to Salisbury the same evening, and returns from Salisbury the next morning at the same hour… Each passenger to pay 23 shillings for their fare, and to be allowed 14lb weight baggage”.

How many more avenues could be followed while enjoying a slow coffee at this small but fascinating little cafe? Do let me know what you ‘see’ next time you visit.

Jacob the Angel is in Neal’s Yard, Seven Dials, London.

*”London Coffee Houses” by Bryant Lillywhite, pub. 1963

Goodbye to the take-away cup

Back in April 2017 I was given a “completely compostable” disposable cup in order to try some batch brew. But how “compostable” is “completely compostable” really? It needed to be tested! And so, once it was empty, the cup was placed into a worm composting bin and left to see how long it took to compost.

Each week I took a photograph of the cup to see how it was composting, the results of which were made into the film below. How long did it take? You can watch the film or scroll past to find out more:

Did it compost?

110 weeks! That is more than two years in the worm bin. Is that how long you thought it would take? When things are marked “compostable”, even when they are marked with a regulatory compostable mark like EN13432 or ASTM6400, this usually means the item is compostable only in an industrial setting. Industrial composting facilities are kept at 58C, very far from the conditions found in a London based worm bin (more details here) or indeed from most people’s idea of a compost bin.

The OK Vincotte label is for items that are supposed to be genuinely “home” compostable. Will this bag from Amoret coffee compost in the worm bin? It is in there now but as we are in winter, the worms have slowed down to such an extent that it would not be fair to start a new #willitcompost just yet.

What about defining labels for a genuine “home” composting environment? The problem here is that a worm composting bin in London will be very different from a more conventional compost heap in a tropical country. How can you define one set of conditions that are universally applicable? One label that tries is “OK Vincotte” but it seems quite rare and indeed I have only seen this once ‘in the wild’: on bags of Amoret roasted coffee (see picture). Have you spotted them anywhere else?

The conclusion from all this? We all need to think about how we each can live more simply and sustainably. Perhaps a re-usable cup will be part of the way that you do this. (Some of them are reviewed by Brian’s coffeespot linked here). Or maybe you’ll opt to drink your coffee to-stay. Whatever else it involves though, it can’t be by putting each “compostable” take-away coffee cup we consume in a worm composting bin in London or imagining that they will somehow compost in a landfill!

Do let me know what you are doing to address the issues of your own coffee waste either in the comments below, on Twitter or over on Facebook. I look forward to continuing the discussion there.

Ringing in the New Decade

Happy New Year!

Nicaragua, direct trade, Amoret, Java beans,
Sometime this week I’ll brew this with a V60 and adapt an ‘examen’ to help me review 2019. I was thrilled to be able to meet the farmers, Dania and Desiree, at Amoret coffee earlier this year. One of the things I’m sure will feature in my ‘gratitude’ examen.

Each New Year is an opportunity to look back at the previous year, anticipate the future year and perhaps make resolutions to improve our lives, or even of those of people around us. Maybe this is even more true this year which is not just the start of a year, but of a decade.

This year I have been lucky to meet, or to continue friendship with, many people who have taught me all sorts of things about life, physics and coffee. There have also been some great finds of some fantastic cafes, trying to make a difference to their local community while serving excellent coffee.

And yet, as the year or the decade turns and we resolve to get fitter, pay more attention to sustainability or whatever seems important to us right now, we will inevitably take our existing selves into the new day and our resolutions will meet the reality of who we are: a bell rings at certain frequencies owing to the resonances of the vibration on the surface of the bell. The resonances of the bell depend on its exact shape and size, it is not easy to change the sound of the bell unless you change its temperature or even the interior to a different gas or muffle. (You can see images of how a violin vibrates at resonance here). The surface of a coffee resonates similarly, if we put it on a vibrating surface with a frequency that matches the fundamental vibrations of the surface. Nonetheless, thinking about these resonances can take us in surprising directions. The mathematics that describes them was developed by Friedrich Bessel (1784-1846) but, Bessel was not thinking about resonances when he formulated what is now known as Bessel functions. And it is possible, his life may have taken a very different direction were it not what happened from 1799.

Resonating coffee.

In the new year of January 1799, when he was just 14, Bessel was apprenticed to an imports and exports company with the hope that he would become an accountant. And maybe we would have heard no more about him had he not got interested in the problem of longitude and solving the navigational issues of the time, important for the company for which he was working. This issue got him thinking about astronomy and he caught the attention of the authorities of an observatory who gave him a job there and encouraged his observations and interest. But it was while thinking about “many body problems” or how multiple massive objects interact with each other via gravity that he came up with the mathematical description that we now know as Bessel functions. It is these Bessel functions that also describe the resonances on a bell and in a coffee cup.

Sun, heat, nuclear fusion
What links coffee to the Sun? So many things! But for the purpose of this post, we can find clues as to the interior of stars by watching the way they vibrate, analogously to a bell. What would Bessel think? Image © NSO/AURA/NSF

What does this leave us with in our thoughts for 2020? That what we are interested by may lead us to discoveries in various tangential and scarcely believable connections? That what we plan for our lives may not be how they have to end up? That it benefits us to stop for 5, 10 minutes, even half an hour and just contemplate our world in our coffee? (ok, that last one did not come from Bessel). Where-ever your paths lead and your interests lie, happy new year! May the 2020s be a decade where we can all slow down, notice, contemplate and appreciate the beauty of this strangely connected world which is our home.

A Weight-y issue

Waves on the surface of of a coffee. But what do we know about gravity driven waves rather than surface tension driven ones?

Ever swung a bucket of coffee round in circles swooping down towards the floor and then over your head? Why would you, you may well ask? Well, the answer may surprise you. It’s all about turbulence.

We have probably all come across turbulence, perhaps by watching how milk is added to a black coffee or seeing the steam interact with the air as it evaporates off a hot mug of tea. But it turns out that there is a lot that we do not yet understand about turbulence and this is where the bucket of coffee comes in.

Waves on the surface of a coffee can be dominated by gravity or capillary effects. Capillary waves are short wavelength (higher frequency) waves that are forced into oscillation by the effects of the surface tension of the liquid pulling the surface of the coffee back into shape once its been distorted. Gravity waves are longer wavelength (lower frequency) waves where the disturbed surface of the coffee is pulled back into shape by gravitational effects rather than surface tension effects.

Benjamin Franklin famously stilled the (capillary) waves on one of Clapham Common’s ponds by adding just a teaspoon of oil to it.

The frequency at which there is a crossover from gravity dominated waves to capillary dominated waves is dependent on both the density and surface tension of the liquid as well as the strength of the gravitational acceleration experienced by the mug of coffee. (We’re getting to the bucket). On Earth, the gravitational acceleration is 9.8m/s, the ratio of a liquid’s density to surface tension is quite similar for many liquids and so the transition frequency between these two regimes is generally in the region of 10Hz.

What this means is that if you wanted to study the turbulence affecting one type of wave only you could measure at higher frequency (and so measure capillary waves) or measure the turbulence in a liquid in lower gravity eg. on the International Space Station (so that capillary waves dominate at lower frequencies too). But both of these types of measurement don’t give any insight into what’s happening to turbulent waves sustained by gravity, such as Rossby waves which travel the whole circumference of planets with atmospheres and affect the weather in different parts of the globe.

So how could you study turbulence in the gravity dominated surface waves of water? It goes back to the bucket mentioned earlier. By putting a freely moving bucket (the authors called it a ‘gondola’) at the end of the arm of a centrifuge of 8 m diameter, the authors of a recent paper created an effective gravitational force on a liquid of up to 20x the value of the Earth’s gravitational acceleration. It’s sort of like the bucket of coffee being whirled around in a circle apart from a lot bigger and capable of moving at up to 67 rpm! This meant that they could measure the effects of turbulence on gravity driven waves up to about 100Hz allowing them a large frequency range over which to compare their results to theoretical predictions.

Coffee, Van Gogh
Turbulence comes in many forms: What do you see in your coffee cup?

And when they did so, they proved one nagging problem for theoreticians studying turbulence: the size of the ‘container’ becomes important, something that models had previously neglected. For the 23cm wide bucket of distilled water used by the authors, this may be something that we can easily visualise but the research has consequences for how we understand the Rossby waves that circle our planet as well as the large wavelength waves in oceans. Slightly more connected with coffee (or at least doughnuts), the results are also important for understanding turbulence in plasma waves in tokamaks.

You may have better things to do over the holidays than swirl a bucket of coffee round and round while watching for the waves on top of it, but if you are stuck for something to do…

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