Author: beanthinking

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General Observations Science history Sustainability/environmental

Reflections, deviations…. coffee

The reflections from the surface of a cup of coffee of a building opposite a central London cafe. Towards the edges of the cup, the coffee bends upwards, revealed by the lines bending that would be expected to be straight.

A “flat white” could be ordered from many a coffee shop. A “flat black” may be a physical impossibility. We can realise this by gazing contemplatively, or perhaps even longingly, at a long black while it cools. Notice that the surface of the coffee is ever so slightly curved. Leaving aside the white mists that you may see skipping across the coffee surface, the coffee is flat in the middle of the cup but rises towards the edges. If you have noticed this, it is most probable that you did so because of the different way the light is reflected over the surface of the coffee. It is most obvious if you can arrange the reflections on the cup to reflect something supposedly straight: a window frame or a beam of strip light for example. The reflection is fairly clear and fairly straight until about 5mm from the edge of the cup where suddenly it bends. You can see an example of this in the photograph on the right.

The reason for the curvature is of course surface tension, which is the same effect that makes droplets form into shapes that are close to spheres. First investigated by Agnes Pockels and Lord Rayleigh in the nineteenth century, surface tension is caused by the fact that molecules at the surface of the water (in the coffee) will feel a net attraction to the other molecules within the water. There being no molecules of water above the surface of the cup, the surface molecules are pulled back towards the liquid in the cup. At the sides of the cup something slightly different is happening. There, the molecules in the water will be pulled back towards the liquid but will also experience the uncompensated attraction (or repulsion) from the atoms in the mug material. Exactly analogous to surface tension, but in the solid, the interaction of the surface energy of the mug with the surface tension of the liquid will pull the liquid into different shapes. It is for this reason that highly waterproof surfaces, such as fresh oak leaves, will form spherical drops of water, but wettable surfaces, such as an oak leaf in autumn, will accumulate flatter, less spherical droplets on the surface.

coffee, red wine, wet coffee stain, coffee spill, coffee ring
The interaction between the surface tension of the water and the surface energy of the solid surface it sits on determines the shape of the droplet. These drops of coffee and wine on paper were for an experiment about coffee ring formation. The droplets are: Drops of coffee (left), soapy coffee (middle) and red wine (right)

We see the effects of surface tension too when a bubble, or a small bit of dust, sits on the surface of the coffee. Again, looking at the light reflections, we see how the coffee, or tea, bends near the floating object showing how un-flat the surface really is. Bubbles are usually large enough that we can see them directly. In the photograph on this page for example, you can clearly see the reflections from the surface of the bubble together with the bent reflections of light from the surface of the liquid. However in the case of the dust, sometimes the dust is small enough that the reason that we see it is because of the change of the path of the light reflected from the surface. For a similar reason, the insects that skate the surface of a pond are visible because of the light patterns they make rather than their intrinsic visibility. Each time we are using the deviation of the light from its expected path in order to deduce the presence and shape of an object hidden to our view.

A similar deviation of the expected path of light is seen in the phenomenon of gravitational lensing which has been used to infer the presence of black holes. Such a deviation even provided experimental evidence for Einstein’s (then) recently proposed General Theory of Relativity, just over 100 years ago on May 29, 1919. The idea that light had weight and would be deflected by a gravitational field was not new, indeed, even the Newtonian model of gravity predicted that light would be deflected as it went past a massive object*. The question was how much and, as an important secondary question, how to measure it. As Arthur Eddington later described in his book “Space, Time and Gravitation”*, according to Newton, any object thrown horizontally on the Earth’s surface would fall 16 feet (in his use of units, 4.88 m in SI) in one second. The same was true for light. However with Einstein’s theory, the predicted deflection of light was 32′ (9.75m). The difficulty for the experimentalist is that in the same second, the light would have travelled nearly 300 000 km. Detecting such a small deflection over such a large distance would be difficult, harder than seeing a grain of dust on the coffee surface. Which is where the light deflection comes in. Because if you watch as the light from a distant star travels past a massive and fairly large object, such as the Sun, you should be able to discern the small, but significant deflection. And on May 29th 1919 a total solar eclipse (which thereby blocked the extra and interfering light from the Sun) offered a perfect opportunity for Eddington and an expedition sent by the Royal Society and Royal Astronomical Society (to Brazil and West Africa) to attempt to measure such a deflection.

tea reflections, bubble on tea, surface tension, light bending
The way that light reflects off a surface of a cup of tea in this case, reveals the curvature of the tea surface. In this case the curvature is clearly due to the bubble in the centre. Sometimes you can see distortions on the surface caused by bits of dust which are difficult to see on their own.

Although the deflection was significant, working with large telescopes and photographic plates, the magnitude of the deflection of the light that they were looking for was still only 1/1500 of an inch on the photographic plate. Two groups at two different locations took multiple photographs of the eclipsed Sun and the stars around it in order to measure the position of the stars as seen behind the Sun and then compare that to the position of the stars when they had been photographed earlier in the year without the Sun between them and the Earth. Eddington describes the experiment:

“There is a marvellous spectacle above, and, as the photographs afterwards revealed, a wonderful prominence-flame is poised a hundred thousand miles above the surface of the sun. We have no time to snatch a glance at it. We are conscious only of the weird half-light of the landscape and the hush of nature, broken by the calls of the observers, and beat of the metronome ticking out the 302 seconds of totality.”

Finally after developing and comparing the images back in London, the team confirmed a deflection of 1″.98 +/- 0″.12 (Brazil) and 1″.61 +/- 0″.30 (W. Africa) for the stars closest to the Sun (NB. 1″ indicates 1 second of arc). Einstein’s theory had predicted a deflection of 1″.74, Newton’s theory had predicted 0″.87. The results of the light deflection were far more in agreement with Einstein’s new theory of General Relativity than with the classical Newtonian model.

The ‘wobble’ of a few of the stars on the photographic plates had confirmed a prediction of the theory of Relativity. Which could lead to the question: What do you see, or not, as the light dances off of your coffee?

*”Space, Time and Gravitation: an outline of the General Theory of Relativity”, Sir Arthur Eddington, Cambridge University Press, first printed 1920, 1968 edition.

Categories
Coffee review General Observations Science history slow Sustainability/environmental

Me time at Hétam

Iced chocolate at Hetam. The chocolate is sourced from Indonesia. At the time of visiting, drinks were only available in take-away cups, hopefully this will change as the cafe becomes more established and the pandemic restrictions that were in place at the time of visiting are eased.

In 2021, a new cafe opened up in Bangsar, Kuala Lumpur, Malaysia. Called Hetam, it is a cafe almost designed for the post-pandemic, Instagram age that we find ourselves living in. At the time of visiting, there was no ‘inside’ to this cafe, everything was outdoors: customer seating was outdoors, even the ordering and the counter were outdoors. Umbrellas provided some protection from the downpours as well as the hot sun that you can get in Kuala Lumpur. You order at a counter which is on the right of what looks like it used to be an ordinary house on the service road parallel to Jalan Maarof (between Lorong Maarof 5 and 6). The house is now the headquarters for the online section of Hetam and is where they package up their online sales. There are a small selection of edibles to the right of the cash till but the main focus is on the coffee, tea and chocolate. The coffee is roasted by Hetam. At the time of visiting, the coffee was a choice of either an Indonesian natural or a Brazilian washed coffee and available as any of the usual espresso based drinks. I found that the Indonesian worked better in the espresso but that when brewing with an Aeropress at home, the Brazilian came out on top. Various Japanese Genmaicha and Hojicha teas were available but each time, I focussed on the coffee. The chocolate also is sourced by Hetam mostly from Indonesia and is well worth trying.

The staff at Hetam were very friendly and knowledgable. When we first arrived, they talked us through checking-in using the MySejahtera (Covid-19) app when we didn’t have data on our phones (as of 1 May 2022, hopefully MySejahtera will be something you don’t need to use any more). This led to a conversation on the origins of Hetam and their hopes for the cafe for the future. We ordered a hot long black and an iced chocolate and took a seat in the side/back garden of the house. The space seems almost made for Instagram. Infact, perhaps it was. Carefully arranged bamboo adorns the sides of the garden. White pebbles form the floor while strategically placed bits of tree are scattered throughout the space leading to a certain, specific aesthetic. The first time that we enjoyed a coffee at Hetam, another couple were already there. As we sipped our coffee, the couple split into model and photographer and, with what appeared to be a well practised routine of recognisable Instagram poses, set about photographing each other against different backdrops. In subsequent visits, we enjoyed the place to ourselves.

The counter at Hetam is helpfully under a shelter, the other seats are mostly under umbrellas. You get a glimpse here of the ‘insta-ability’ of the cafe. Random dead logs form a counterpoint feature to the white pebbles of the seating area.

The name “Instagram” is apparently a derivation of a combination of “instant camera” and “telegram”. The idea being that a message is sent through an image acquired by an instant camera. The word camera is in turn derived (from both Latin and Greek) from the word for a chamber or a vault. Presumably this was a suitable name for the camera because early photographs were taken through a pin hole into a vaulted dark chamber. Which brings us into the realm of physics as the photograph is literally that which is written by light. Film cameras and even the old Polaroid instant cameras, could still, legitimately be said to take photographs. The light would fall onto a chemically active film and change it based on the exposure levels so that the image was written directly by the light. When it was developed, the negative would be the reverse of the places on the film ‘written’ by the photons of the light (for a description of the process and a recipe for developing film with coffee click here, opens as pdf). This is not true of the sort of “instant cameras” most would now use to upload an Instagram post. In the case of digital cameras, the photons of the light still activate a light sensitive electronic chip behind the camera lens, but much of the interpretation of the image is done using computer software. For example, many of the light sensitive cells in the camera are not colour sensitive, they are only sensitive to the number of photons that fall on them (the intensity of the light). Colour images are formed by considering neighbouring cells which each have a different coloured filter covering them. The relative intensity of the electronic response within each group of cells is then interpreted by the software as a different colour. At this point can it be said that the image is written by the light? The final image is a mixture of the light falling on the photoactive cells and the interpretation of that electrical data by the software in the phone or digital camera. The light directs the electrons within the device but does it write the image?

Table, pebbles and bamboo in the seating area of Hetam, KL.

There’s also the issue of what it means to have the image and to share it. The picture on the phone, the image shared through the screens, is a collection of data points that no one can hold. A photograph printed from film or even the negative is, in that sense, more tangible. In the case of the negative, what you hold is what was written by the photons, by the light, at the point at which the subject was seen. In either case though what does it mean to have, or even to share, that image? Erich Fromm in his book “To have or to be” contrasts a poem of Tennyson with a haiku of Basho*. In the former, Tennyson ‘plucks’ a flower out of a wall in order to study it. Basho in contrast looks “carefully” at the flower; paying attention to it but not possessing it. Fromm questions our mode of being, suggesting that Tennyson could be compared “to the Western scientist who seeks the truth by means of dismembering life.” Is this fair? Does our desire to possess an image, pluck a flower or to ‘capture’ a moment and thereby ‘keep’ it necessarily imply that we would seek truth by means of dismembering life?

Which may take us to a consideration of those dead tree branches on the gleaming stones. They appear like petrified wood, wood that has been preserved for years through a process of fossilisation. We cannot own such objects, they outlast us. If we photograph it we cannot keep that moment, what does it mean to us if we don’t look carefully at the instant but rather try to pluck it for posterity?

So finally back to Hetam. While it may be ideal for Instagram, and while it will definitely be worth a few good photo ‘captures’, the space is also ideal for contemplation. For sitting with a coffee, enjoying the moment, appreciating the surroundings, both aesthetic and people, and for being rather than having. A friendly, outdoor and relaxed cafe, what more could you want?

Hetam is on Jalaan Maarof just next to the Petronas petrol station on the service road to Jalan Maarof.

*”To have or to be” by Erich Fromm, Jonathan Cape publishers, 1976 (1978)

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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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Home experiments Observations slow Tea

Time for tea?

Matcha, tea in Japan, frothy tea
A Matcha tea in Japan. A lot to contemplate here.

A recent article in Caffeine magazine caught my attention. Emilie Holmes of Good and Proper Tea was writing about the joys of appreciating loose leaf tea. While tea is a little diversion from coffee, January is traditionally a time to look forward as well as back and maybe, BeanThinking should occasionally cross over to the tea side. It was one line in particular of that article that puzzled me. Writing about the ‘naturally “slow” nature of the tea ritual’, Holmes observed that while brewing loose leaf tea you would be able to see “the leaves in a glass pot emit wisps of colour as they infuse…”

It was great to read someone who clearly had spent time carefully observing their tea. And yet that sentence prompted a series of questions in my mind. It was not that I doubted the observation, indeed, thinking back to teas I have made and enjoyed, I realise that I have seen these wisps before. It was more a question of why would it happen, why would the brewing tea emit lines of colour from the leaves? These lines must be telling us something.

diffusion, convection, tea brewing
A tea bag in hot water. The lines of tea are difficult to see in the photo, you’ll just have to do your own experiments, but, streaming from the bottom of the bag, you can see wisps of darker tea-water.

We need to think about how tea brews. A first mechanism would be through turbulence. Hot water poured onto a bed of tea leaves would stir them up and the resulting movement within the pot would mix the leaves with the water leading to a properly brewed cup of tea. This is very much the lazy tea brewers bag-and-cup method (which I can share). It would lead to a brewed tea, but it could not lead to a situation in which you could sit back and see wisps of colour. That requires calm and the quiet moments of a pot of tea brewing while you can enjoy the process.

A second mechanism would be through diffusion. Ultimately the same mechanism as the principle behind how LEDs work, diffusion is where the soluble parts of the tea leaves would travel, through the process of a random walk, throughout the water of the pot. This is a very slow process and we would expect that the concentration of colour would be most intense around the leaves and then fade out gradually with distance from the leaves. We would not expect ‘wisps’ nor lines of tea, that suggests something else.

It suggests the third mechanism of the tea brewing: a mix of diffusion and then convection within the hot water of the pot. The lines of tea are indicating that within the cup, regions of the hot water are at slightly different temperatures. Owing to the hot water being in contact with cooler air surrounding it, the surface of the water is cooling down and sinking, leading to a convective motion within the water inside. As the water moves it carries the diffused tea with it into new areas of the water, a movement of hot water to cooler water and back again. The tea is carried in a line because the convection patterns are occurring in small cells within the tea pot, small regions where hot tea is moving towards cooler tea which is warmed and itself moves. The convection does not happen as if the hot water is one big mass but a series of smaller ‘cells’. We see similar cells on the surface of the Sun. The lines are telling us of the movement in the tea pot and the amount and speed of their movement reveals more about how hot the water is relative to the air outside the pot.

diffusion only
A tea bag in cold water: This time, there are no wisps of tea as the drink brews. Instead, there is a slow diffusion of tea infused water from the bag outwards.

Testing this idea I required tea bags. My tea pots are opaque and so would not help me to appreciate this detail of brewing a cup of tea and so it was back to the bag-in-cup method. However, in order to avoid turbulence, I poured the water (hot or cool) into the mug before adding the tea bag. It was not the best way to make a tea, apologies to tea lovers, but it was a tea that I do not enjoy anyway, so it was good to use it up. Sure enough, when the tea bag was put into the hot water, within a very short time, wisps of coloured water formed lines curling underneath the bag. Why did they flow down? Was it because the tea in the bag was slightly cooler than the hot water and so, as the tea diffused out of the leaves it moved with convection downwards because of gravity and the fact that cooler water is denser? A tea bag in cool water however behaved differently. The water in the cup had been taken from the tap and then left in the cup for a couple of hours so that the water was definitely at the same temperature as the room. This time, the tea bag first floated and then sank to the bottom of the cup. There was no obvious infusion of the tea-coloured water into the plain water but slowly the region around the bottom of the tea cup at the bag turned browner with the tea. As time went on, this region expanded to give a tea layer and a water layer.

The wispy lines of tea only happened when using hot water. Which suggests a further experiment. How do these wisps change when brewing for black teas as opposed to green teas (which use a lower brewing water temperature)?

After about five minutes the tea brewed in hot water (left) was fairly evenly distributed throughout the cup whereas the tea brewed in cold water (right) showed a distinct layering between concentrated tea at the bottom of the cup and plain water above that layer.

One last observation with these tea bags in the hot water. Some of the tea floated within the bag, some sank, as time went on, more tea leaves fell towards the bottom of the bag (which was itself floating). What was happening there? Maybe if you experiment with your tea, you can let me know in the comments below, on Twitter or on Facebook. There are definite advantages to slowing down and brewing a proper cup of tea.

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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.