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What is Project 68? Kings Cross, London

Project 68 on Tavistock Place. Most of the seating is inside with four benches outside.

Opposite a post box that is more than a century old, there is a cafe on Tavistock Place with the unusual name “Project 68”. Located along a well-used cycle route, when the cafe appeared last year, an obvious question kept springing to mind: what is Project 68? The opportunity to find out and try the coffee came the other weekend on a cycle up to Highbury. Jumping off the cycles at the junction, we had to walk past a couple of black board signs advertising the cafe. “Come this way for really, really good coffee” the signs said. Should we believe it?

The coffee is roasted and supplied by SEND which stands for “Special Educational Needs and Disabilities”. This coffee roasting company works to help people with special educational needs and/or disabilities gain employment as baristas within the coffee industry. As well as working with a few London cafes to train and mentor prospective baristas, the company also sells its coffee to other cafes, such as Project 68. The counter at Project 68 had a large supply of pastries on that Saturday morning which were tempting but we were on our way for lunch. Along with the usual suspects of coffee ‘types’, hot chocolate was available as three varieties, ‘white’, and two differing cocoa percentages. The chocolate was supplied as large buttons, so one hot chocolate was two buttons. Presumably this would allow you to mix the chocolate types should you feel the need.

Inside felt very wooden, in a good way. Light shades of wooden seating complemented the wide windows onto the street. The fact that we had our bikes meant that we went back to sit outside to observe what was happening in this cafe. It seemed appropriate to start by pondering the name. What would “Project 68” refer to? A quick (pre-visit) look on duckduckgo suggested that project 68 referred to a set of Soviet era battle cruisers. This seemed very far from the feel of the cafe. An alternative could be the social riots that occurred in Paris in ’68. And although Project 68 is fairly close to some major London universities again, this didn’t seem to be the feel of the cafe. Looking around, and at the door of the cafe a more mundane explanation suggested itself. The cafe is located at 68 Tavistock Place. Was the name merely a reference to the address?

A board on Hunter St advertising Project 68 just around the corner. Could there be physics connections here with font types or stripy bollards?

Returning to the coffee. We sat outside with our bikes and then the barista brought out our long black and oat milk hot chocolate. Perfectly enjoyable, we sat back and continued to notice what we could observe. The aforementioned postbox almost jumped out at us. The cypher on the front suggesting it dated from the short reign of Edward VII (reigned 1901-10). Among other things, Edward VII had been known to run the clocks at his residence in Sandringham, 30 minutes fast. Much like daylight saving time, this extra half hour meant that there was even more daylight for hunting for him. In the UK, the clocks are brought forward and put back by one hour in March and October respectively each year. It is a desire for our own convenience set against the unchanging regularity of the Sun being directly overhead at mid-day. We could pause to think on that or get distracted by a musical connection to Edward VII as he opened the Royal College of Music in 1883. For while we are considering the music, we may start to notice the connection between this cafe, music, and the stars, and we noticed it just beneath our feet.

Many of London’s older houses and shops have an under-pavement cellar. In the past this would have been used as a coal storage area, now they find many diverse uses. A grate beneath our feet indicated that we were above such a cellar. Suddenly, just after a sip of coffee, there was a whir from beneath the pavement. A rhythmic motor-like noise below where we were drinking our coffee. A stray leaf on the mudguard of my bike indicted a draft coming from the grate too. What was this machine that had sprung into action? It is telling that we can discern many things from how something sounds. We know if our coffee grinder is working correctly by the characteristic hum of the rotation of the burrs. A problem with a vacuum cleaner is often indicated by the regular sound of the motor being disturbed by the clatter of a coin going up the vacuum pipe. Sounds can reveal what is happening in much the same way that we use our eyes to see. But fundamentally a sound is a vibration. Something is vibrating that causes the air adjacent to it to oscillate sending forth a sound wave of a varying note. The mechanism is the same whether we are considering the rotation of the drum of a motor or the subtle vibration of the internal cavity of a violin. Sometimes we can see the cause of the sound as when we notice the fast vibration of a guitar string after it has been plucked. Sound can be ‘heard’ through at least three of our senses (hearing, seeing, feeling).

Finally, it is all about the coffee. A long black balanced on one of the benches. You can see the grate on the pavement to the left of the photo.

On Earth we use sound to detect earthquakes around the world, listening for the vibrations through the ground. Often these vibrations are amplified by detectors changing the subtle oscillations of the Earth into wild wobbles of a pen drawn on paper (or now, into a more digital form). In this case, the sound is transferred to what we see. Recently, there has been the opposite case. Astronomers have noticed, by looking at different stars how each star can vibrate with different sorts of ‘note’. Just like a violin, the internal vibrations in the star will cause the surface of the star to wobble and oscillate according to different types of vibrational sound. And, just as with the Earth, how those vibrations travel around the star and persist with time reveals the composition of the interior of the oscillating sphere. Astronomy provides a further connection to Tavistock Place, if not quite Project 68. The site of Francis Baily’s home is just down the road from Project 68, at number 37. Although he does not seem to be commemorated with a blue plaque (yet?), there are plaques nearby to Lenin and Jerome K Jerome. Clearly this street has quite a history. Baily was an astronomer mostly commemorated now by “Baily’s beads” a phenomenon visible during total or annular solar eclipses. Observed by Baily in 1836, the beads are bright spots of light around the rim of the dark circle of the Moon blocking the Sun. Caused by the mountains and craters on the Moon’s surface, it’s a visual, rather than audible clue as to something slightly off-tone, or irregular, on the Moon’s surface.

Back at the cafe, the sound continued under our feet. Fairly uniform, it seemed like a machine running regularly. We guessed at what could be producing the noise. Nothing was conclusive. Perhaps it is more intriguing if the cause of the sound remains a mystery, just as the name of the cafe.

We returned our coffee cups to the counter inside the cafe and wandered off across the road to Judd Street, ready to cycle off to Highbury. As we were just about to peddle off, a couple of people were walking towards us. “What is Project 68?” one of them asked the other. There can be no such thing as a stupid question and if you would like to find the answer to this one with a lovely cup of coffee with a story, it is worth a diversion to 68 Tavistock Place.

Project 68 is at 68 Tavistock Place, WC1H 9RW

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If you are tired of London

Jonathan's coffee house plaque
Jonathan’s Coffee House was the site of the first London Stock Exchange. A link between coffee and the financial markets can be made via the maths describing Brownian Motion.

London has a long history for tea and coffee drinkers. The first London coffee house opened in around 1652 in St Michael’s Alley*, followed quickly by a large number of coffee houses all over London. Isaac Newton, Edmond Halley and Abraham de Moivre were all known to drink coffee in the various establishments dotted around the city (de Moivre even charged a small fee for mathematical advice at Old Slaughters on St Martin’s Lane). Famous institutions such as the London Stock Exchange and even the RSPCA were first formed at London Coffee Houses. On the engineering front, Thomas Telford established the Institute of Civil Engineers at the Salopian Coffee House just off Trafalgar Square. Yet the story in this post comes not through the coffee houses but from historic tea drinkers and, in particular via a “prodigious drinker of tea”, Dr Samuel Johnson**.

The story is detailed in Boswell’s “Life of Samuel Johnson” and concerns a friend of Johnson: Mrs. Anna Williams (1706-83). Williams had gone blind in the 1740s and, in the later part of her life lived with Johnson in his various dwellings around Fleet Street. You can now visit the Samuel Johnson museum in one of those properties at Gough Square, just off Fleet St. Between 1759 and 1765 Williams was living a short distance from Johnson but Johnson regularly popped around for tea in the evening. It was on one such occasion that Boswell joined them. Williams would pour the tea for all the guests but Boswell wondered how she knew when to stop pouring given that she couldn’t see. Boswell thought it was possible that she had subtly dipped her finger into the tea to feel when it was about to flow over the edge of the cup. As he later found out, this was not true. Apparently she had “acquired such a niceness of touch as to know by the feeling on the outside of the cup how near it was to being full.”

The question then arises, how could she have known this just by touch?

a heat sensitive coffee mug
This coffee mug changes colour as it gets warm. The constellations slowly appear as the heat is transferred from the hot coffee inside to the outside of the mug.

A first solution would be that she was feeling the heat of the tea through the porcelain of the cup. Is this possible? It is certainly true that, once a cup is full, it feels warmer to the touch than an empty cup, but heat does not travel through a substance, such as a cup, instantaneously. Would Williams have had enough time between feeling that the cup had become warm to stopping pouring to prevent the cup from overflowing? A quick experiment with my colour changing cup (pictured) suggests that this may be unlikely. Even if she had the sensitivity in her fingertips to quickly discern a change in temperature, it took 20 seconds between boiling water being introduced to the cup before the top started to change colour. Although porcelain is thinner than my mug, and the change is probably quicker to feel than to see with the cup, it is likely that she would have carried on pouring the tea beyond the rim of the cup before realising that the cup was becoming full. Although it is possible that she could compensate for this, for example by touching the cup further towards the base, perhaps there are other ways that you can discern a cup is almost full without seeing it?

A second solution could be that her discernment occurred through a mix of touch and sound. There are a few ways in which audio signals will suggest that a cup is almost full. Coupled with the feeling of increasing warmth on the side of the cup, this may give the tea pourer increased confidence that the cup is comfortably full. One such clue is familiar to those of us who fill water bottles up from the tap, the change in pitch as the tea is poured. Similar to a Helmholtz resonator, a pourer would know when to stop as the musical note from the pouring tea changed. But the tea cup is open at the top so this effect is weak. Perhaps the sound clue instead comes from the sound that the poured tea makes as it impacts the surface of the cup of tea. When we hear a dripping tap, what we are actually hearing is the bursting of an air bubble just below the water surface. We would expect the sound of that to depend, for shallow containers such as the cup, on the depth of the tea. Consequently, if we poured the tea slowly and heard the drops of tea as they entered the cup, we may expect to gain an idea as to how full the cup is. As Boswell assumed a slight of hand (and of hygiene) on the part of Mrs Williams, it is probable that she did not pour the tea slowly enough for this to have been her primary route for knowing that the tea cup was nearly full. Perhaps Mrs Williams did not use sound after all, but relied on something else?

Bubble tea? There is a lot of physics to be found in tea as well as coffee. Take time out with your brew. What do you see, hear, feel?

There is one more clue that you could get about the relative fullness of a cup of tea using touch. If you very gently push against the cup with your finger, you can feel the resistance of the cup to movement. As the cup became heavier with tea, the resistance to its being pushed would increase (Newton’s second law). After pushing at the top of a few mugs with my finger, this seems to be possible. Unless the person watching the tea being poured was very observant, it is not clear that they would notice this. Can we gather anything of Boswell’s skills in observation? Throughout his book he describes people and social situations well and yet, he was unsure whether Williams had dipped her finger into the tea cup to know when it was full enough. I think we can probably gather that on this particular point, Boswell may well not have noticed had Williams been pushing gently at the tops of the cup to see how easily they moved. Although there is a risk of pushing the cup over, this does seem to be a very feasible way of filling a cup without using your sight.

There is however one last, maybe more boring possibility. Generally the speed at which liquid comes out of a tea pot is quite reproducible. Through the fact that she was pouring tea every evening for Johnson and his guests, it is quite possible that she knew how long to pour for each fixed angle of pour before the tea cup was properly filled. We’d still have to ask how she measured the time given that she wouldn’t have looked at her watch, but perhaps here we would have a clue from Galileo. He is said to have sung songs with a known rhythm in order to measure time (the use of the pendulum to measure time came later, partly as the result of Galileo’s work). The idea was that the rhythm is a surprisingly reproducible method for comparing time intervals. Maybe Mrs Williams sang to herself while pouring the tea for the guests.

What do you think? Perhaps you can think of another effect that could be used to determine when your tea cup is full, without any visual clues. Or maybe you just disagree with my deductions. Whatever your thoughts, do let me know in the comments below or on the various social media sites (FB, Twitter, Mastodon), I look forward to learning more and maybe, being able to pour my coffee with my eyes closed.

*Information on the London Coffee Houses can be found in the excellent “London Coffee Houses” by Bryant Lillywhite published in 1963

**The story of Mrs Williams, Dr Johnson, his tea drinking and the notes of Boswell is described in “Life of Samuel Johnson” by James Boswell

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A Need for Roots, Hermanos Coffee, Portobello Road

Hermanos Colombian Coffee Roasters on Portobello Road. There was a fair amount of graffiti, which could offer further avenues of thought but is probably owing to the Notting Hill carnival which had happened just before my visit.

There is nothing quite like wandering through a market street in the hour or so before the market opens. It feels as if you are there as part of the city is waking up, things moving into place, ready to start the hustle and bustle of the day. It is one of the things I like to do when exploring a new city: wake up earlier in order to walk around and try to find breakfast, listening for the character of the place. Sometimes though, it is good to try this in your own city, it is a chance to see your home, your space, in a new light; a different aspect of its essence. So it was that I ended up wandering along Portobello Road shortly after 8am one week day morning. Market stalls were being moved into place, office workers were cycling or scooting on their way to work and a small little coffee shop with an open door seemed to invite customers in to sample their coffee.

Hermanos Colombian Coffee Roasters cafe can be found at 127 Portobello Road. Actually, it can also be found at 7 other locations including Kings Cross and Victoria Stations. Does this qualify as a coffee chain? Regardless of the number of cafes, Hermanos Coffee on Portobello Road is somehow clearly integrated into that community, so much so that this initially appeared to be a small scale single shop embedded within Portobello. When you sit to drink a coffee while the street around you ‘wakes up’, it seems as if people around drop their guard a bit, you see glimpses into relationships that later in the day will be hidden by the rush of people. Perhaps it was the time of day that made this cafe seem particularly friendly, I stepped inside to order my coffee.

Perhaps unsurprisingly, the coffee is roasted by Hermanos Colombian Coffee Roasters and they have a large selection of their coffee (and coffee making equipment) for sale on your left as you walk into this small cafe. The counter is just ahead of you (also on your left) with the menu behind the counter showing the usual selection of coffees available. I am fairly confident that they also offered a pour over coffee on the menu, but it being the start of the morning rush, I didn’t want to preoccupy the barista with making a filter for me and so I ordered a long black. Beyond the counter, there were a few seats with people already inside enjoying coffee and conversation before the start of the day. As I indicated that I wanted to sit outside, the person behind the counter asked me if I would prefer “china or a take-away cup”. It’s a small detail but it was nice to be asked. I picked up my cup of coffee from the end of the counter, wandered outside and took a seat waiting to watch what went on for the next half hour or so.

Coffee on a wooden table. Looking at the lines formed by the wooden panels, you can see which one was wonky.

I listened as the barista spoke to each new customer, some of whom were clearly regulars. “Back to hot coffees as the weather turns colder then?” I heard, as a customer walked in off the street. I then watched as the barista came outside, took a short breather during a lull in customers and greeted the person who was opening the hat store next door. Another person walked into the cafe to pick up his bag that he had left there briefly as he knew it would be ‘safe’. This is (unfortunately) not something that you would normally assume of Portobello Road. It was not just the community aspect that jumped out at the interested observer of this cafe. The science started to appear everywhere. This is of course in one sense always true. The mere fact of seeing something involves multiple elements of physics and biology without thinking about anything deeper. However, some cafes produce something in the physicist akin to writer’s block, it can be hard to link to what is around you. This is very far from the case at Hermanos.

The recent rain on the wooden table top suggested the phenomenon of coffee rings and what makes a surface wettable, while the table itself with one wonky wooden slat immediately prompted considerations on defects in crystal lattices. One of the many people who rushed by was talking to someone on a mobile phone set to speaker. This meant that the entire conversation was audible to the nearby coffee drinker. You could ponder what they were talking about or you may start to think about why it is that someone through a phone line sounds different to that same person speaking in front of you. Every sound is made up of several frequencies of sound wave. The more complex the sound, the greater the number of frequencies used to convey it. When you speak to someone face to face, all of those frequencies will be transmitted from the speaker to the listener. When a sound is transmitted through a phone line, it is necessary to limit the frequencies that the phone line can carry. For some sounds, some of the higher frequencies that contribute to that sound’s ‘sound’, will get blocked off by this bandwidth limitation. Consequently certain sounds, like “s” and “t”, will sound slightly different over a phone line than when speaking face to face. The conversation will be perfectly intelligible, but we pick up on subtleties in people’s voice and know that they sound slightly different through the phone network.

The Hermanos signboard photographed with the hat store in the background.

Elsewhere, the sign of the cafe puts a square around the “H” of “Hermanos” which prompts recollections of the element hydrogen which has that symbol on the periodic table. It was an appropriate connection because the raindrops on the signboard were reflecting the Sun which is mostly made of hydrogen being formed into helium through nuclear fusion. Looking again at the sign, the reflections and the table, it was clear how our eyes interpret lines and angles into information about distance. These are observations that scientist-artists of the past have used to formulate the rules of perspective. It caused me to look again at the wonky wooden slat and think about how I knew it was wonky (without putting my coffee on top of it). Perspective then surfaced again as the worker in the hat stall next door brought out the days collection of hats and goods. As he was standing behind a hat stand arranging a viewing table, it appeared that he was wearing one of the hats on the stands. Only the fact that he moved and the hat didn’t showed that actually my eyes had been deceived. A parallel meaning of perspective was evident around as the street changed while I was there: by the end of my coffee, the shops were open, parents were rushing children to the school around the corner, the street was alive. What would I think of this cafe if, rather than enjoying my coffee at around 8.30am, I was trying to drink it in the middle of the Saturday tourist rush?

The coffee was good. A very drinkable cup with which to enjoy my time on Portobello Road. The cafe was equally good, showcasing many different aspects of what being a cafe is about. Hermanos Coffee is definitely worth a visit and I hope to go again next time I have the urge to explore Portobello Road before the crowds arrive. It seems appropriate however to conclude this cafe-physics review with a quote from the Hermanos brothers themselves about their cafes and coffee. They say “We understand that our journey at Hermanos is also the collective journey of so many, who each in their own way contribute to and benefit from the world of coffee.” A journey that is so much easier to appreciate if you pause to sit and people watch at this lovely little cafe.

Hermanos Coffee is at 127 Portobello Road, W11 2DY and at multiple locations around London.

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Kuro Coffee, Notting Hill Gate

Kuro Coffee London
Outside Kuro Coffee on Hillgate Street.

Just off the busy Notting Hill Gate itself, on a side street, is a small corner cafe called Kuro Coffee. White brick with a couple of benches outside, the cafe is at the corner of Hillgate Street and Uxbridge Street. The area is relatively quiet and sitting on the bench outside while enjoying a coffee is certainly, strangely given the location, fairly peaceful. Kuro Coffee seems to come in two guises. The first, which we tried, was the coffee/cafe guise. This Kuro serves coffee, tea and matcha with a good selection of pastries. The second is the Kuro-late version, which has a license. We’ll have to return to try that another time.

Assuming that you are going for the coffee, you have the usual choice of drink types served either with the regular coffee or a guest single origin. On our visit, the single origin was an El Salvador with an interesting set of tasting notes, so trying the single origin long black became too tempting. It is worth remembering that the entrance to the cafe is up a couple of stairs. While this was no problem on the way in, forgetting the steps on the way out led to a slight coffee loss, though I recovered in time to save both coffee and some dignity. The cafe itself is a small room with the counter on the right as you enter. The large window at the front of the cafe gives the space plenty of light and, if you wanted to stay inside, there is seating upstairs.

Returning to the bench outside with our coffees, we watched as the traffic went past as well as the clients of the nearby dog grooming shop. The buildings around Kuro Coffee certainly give a hint as to the complex history of this area of London, together with a nod towards some of the more colourful buildings that it has become famous for. The Coronet Theatre just opposite the cafe dates from 1898 and is richly decorated in the style of the time. Towards the main road, the buildings take on an appearance far more characteristic of the 1950s-60s when the main road was widened and a lot of the historical area demolished. These block-like buildings contrast with the intricacy of the decoration on the theatre and the individuality of the houses within this part of “Hillgate village”. It has been said that the architecture of an era and location reflects the values of the society that builds it. What does this say as we look around, and which buildings, if any, resonate with us, which do we find beautiful?

Coronet Theatre, Notting Hill
The Coronet Theatre as viewed from the bench outside Kuro Coffee. The statue on the dome of the theatre is a fairly recent replacement of a much earlier statue that had not been there for years.

It is a question with relevance to physics. Many theories in physics are considered to be ‘beautiful’ but what does this mean, particularly when applied to physics? Michael Polanyi captured part of it when he wrote “The affirmation of a great scientific theory is in part an expression of delight. The theory has an inarticulate component acclaiming its beauty…”(1) We may not be able to define beauty, but the delight we feel discovering it as we learn about some parts of physics is something that we can certainly sense.

One of the theories that is considered beautiful in this way is that of relativity, one part of which has become part of our common knowledge, E = mc2. Special relativity holds that the speed of light in a vacuum is the same for all observers. This is remarkable partly because it contrasts so much with our every-day experience. When we think about our every-day, if we were to be travelling on a train and throw a tennis ball forwards, we would see the ball move away from us at, say 15 metres per second. Someone standing on the station platform watching as the train goes by would see the ball move at 15 m/s plus the speed of the train. Perhaps it is more dramatic if we threw the ball backwards, it may appear to the observer on the station platform that the ball was actually stationary as the train moved past the platform. This is not true of light. If I was on a train and could measure the speed of light travelling away from me, I would measure it travelling at 2.998 x 10^8 m/s. Someone on the platform watching light travel away from me would measure it to have exactly the same speed, we call it c.

You may say that trains are (relatively) slow, even the high speed ones, and so maybe this is just within error but it is true irrespective of the speed of the train. The famous example is of Einstein wondering what would happen if he was driving his car and looking in the internal mirror at his reflection. To begin with, everything is fine, he can see his reflection, but as the car’s speed increases to close to the speed of light what would happen then? He would see his own reflection! As if nothing has changed, the speed of light relative to Einstein would be the same, c. Someone watching and seeing that the car was travelling at, say 0.9c would not measure the speed of light in the car to be c + 0.9c = 1.9c. No! They would measure the speed of light within the car to also be, c.

The view to the right of Kuro Coffee. The concrete buildings were built when the road was widened around the position of the old toll gate itself.

The solution to this seeming paradox is how we arrive at the idea that moving clocks go slow and, of course, the famous equation E = mc2, the idea that the energy (E) of an object is equivalent to its resting mass (m) multiplied by the speed of light squared. These ideas have been tested by comparing a stationary and a fast moving atomic clock and, in the case of the energy-mass equivalence in the atomic bomb where a very small amount of mass translates to an enormous about of explosive energy. Another test of the idea is comparing the speed of light on Earth along the direction of the Earth’s rotation around the Sun and perpendicular to it. If light waves did behave like the tennis ball on the train, there should be a difference between the light speed measured in these two directions (which can be done by a technique called interferometry). The result of this experiment, now known as the Michelson-Morley experiment, supported the theory of (special) relativity: light did not behave like a tennis ball in a train(2)!

The beauty comes as we explore the physics, and the maths, that allow these equations and results to emerge. Nonetheless, it is still perplexing and boggling, perhaps even a little bit weird. Beauty can definitely be disconcerting, but it retains an ability to push the intellect into an “expression of delight”. Where else do we experience this “expression of delight”, do we recognise beauty similarly for beautiful physics and beautiful buildings? As we sit on the bench which looks towards the west, we can know that the light reflected back from the buildings is travelling at the same speed whether we look ahead of us or immediately to our right; in opposition to the additional speed of rotation of the earth or neutral to it. The buildings immediately in front of us or to our right however are certainly not of the same level of beauty and aesthetics. What makes it so? Perhaps it would be a good time to go and get yourself a coffee and a space on the bench, and just enjoy the moment as you experience the present, ahead of you and to your right.

Kuro Coffee is at 3 Hillgate Street, W8 7SP

1 Michael Polanyi, “Personal Knowledge, towards a post-critical philosophy”, University of Chicago Press, 1958

2 There is some discussion about whether the Michelson-Morley experiment prompted Einstein to think about his idea of relativity or not. As I am not a historian of science, I won’t get into this as it is incidental to the story. Einstein was certainly aware of the Michelson-Morley experiment and thought it helpful as an experimental support of his theory, the discussion of its importance in the development of the theory can be found in Polanyi cited above.

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Gallery of fluid motion 2021

Each year, the American Physical Society hosts the Annual Meeting of the Division of Fluid Dynamics. A highlight of this is the Gallery of Fluid Motion, a competition of videos showcasing fascinating science into all aspects of fluid dynamics. You can find a link to all of the videos, including this year’s prize winners here. Listed below however are a few videos with links to coffee, cafes or just generally beautiful physics that you may be able to replicate in your kitchen.

Beautiful Physics

How do fish know how to swim together in a school? An illuminating study that helps us to find out:

The strange and wonderful patterns formed by dropping a small amount of dyed water onto glycerin:

Can you bounce a liquid drop on a liquid? Yes, it even explains something we may have noticed while brewing pour overs, but what about bouncing liquids on a solid:

Finally, although it refers to something we may not want to think too much about, there is some beautiful physics going on as people exhale, with and without masks:

Coffee/Cafe Physics

You may recognise the sound of something that is deep frying. But what fluid physics is causing it? You need to “listen to your tempura”

It stretches it a bit to call this “coffee” or “cafe” physics but many people have tattoos and surprisingly, no one has really ever investigated how the ink gets under the skin. Until now of course:

The Leidenfrost effect is something that you will have seen often while frying eggs. This takes a closer look at the Leidenfrost drops:

Kitchen Physics

Experiments you can do at home. The first is to look at the patterns formed as a drop of food colouring spreads on a mixture of water and xanthan gum (available in many supermarkets for gluten free cooking).

Secondly, how does water flow out of a bottle?

And Finally

Do take a look at the full gallery (here) and even have a go at one or two of the experiments. It would be great if you would share your photos of fractal patterns formed by food dye or even if you’ve been inspired by any of the other videos. Whatever you do, enjoy your coffee.

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Conscience Kitchen, Notting Hill

Conscience Kitchen Restaurant and Coffee House on All Saints Road in Notting Hill.

It was 8am on an unseasonably warm morning in November. There were two cafes open on All Saints Road in Notting Hill, but Conscience Kitchen had an open door and comfy looking seating outside. Conscience Kitchen describes itself on its sandwich board as a “restaurant and coffee house”. At 8am in the morning, I wasn’t going to try the restaurant bit (though there were croissants available), but I did enjoy the coffee house bit. Seating had already been arranged outside. There were comfortable and cushioned seats immediately outside the cafe, a set of table and chairs on a converted parking space diagonally in front and a covered section in the parking space immediately outside the cafe. There were also plenty of seats in the spacious interior. The cushioned seats just by the window however offered a perfect spot to watch the world go by.

As it was a week day, plenty of people were either commuting to work or taking children to school. It is interesting how much you can discern about someone passing by when you listen to how their footsteps sound. Confident and clipped, shuffling or lethargic, or occasional combined with the whirling of the wheels of a scooter. A large number of characters passed by as I sat with my coffee. The coffee was an El Salvador single origin roasted by Round Hill coffee roasters. There was also a guest coffee on offer that day from a roaster in Amsterdam but as I didn’t realise this until I was paying I missed the opportunity to try both sorts.

Conscience Kitchen signed the lease on the shop in March 2020. What timing! Shortly after opening they had to close with the lock-downs and so the past eighteen months have been a series of adaptations as they renovated and grew their business. It does seem that their focus on good, organic food has attracted a loyal local following. At times during the second lockdown, the coffee house was turned into a produce store and while those days are far behind us (hopefully), that time did allow the locals to appreciate the care that Conscience Kitchen took over their ingredients. The pandemic times have also affected the seating arrangements with both the aforementioned parking space seating and the outdoor heaters a sign of our times. It was fairly warm that day and so I declined the offer of them turning the heating on for me, I had a hat and a coat after all. But this did give me a reason to look at the heater a bit more closely.

The heater at Conscience Kitchen. You can see the coiled element and the reflecting domed surface.

The heater consisted of a strange light bulb like fitting which led to a coil of what I had assumed was wire, enclosed in a tube and backed by a silvered domed surface. Investigating such heaters later, the ‘wire’ was more likely to be a weaved carbon fibre element. Regardless of what the heating element was made from, the mechanism of heating is the same. The power emitted by the element is the product of the electrical resistance of the element and the square of the current going through it. This relation, known as the Joule-Lenz law was discovered independently by Emil Lenz and James Prescott Joule in the 1840s. So why use weaved carbon fibre as a heating element? There are presumably a few reasons. Firstly, as a weave, a network of fibres, the heater will be more resilient if one of these, for any reason, breaks. If we had a single tungsten wire (as an extreme example), and it broke, the heater would no longer work. This makes the heater more long lasting. But there is a second, more physics based reason for using carbon fibre.

The power rating of the heater is defined as the energy emitted per unit time. When you subject a material to a given amount of energy, it is heated. The increase in temperature of the material is proportional to the amount of energy you put in, divided by the specific heat capacity of the substance which is material dependent. The specific heat capacity of woven carbon fibre is approximately twice that of copper and five times that of tungsten. This means that, for the same amount of energy the carbon fibre will heat up less than the metal wires. This provides the clue for the silvered dome. The heat from the heater is not really just coming directly from the electricity passing through the heating element. The second component is the infrared radiation emitted as a consequence of the temperature of the heating element. As the carbon fibre is not so hot as a metal element of the same power rating, the infrared radiation is at a different wavelength which turns out to be more efficient at keeping us feeling warm. The silvered dome was there to reflect the heat back towards the people on the terrace, further increasing the efficiency of this heater.

Looking further around, I noticed the hashtag on the Conscience Kitchen sandwich board: # Less is way more (unsure about the spacing!). Does this have an analogue in physics? Since the time of Joule and Lenz, physics has undergone increasing specialisation. In Joule’s time, physicists could investigate any number of topics which were also related to each other: heat, optics and electricity, or magnetism and fluid dynamics. Experiments with electricity informed our understanding of thermodynamics for example, while mathematics provided connections between magnetism and fluid dynamics via vortices. Researching one of these fields could, and did, lead to fruitful advances in other fields.

All about the coffee.

Since then, physics has become increasingly specialised and our research focus very narrow. In my field of magnetism, it is highly unlikely that I would get to investigate any aspect of fluid dynamics except for fun over the coffee table. It has been joked that, as individuals at least, we know ‘more and more about less and less’. This specialisation has however led to an enormous growth in our understanding of each of these sub-fields, and, correspondingly, a growth in the technological applications of the research. For example, dedicated research into a specific small detail of how electrical current travelled through layers of magnetic materials led to the sudden increase in the storage capacity of hard disks in the 1990s (and to a Nobel prize). The increased ability to store data has led to other fields being able to investigate highly data intensive areas and so produce advances in their subfields too. These are advances that could not have been made without specialisation.

Is this the ‘more’ of the “less is way more” equivalent for physics? Or is there perhaps a ‘way more’ about it?

The science historian LWH Hull described our situation as if the varying specialists were like people exploring the branches of neighbouring trees, “A man cannot understand other people’s problems by interrupting his own work to climb a few feet up their trees…”* Where then does this leave science? No physicist can any longer be a practitioner of the entire field of physics. Certainly no scientist can any longer understand ‘science’. And yet physics progresses because we work together in an inter-disciplinary way using our community to build a deeper understanding of the whole. This can only work because there is trust in other scientists and in the integrity of the work that they do. A trust that builds community which has consequences for our approach to society. Michael Polanyi took it further “Fairness in discussion has been defined as an attempt at objectivity, ie. a preference for truth even at the expense of losing in force of argument. Nobody can practise this unless he believes that truth exists.”**

“Less is way more”, but how “way more” do we want to take it?

Conscience Kitchen is at 23 All Saints Road, W11 1HE

*Quote from History and Philosophy of Science, LWH Hull, Longmans, Green and Co Ltd, 1959 – it is possible that it is not a verbatim quote as I only have my notes of this book with me at the moment and not the full text.

**In “Science, faith and society” by Michael Polanyi, Oxford University Press, 1946

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Vacuum fillers

inverted Aeropress and coffee stain
The Aeropress on top of a mug, with a coffee spill. Clearly a badly performed inversion method brew.

The Aeropress is a lovely way to make a fairly quick cup of great coffee. Part filter, part immersion, it is a coffee brewer designed by an engineer. There are many ways to make coffee with an Aeropress but common to all is the ‘press’ towards the end where the plunger is pressed down onto the coffee pushing the liquid through the filter paper and into the mug. As you remove the Aeropress at the end, it can drip leading to coffee stains on the work surface. However there is a trick to prevent this. While watching James Hoffmann videos to improve my Aeropress technique, he mentioned that after pressing, he pulls the plunger back up a bit and this helps to prevent dripping. Genius. You can see his recommended Aeropress technique here.

The trick presumably works because the plunger has a rubber (or silicone) seal into the Aeropress base. This ensures that when you pull back the piston, there will be a slight vacuum created just behind the filter paper. As the air flows back into the space behind the filter it will primarily do so via the filter-end of the Aeropress (not the seal) and so any drips that were forming will be pushed back into the brewer. Rubber seals are not fantastically air-tight and will let air in eventually but for the few seconds that you need to take the Aeropress off of the mug and replace it upside down on the work surface this level of vacuum is sufficient.

So how air tight is a rubber seal? The seal in the Aeropress will not be very air tight at all. The seal created is purely through the rubber piston being pushed into the Aeropress body. However purpose-built rubber o-rings can support fairly respectable vacuums. Normal air pressure is 1 Bar or 1000 mBar. Using rubber o-rings that are clamped into place between separate parts of a vacuum chamber, it is perfectly possible to achieve vacuum levels of around 0.01 mBar**. For higher vacuum levels (or equivalently lower pressures), you would need to use a metal seal such as copper. As copper is slightly malleable, if you use it as a join between two parts of a vacuum chamber and clamp it together, you can create a very good (air-tight) seal. In this way you could pump the vacuum chamber to pressures of 10^-8 mBar or even lower. You would need this level of vacuum to make some of the components that are contained in your mobile phone or laptop, possibly even some of the components in the measuring scales you use to weigh the coffee. You would not need that sort of vacuum to make coffee itself.

How to brew a perfect cup? Would a bit of physics help with the clean-up?

The Aeropress is a fairly recent invention and yet similar problems, and solutions to Hoffmann’s trick would have been noticed in the past. And yet there is a common saying that “nature abhors a vacuum”, originally attributed to Aristotle. If we think that the explanation for the effect above seems sensible, how do we reconcile these two ideas? Descartes noted a similar problem in a wine keg. It is like the lid of a take-away coffee cup: for wine (or coffee) to flow out of a hole in a container, another hole is needed. Did the extra hole allow the wine to avoid the vacuum? Instead, Descartes explained it differently:

“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…”*

The idea was that everything including space was absolutely filled with matter. So the extra hole in the wine keg allowed this extra matter to flow into the keg and the wine to flow out; if the Aeropress plunger is pulled back, matter would immediately flow back into the space created. The drops would be pushed back into the Aeropress and it would not drip. A very similar mechanism to the reason suggested for the behaviour above. It perhaps could cause us to question, what evidence do we have from our own daily lives about the existence of vacuums? How could we personally prove that they exist even as we rely on their existence for our consumer electronics?

Joseph Wright ‘of Derby’ An Experiment on a Bird in the Air Pump 1768 Oil on canvas, 183 × 244 cm Presented by Edward Tyrrell, 1863 NG725 https://www.nationalgallery.org.uk/paintings/NG725

There is a famous painting from the eighteenth century that demonstrates the creation of a vacuum in a home-setting. In “An experiment on a bird in the air pump” (pictured), Joseph Wright depicts the moment that air is taken out of a vacuum chamber containing a bird. The bird collapses in the vacuum as the audience looks on. We know the vacuum exists because the bird no longer has air to breathe. At the moment that we encounter the picture the scientist demonstrating could either let air back into the chamber and allow the bird to live or continue reducing the air pressure at which point the bird will die. What will he choose? The audience display a variety of reactions from the indifference of the couple on the left to the impotent horror of the girls on the right. Only two of the audience seem to be paying attention, even the experimentalist appears to be performing, and not participating in, the experiment. It could be argued that the painting speaks to us of the scientific method and the idea of being detached, outside of and observing the natural world. Imagining ourselves “independent observers” of a situation we are participating in. We are all detached and looking on, both controlling the life of the bird as well as claiming indifference to its fate.

As this was written, COP26 was continuing in Glasgow. We are at a specific point in time, just as with the “experiment on a bird”. Are we going to continue as we are or will we intervene and allow life to recover? Do we tell ourselves that we are indifferent observers or are we co-inhabitants of a common home? These are perhaps not considerations for a website about the physics of coffee. They may be considerations to have while enjoying, or certainly contemplating, a coffee. Whether or not you use a trick from vacuum science to help you clear up.

*Descartes, “The World”, ~1632

**Basic Vacuum Technology, 2nd Ed. Chambers, Fitch and Halliday, Institute of Physics publishing, 1998

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A reason to add milk to your coffee

stirred cup of coffee with streak lines
It is astonishingly difficult to photograph the swirls of a stirred black coffee, still harder to capture the shape of the surface. This was an attempt with a strong light reflected on the surface.

You sit down to savour a well rounded, freshly roasted and just brewed pour over. Is there a good reason to add milk to it? Well, besides anything else, it may be a good test of an idea suggested by coffee-cup physics.

It’s about what happens as we stir our coffee. Many of us have contemplated our drink as we have stirred it either to cool it down or indeed to add milk or sugar. The surface of the coffee forms a depression at the centre, while at the walls of the mug, the surface forms a fairly steep slope. What is causing this shape and could it have any influence on how we appreciate our brew?

The shape of the stirred coffee surface is a consequence of the balance of forces acting on the surface. In addition to the force of gravity, there is the centripetal force on each bit of coffee swirling around the centre of the drink. These two forces have to balance at the surface of the water (assuming constant air pressure above the surface). If you make the further assumption that the coffee liquid rotates as one mass, so that the coffee at the edge of the mug rotates at the same angular velocity as the coffee at the centre, the centripetal force increases with increasing distance from the middle. This means that gravity dominates in the middle of the coffee whereas, towards the edge, the larger centripetal force is having a far greater influence. It is this that leads to the depression at the centre of the coffee and indeed the parabolic shape of the surface (click here for a mathematical derivation). The parabolae formed by such rotating liquids can be so perfect that liquid mirror telescopes have been developed to closely scrutinise specific parts of the sky. One problem with these liquid mirror telescopes is that the rotation of the liquid (often mercury) has to be perpendicular to the force of gravity. Which means that the telescopes are not able to move to different regions of the sky but instead only look ‘up’. Nonetheless, this does mean that they scan the same region of sky each night and so can be used very effectively to compare changes in that region of sky.

vortices, turbulence, coffee cup physics, coffee cup science
When a cup of water is first put onto a rotating platform, the liquid at the centre does not rotate at the same speed as the walls of the mug (that comes later). During these times, turbulent boundary layers appear at the walls of the mug which can be visualised with ink as has been done here.

Stirred coffee in a mug though is not a rigidly rotating liquid. Instead, the friction at the walls of the mug means that the coffee at the outer edge is slowed down and so the rotation is faster at the middle of the coffee than the edge. To form a parabola on the surface of a mug of coffee, it would be better to put the whole mug onto a record player played slowly. How does the shape of a stirred coffee differ from the surface of a coffee placed on a record player?

Initially, as the spoon is forcing all the liquid around together, the curvature will be approximately paraboloid. The interest comes once the spoon is removed and the friction between the coffee liquid and the sides of the mug becomes important. Towards the walls of the mug, the rotation will be slowed down which means that the centripetal force will decrease. Gravity will then dominate the combination of forces and the coffee surface will become flatter. As more of the coffee slows down, progressing from the edge of the cup towards the centre, the coffee surface will further flatten until the central depression is all that is left. As the friction slows more of the liquid down, so the depression at the centre of the coffee will also eventually disappear.

This is where the milk comes in. Assuming that you add cold milk to the centre of the rotating (hot) coffee, what should happen is that the milk (which is denser than the coffee because it is cold) will sink down towards the bottom-middle of the cup. As it sinks, so it will drag some of the swirling coffee down with it causing the coffee at the centre to accelerate and rotate faster around the centre of the cup*. The faster rotation will increase the centripetal force and so the central depression will become a bit more obvious again. This is the prediction anyway. So far, using chilled water and food dye, I have not been able to convince myself of the effect. But perhaps you will have more luck. Do let me know in the comments or over on social media, what results you get with this.

vortices in coffee
Vortices behind a spoon dragged through coffee. Experimental physics is a great excuse for playing with coffee.

Returning to the just stirred coffee, there may be one more thing to notice. At the interface between two moving fluids, a turbulent layer can form. We can see this when we first put a coffee on a record player (link here), or with the appearance of certain clouds (link here). This leads to a suggestion. As the coffee will be rotating faster at the centre of the cup than at the edge (owing to the resistance of the mug walls), the turbulence in the air over the centre of the cup will be greater than that at the sides. Fast moving fluids flow at lower pressure than slow moving fluids (Bernoulli’s equation). And although strictly speaking this is only valid for non-turbulent air flow, the principle can explain how planes fly and it may also have a consequence for our coffee.

As the air above the coffee at the centre of the mug will be moving faster than the air outside the mug, the air above the centre should be at an ever so slightly lower air pressure than that outside the mug. We know that water evaporates more quickly at lower atmospheric pressure. Consequently, more coffee aromatics will be evaporating from the centre of a just-stirred cup of coffee than from one you have left to sit still for a similar amount to time. To phrase this in a slightly different way, stirring your coffee should make it more aromatic and fragrant.

There are of course questions. Would the air pressure really decrease so significantly to affect the evaporation rate? How do you account for the fact that stirring coffee cools it relative to a coffee that is left to sit and wait? (Though why stirring a coffee should cool it is a whole other conversation). Nonetheless, it would appear to be a perfect excuse to brew and enjoy more coffee. Inhale deeply, stir contemplatively and, perhaps, add a little milk.

*In “Vortex flow in nature and technology”, HJ Lugt, John Wiley and Sons, 1983

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The Dark Arts at Amar, Chelsea Green

Amar Cafe, Drinking coffee on Chelsea Green, Colombian Coffee
Amar Cafe on Chelsea Green. The small terrace area outside is on the spot of two car parking spaces.

Amar Cafe means to love coffee. Though this is in Spanish. In Bengali it apparently means “my cafe”. This could perhaps lead to a short meander onto a language inspired thinking trail about how a cafe that you love to frequent becomes “yours” in a certain sense. Though to return to the coffee, the cafe itself is a small space located on Chelsea Green. There are other branches of Amar cafe in telephone boxes around London and in Stratford upon Avon. A couple of parking spots just outside the cafe have been converted to an outside terrace complete with small olive trees at which you can enjoy your coffee in the fresh air of the Green. Although this is clearly a Covid-19 related temporary measure, it would be good if some of these outside places can remain on a more permanent basis. They do add to the character of the Green. Amar cafe specialises in Colombian coffees that they source themselves. All of the usual espresso based drinks are available as well as the option of a pour over. A small selection of pastries including empanadas are available for breakfast. There are about four tables outside and a couple of tables (along with a window bench) inside. As you enter the cafe, the bar is immediately in front of you. Outside, the cafe is painted a bright yellow colour which makes it stand out among the independent shops that are in this little quarter of Chelsea.

On the two occasions that we have visited, I have enjoyed a really well made pour over each time. Although I am not good at generating my own tasting notes, I would say that the coffee was sweet and syrupy, with a fruitiness and complexity that was very enjoyable. It was presented in a V60 jug together with a black, handle-less, porcelain cup.

V60 at Amar Cafe, Chelsea
Carafe of coffee and cup. The blackness of the coffee is similar to the blackness of the cup. On the carafe, condensation has formed on one side only. How did that happen?

Gazing into the filter coffee, there were patterns on the surface of the coffee that you could see reflected at different angles. But looking more deeply, it looked black, within a black porcelain cup. Where did the cup end and the coffee begin? How could you see black on black? Which was blacker?

A material appears black to us when it absorbs the majority of the visible light incident on it and doesn’t reflect or emit any visible light back. Until 2019, the world’s blackest material was “Ventablack”, but even this material only absorbed 99.96% of the light shining on it. Late in 2019, a new material was discovered that absorbs 99.995% of light shone onto it. And not just that, it absorbs 99.995% of light from the ultraviolet to the THz (between infrared and microwave). The material is truly black. But the inventors of this material were not trying to make a black material, they weren’t even really interested in optics. The invention came courtesy of a collaboration with an artist.

At the time, Diemet Strebe was the artist in residence at MIT. The scientists at MIT who would go on to discover this new black material, were interested in the electrical properties of carbon nano tubes (CNTs). CNTs are a layer of carbon atoms (arranged in the hexagonal structure familiar for layers of graphite) wrapped into a tube. Each tube may be just a few nanometers diameter but they can grow hundreds of micrometers long. Depending on how they are wound into a tube, CNTs can have a very low electrical resistance. But this electrical advantage is lost when you try to attach them to a metal like aluminium because the surface of aluminium always oxidises. Unlike aluminium, aluminium oxide is a brilliant electrical insulator. Which is great if you want to study effects in which the electrical current is blocked, but terrible if you want to utilise these fantastically conducting CNTs. This was the problem that the scientists at MIT wanted to solve. Their solution was to remove the oxide using salt water and then deposit the CNTs on top. (When phrased like this it sounds such a simple idea, “why did no one do this before”, but there are many experimental steps needed to be able to grow CNTs onto substrates such as aluminium and it has taken many years to get to this point). Once the CNTs were deposited, the authors found that not only did they have a good electrical conductor, the material was really black.

Coffee love. Some evidence of foam ripening at the surface of an oat milk latte.

What happened next is where the art comes in. Professor Brian Wardle who led the study was quoted as saying “Our group does not usually focus on optical properties of materials, but this work was going on at the same time as our art-science collaborations with Diemet. So art influenced science in this case.”

Thinking about how black the material was, the team decided to measure its optical absorption, which is when they discovered that they had broken the record previously held by Ventablack. And it was then that the art came back in. Strebe took a $2m natural yellow diamond and covered it with this ultra-black material. The result is striking (link). The composition, called “The Redemption of Vanity” could cause us to pause and ponder what we value as a society. What makes a sparkling diamond so valuable? How do we start to see objects by the fact that we can’t see them at all? If we extend this contemplation to our surroundings of Chelsea Green, we may wonder at this small little triangle of grass with its couple of benches. What does it reveal or hide? Does it help us to know that this is the last remnant of Chelsea Heath*, a bit of common land in which occupants of the surrounding manor houses down on (what is now) Cheyne Walk had the right to graze their livestock. Throughout this green, cows wandered up from the Thames as recently as the seventeenth century. A part of London that has disappeared, obscured by modern buildings yet held in memory by the street names and, the names of housing blocks.

As for why this material absorbs so much light, it is still an open question. It is known that the arrangement of the CNTs (including their alignment) can make a material coated in them very black. Even Ventablack is made from CNTs. It is a question that will probably continue to be discussed over many coffees. But is it a question that we would be asking again at all if it weren’t for the interaction in this case of art and science? Another point of contemplation that we can enjoy while looking into our coffee and just wondering ‘why’?

Amar Cafe is on Chelsea Green, 15 Cale Street, London, SW3 3QS and at several other telephone box locations.

*London Encyclopaedia, 3rd edition, Weinreb, Hibbert, Keay and Keay

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Under pressure

What do you notice about this iced latte? The cup is rich with physics, but for this post, the important bit is the floating ice on top.

A coffee should be a time for relaxation, for reflection. As we come to the end of summer here in the northern hemisphere, we may want to enjoy one last iced coffee before we return to the warming coffees of winter. If on the other hand you are reading this from the southern hemisphere, the equatorial region, or some time after it was originally posted, you may be just starting to enjoy your iced coffees again. Either way, ice is remarkable and it is good to make some time to enjoy it. One of the things that makes it remarkable is what seems to be its very ordinariness: it floats.

Ice floats because the solid form of water is less dense than the liquid form. This is actually fairly unique to water. Most liquids get more dense as you cool them. As they transform into solids, they get denser still. This would mean that if you were to cool a liquid until it starts to solidify, the solid would sink, not float, on the liquid. If water were like most other liquids, all the ice in our iced-coffee would be at the bottom, not jiggling at the top. In addition to what would be an almost aesthetic problem for the coffee, this has consequences for life itself. When a lake or a pond freezes over, the fish and other aquatic life, can survive under the ice in the denser water. This odd property of water has helped life to evolve.

The reason for this strange behaviour lies in the way that water molecules bond together. Each water molecule can bond to a neighbour through a hydrogen bond. This optimises the structure to a layered form of well spaced hexagons (link here for an interactive model of water ice). Each corner of the hexagon is an oxygen atom. The size of the hexagon means that, if they weren’t arranged into a regular lattice, the water molecules could get closer together than they do in the solid phase. Which is another way of saying that the liquid can get more dense than the solid. Ice will float on water.

The layered structure of the ice crystals also means that each hexagonal face will tend to glide over the one below it or above it. It is this property of ice that means that we can determine the direction of glacial flow in centuries past. When fresh snow falls on top of a glacier, the density of the snow layer is about 50-70 Kg/m3. For comparison, the density of water is 1000 Kg/m3. Although each snow crystal is hexagonal, they have random orientation as they fall. As new snow falls, it pushes down on the old snow and compacts it until, about 80m down into the glacier, the density of the (now) ice is 830 Kg/m3. As the depth increases still further, the density increases to 917 Kg/m3 which is as dense as a glacier can be but is still much less than the density of water; a glacier would float. When the snow crystals are pressed down, the hexagonal layers of ice will glide past each other in the direction of push and the crystals will re-orienate. They will also grow as they merge with other crystals and as a result of the heat from the bedrock beneath them. This means that deep in the glacier, more of the crystals will be orientated in the direction of the push. Taking a vertical core of ice and looking at the orientation of the crystals in 0.5mm thick cross sections therefore reveals how they have been pushed as a function of depth. This in turn reveals which way the glacier has flowed in the past.

Sun-dog, Sun dog
A ‘rainbow’ of colour as seen in a ‘sun dog’ observed in central London. Note the order of the colours.

The structure of ice has one other surprise for those of us who are enjoying more coffee outside. Depending on the weather conditions, high up in the atmosphere, hexagonal ice crystals form. Because they are hexagons, they are, in effect, a section of a 60 degree prism. This means that light entering through one face, will be refracted twice to emerge from the crystal at 22 degrees relative to where it came from. If there are enough of these crystals high in the atmosphere, a bright circle will form around the Sun. For reasons that are probably obvious, it is known as the 22 degree halo. It seems fairly difficult to observe this halo. What is far more common to see are two bright regions of light at the 9 o’clock and 3 o’clock positions on the halo. In addition to being brighter than the rest of the light circle, these two regions often appear like a ‘rainbow’, but with the red on the inside of the halo and the blue on the outer edge. Known as “sun dogs” or parhelia, they too are a consequence of the ice crystals. As the ice crystals fall, they are more likely to fall flat so that each hexagonal face is horizontal. More of these ice crystals means that there is going to be more light refracted at the position horizontal to the Sun and so the light there is intense. They appear as separated colours for the same reason that the colours disperse with a prism: each wavelength of light has a very slightly different refractive index and so gets ‘bent’ by a slightly different amount. The ice crystals are bending the red a bit less than the blue.

This is a good time of year to keep an eye out for Sun dogs and haloes. And if we can do so while enjoying a well made iced coffee with the ice cubes floating at the surface, all the better.

Please do share any photographs you have of coffee with 22 degree haloes or sun dogs, either here or on Facebook or Twitter.