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.

Drawing a Blank?

Blank Street Coffee on the corner of Charlotte Street and Goodge Street.

In the summer of 2020, a new neighbourhood cafe started up in New York. Now, at the end of summer 2022, Blank Street coffee has over forty locations in the US with two in the UK and the ambition of 24 in the UK by the end of 2022. This is not your usual small coffee shop. Nor is its financing. After initially raising $7m from venture capitalist firms, a recent fundraising round raised a further $25m. It is worth asking: why? What is it about this chain of cafes that makes financiers value it at more than $98m?

I came across Blank St Coffee at 44 Charlotte St on the junction of Googe St by accident. After a good Penang Assam Laksa down the road at Laksa Mania, we were after a coffee and this cafe looked, at first sight, like a small neighbourhood cafe. Judging from headlines in the New York Times, this post is going to date very quickly and conceivably, in just a couple of months we are going to wonder how it was that we had never heard of Blank St. As we got closer, the merchandise visible through the window, and other aspects of the coffee shop which are harder to pin point, suggested that this was a coffee shop backed by quite a lot of finance. There are a few such cafes around and it is not always easy to discern what it is, exactly, that indicates that they are far from my usual focus on neighbourhood cafes. However, on this occasion, we were after a coffee and thought we’d give it a try.

We ordered an Americano and an oat milk hot chocolate. The coffee is roasted by Origin Roasters and all the coffee is served in disposable cups. As we were drinking ‘in’ (on the benches outside), I refused a lid for the cup which was a mistake as the Americano had been filled to the brim of the cup. Shortly after taking the coffee from the counter and stepping outside I found, experimentally, how easy it is to spill a black coffee (as opposed to say, an oat milk hot chocolate). The over-filled Americano turned out to be an interesting feature because I had naively attributed it to human error. As with much else at Blank Street, such impressions can be deceptive.

Over-filled Americano and oat milk hot chocolate with a lid. You can see which one spilled by the liquid around the bottom of the cup.

I’ll declare a bias here. I think society works best with human interactions and community. It is why I have focussed on reviewing small, locally run, community cafes in the past. Blank Street Coffee has a different ethos which is to make good coffee affordable. While this is not necessarily a bad aim (click here for a discussion on the pressures and ethics involved in coffee pricing in cafes), Blank Street has a particular approach to cost cutting. Firstly it rents smaller spaces for its cafes. It also automates much of the coffee preparation. The baristas no longer have to make the coffee, they just push a button and the coffee comes out of the machine. This makes the over-filled Americano odd because it is an automated, not a human process, have they really designed the Americano to fill to the brim?

Blank Street Coffee explains that the fact that the baristas just have to ‘push a button’ for the coffee means that they have more time available to chat with customers. This does not make sense to me. I like knowing that the barista knows coffee and knows (and cares) how to make a good coffee. I like the fact that the barista knows more about coffee than I do and so can talk to me about different coffees and issues within the coffee chain. I do not see that the baristas can have the same in depth understanding of coffee if they are only required to push a button to make it. Nor do I think that this will reliably produce a good coffee as the coffee dosing needs to be adjusted throughout the day by experienced baristas in order to keep the espresso flavour consistent. There is a similar problem in experimental physics. In order to get more results in a given time frame (such as overnight), many pieces of equipment are now automated. This starts off as a great idea but has the result that the experimenters lose familiarity with the electronics behind the computer interface. It is hard to troubleshoot when something goes wrong if you don’t have the feeling for what different bits of the equipment do to begin with. On a practical point with the cafes, baristas are needed now because they are expected. If the aim is to provide good coffee cheaply, what is to stop getting rid of the barista entirely and allowing the customer to press the button?

Opposite Blank Street a properly blank sign. What used to be written on this sign?

The coffee itself was ok. It was nice to get a drinkable cup of coffee in a space in central London where you could sit on a bench and people watch. And in terms of the physics aspect of a cafe-physics review, there was also an appropriate point to consider. Just opposite the cafe there was a space on the wall of the shops for a shop sign. It was a type of nineteenth century panel built into the shop fronts which would in the past have been painted with the name of the shop below. Only this one was, fittingly, blank. Not really a ghost sign, it was so ghostly as to have disappeared altogether, it was a blank space. The presence of the sign was announced by its absence. A similar absence is revealing in space; “in space, no one can hear you scream.” Does this suggest that space is a vacuum? For sound waves to propagate, and so for your scream to be audible, the sound needs to create a pressure wave within a substance, whether that substance is a solid or a gas. If a sound wave cannot propagate and we take the movie-tagline literally, it would mean there is no substance in space, it is a vacuum. Depending on where you measure it, this is nearly true. In interstellar space, there is approximately 1 atom per cubic centimetre compared with 3×10^19 atoms per cubic centimetre on Earth’s surface. In intergalactic space there are even fewer atoms in a given volume. Even in interstellar space though, there are small fluctuations in the density of atoms with some regions having what appears to be a bunching up of the atoms present into waves as the shock fronts of things like distant supernovae come through. The spacecraft Voyager 1, launched in 1977, crossed the boundary into interstellar space in 2012. Voyager continues to take measurements of what it encounters and is now being used to understand the density of interstellar space, partly by measuring these bunched up bits as they flow over the spacecraft.

Voyager measures the density of space, partly by revealing the very absence of measurements for the most part. Which brings us poetically back to the name: “Blank” Street coffee. It is announcing something by its very anonymity. This anonymity is continued even inside the cafe. Painted a shade of green which is fairly instagrammable but somehow generic, a copy of other Blank Streets elsewhere. The space offers plenty to think about: what does it mean to be empty? Do we value something purely by its economic cost? And what does it mean to be anonymous, or even a unique individual, if you order coffee using an App on your phone?

Coffee at Blank Street was an experience. It can prompt many reflections on society and on physics. Yet, there are issues in its apparent anonymity and generic layout. Two weeks after visiting Blank Street, I visited a small, local cafe where the community and the physics jumped out at me. Offered the choice of many generic ‘blanks’ or a few memorable cafes, which would you choose?

Blank Street Cafe can probably, by now, be found all over London but was reviewed at 44 Charlotte Street.

Cracking Magnets

Rare earth magnets are very strong despite their size. These magnets are several times stronger than an ordinary fridge magnet.

Can you hear it? The first, second and then third and fourth cracks as a magnet is brought near a magnetic (but not magnetised) material, such as a piece of cutlery? Unlike the first and second cracks during coffee roasting, which are clearly audible, it is unlikely that you would have actually heard the cracks of a magnet. To hear them you would need to amplify the effect and connect it to a loudspeaker (there’s a link to how you can do this experiment here). Nonetheless, if you were to do so, you would hear the cutlery cracking. And while these sounds are not connected to the first and second cracks in coffee roasting, they are connected, via physics, to coffee. To see why we need to think a bit more about what is causing these magnetic creaking noises.

The effect is known as the Barkhausen effect after Heinrich Barkhausen who discovered it in 1919. It turns out the the effect reveals quite a lot about how magnets work because it reveals what is going on at an atomic level in the kitchen fork. Some metals are attracted to magnets but not others. So a fridge magnet would stick onto materials containing iron but would not stick to a sheet of aluminium; we can pick up pins, paper clips and some cutlery with a strong magnet but we could not pick up a piece of kitchen foil. These iron containing metals are magnetic but not magnetised, they will be attracted to a magnet but they will not ordinarily attract other items to themselves. We may remember from school that we can make them magnetised by continuously stroking a strong magnet along the length of the pin (or fork, or paper clip) until the pin itself is able to attract other pins to it. We may even remember the explanation for this which was that for something to be magnetised, it had to have a clear magnetic orientation of North-South throughout its structure. Within the pin (or fork or paper clip) there are many small regions, called domains, which within themselves have a north-south orientation but they do not all point in the same way throughout the fork. Each little region points in a different direction to the others and so the net effect is that there is no overall North-South magnetism in the fork as a whole. As the strong magnet is used to stroke the fork, so the small regions move to align to the direction of the stroke of the magnet. The regions stop cancelling each other out and align so that the fork itself becomes a magnet with its own North-South.

inverted Aeropress and coffee stain
The link between coffee and the Barkhausen effect in magnets can be seen in this photo: a coffee spillage. It is the way that coffee evaporates and that coffee stains form that forms this physics connection between coffee and magnetism.

To return to our un-magnetised fork, you can imagine that where all these domains meet, there will be an area of confusion where the direction changes from one orientation to that of the neighbouring domain. This is called a ‘domain wall’ and it is these domain walls that are responsible for the Barkhausen effect. You can feel the effects of domains and domain walls in this experiment taken from the Institute of Physics Spark series: take two flat fridge magnets and turn them over so that the magnetic side of each faces the other. Move one of the magnets along the length of the other one. Think about how it feels to move it. Now move the same magnet perpendicular to the direction that you initially moved it in. Try it again. You will find that in one direction the movement feels smooth whereas in the other the magnets judder against each other, the movement is not smooth at all. You are feeling the effects of moving across a series of domains and domain walls, you can read more about the experiment here.

What actually happens as you bring a strong magnet towards an object such as a fork is that those domains in the fork that are aligned in the same direction as the magnet will tend to grow slightly at the expense of the ones that are not aligned with the magnet. The initial growth happens as the aligned domains get a bit bigger, a bit rounder and fatter. The domain walls bend a bit and the domains of the non-aligned regions get a bit thinner, a bit more squished. As the magnet is brought closer still, the aligned domains will actually start to grow at the expense of the non-aligned: the domain walls of the aligned domains will start to move outwards ‘eating’ into the neighbouring regions. It is at this point that you can pick up the Barkhausen effect because as the domain walls move, they can get stuck on defects in the metal rather like an elastic band would get stuck on an obstacle. The defect could be just one or two atoms that are out of place but the effect is that, just like the elastic band, the wall around the obstacle continues growing and the domain wall stretches more like an elastic band until pop – crack – the wall moves releasing a bit of energy that you pick up on the loudspeakers. This is what you hear as the Barkhausen effect. As the walls continue to grow so they will repeatedly get snagged on different defects in the metal and repeatedly ping – crack – into growth. Eventually, as the fork itself becomes magnetic* the last few non-aligned domains also start to align with the approaching strong magnet and the whole fork acts as if it is one magnet.

coffee ring, ink jet printing, organic electronics
A coffee stain. There are many experiments you can do at home with these.

The pinging domain walls have a direct link with an effect you can see in coffee, or more specifically spilled coffee. When you spill a few drops of coffee on a movable surface, you may have noticed that you can angle the surface a surprising amount before the drop starts to run down the side. You could try it now on a coaster if you have one available to you. The drop does not move because the edge is stuck, ‘pinned’, on defects on the surface of the coaster. These defects could be a crack in the material or a bit of dust or even a slight irregularity on the surface. Whatever it is, this defect acts to keep the edges of the drop in place. The first effect you would notice is that you can move the drop to a near vertical without it moving, the drop shape gets distorted but the drop itself does not move. The second effect is more subtle and is what happens if you leave the coffee drop there to dry.

Once spilled, the water in the droplet starts evaporating and eventually the droplet will dry leaving a coffee stain. The consequence of the pinning that you have just noticed is that the edges of the drop are quite stuck: the drop can’t just shrink. Instead, as the water evaporates, the drop will get flatter and because the water evaporates more quickly from the droplet edge (to see why click here), there will be a flow of water inside the drop from the centre to the edges. As the water flows outwards so it takes the coffee sediment with it which means that the dried coffee becomes a ring of sediment at the edge of the dried droplet.

Although it is on a different scale, it is the same sort of pinning that is happening in the coffee ring and in the Barkhausen effect. There are connections between physics and coffee to be found in many surprising places. Where will you find one today?

*This is an instance in which scientific English is not the same as English-English. In scientific-English, the fork is always a magnetic material it is just not fully magnetised. In English-English we tend to use the word ‘magnetic’ only for those materials that attract iron etc. to them. For ease of reading I have kept with the English-English usage here but if you are interested, you can read more in these links about magnetism and magnetic materials.

Coffee: emissions or waste?

Plastic free coffee? Is it possible? Is it desirable?

Plastic free July is nearly with us once again, prompting us to look again at the sustainability (or not) of the coffee we enjoy each day. Whether we drink our coffee at home or in a cafe, it is easy to see plastic items associated with the coffee. In a cafe there are the take-away, disposable, cups, together with the cartons of milk and perhaps, plastic cutlery. Even at home, we see the plastic packaging in the coffee bags that we buy or receive through the post and perhaps with the milk (or mylk) cartons that we ourselves are using. Some have argued that given that we are in a climate emergency, at this point we need to focus on the most urgent problem of all, the reduction of greenhouse gases. These are valid concerns and it is certainly true that the emissions costs associated with packaging and transporting items using low mass containers like plastic are lower than if the same product were transported in heavier, but more obviously sustainable, materials such as glass. Should this stop us from trying to reduce our plastic consumption and what hope is there that new technology can help us? These are questions we may like to ponder with a cup of coffee.

What is the problem for those of us who mostly brew and drink coffee at home? Here, the major sources of plastic packaging are likely to be in the bags that the coffee arrives in and in the packaging of the milk that we may use to accompany our coffee. Many speciality roasted coffee beans now arrive in LDPE plastic packaging. This is recyclable although most household collection services will not be able to process it. Fortunately there are places that you can take your coffee bags for recycling such as supermarket plastic bag recycling points. Alternatively, some coffee suppliers or roasters (eg. Dog & Hat) will offer to arrange recycling for you via Terracycle if you return the empty bags to them.

If you decide to buy your coffee in compostable packaging, look out for the “OK compost, home” label by Vincotte

Another popular packaging material for coffee bags is “compostable” packaging. This falls into two types, the compostable according to EN13432 and packaging that is fully home compostable (occasionally certified with the Vincotte OK Compost label). You can read more about the relative compostability of each type of packaging here, but it is worth noting that the majority of us do not have access to an industrial composting facility, nor can these bags be recycled with other plastic. Packaging that is only certified to EN13432 will not compost easily in a home composting environment. Be very careful when buying “compostable” packaging. Less commonly, coffee could arrive in cans, paper or even (in the case of supermarket instant coffee) glass packaging. These can be easily recycled but may have higher carbon emissions costs associated with them, both in terms of the cost of transport and in terms of the cost of manufacture of the packaging.

Milk cartons are often recyclable or even, in the case of glass milk bottles, reusable and then recyclable. Those of us using non-dairy ‘mylks’ could try to eliminate the plastic altogether by buying oats or soy beans from a zero waste shop and then making our own according to recipes you can find online.

So it seems that we have a problem: we still have a lot of waste material when we drink coffee. Can technology help us? Recently, a new form of plant based ‘plastic’ material has been demonstrated. This material is able to be formed and used in a similar way to commercial plastics and may be suitable for food packaging. Not only that, but the material is fully degradable, though more details are needed here before we can know whether this is degradable as we would understand it or degradable under certain conditions similar to the ‘industrially compostable’ packaging. Nonetheless it is highly encouraging that work is being done on materials that will provide the low carbon footprint and food-safety aspect of plastic combined with the reduced problems of landfill waste. While still a few years off, such technology may provide a longer term solution to our current waste/emissions problem.

There is a good (physics based) reason that dairy milk is often supplied in semi-opaque bottles: the packaging protects the milk within from UV light that would otherwise cause it to spoil.

Which leaves us with the initial objection. Given the state of our climate emergency with regards to greenhouse gases, should we really be concentrating on reducing greenhouse gas emissions rather than on reducing plastic waste? Fortunately, there are choices that you can make that will reduce the carbon cost of coffee, regardless of the packaging it arrives in. There are some well researched and thoughtful articles available about the carbon footprint of coffee and what you can do, including this series from United Baristas. However, it is worth noting that even if we do decide to prioritise greenhouse gas emissions, recent work has shown that the methane emissions of plastic as it degrades has been previously under appreciated. Perhaps what we can do is make choices that reflect our concern for the planet that we call home. Clearly this means reducing as far as practicable our greenhouse gas emissions, but it also means living as sustainably/zero waste as possible. We each need to find the balance that is suitable for the situation that we each find ourselves in. A significant benefit of Plastic free July is that it gives us an opportunity to examine how much of our lives are made easier, or in some cases possible, by the use of plastic, in short, how much we rely on it. It is not necessarily about changing our lives forever, though maybe we will find some changes good to carry on with, but becoming more aware of the problems that we really have.

Have you signed up for Plastic Free July? What are you doing to reduce the environmental impact you have while still enjoying a cup of coffee? And do you have a good recipe for oat milk? Do let me know either on Twitter, Facebook or in the comments below. In the meanwhile, a happy Plastic Free July to you all.

Worth dying for? A glassy tale.

Pureover, pureover in packaging
The PureOver in its packaging. Glass and cardboard, no plastic in sight. The PureOver is designed to brew filter coffee but without the need for filters.

It was the middle of the afternoon and we had friends over, friends who wanted coffee but, “only a small cup”. What were our options? We could make a V60 which would be a bit of a waste or an Aeropress which, while great for a small coffee for one person, is pushing it a bit for two people (even if one only did want a “tiny” bit). It was time to dust off the PureOver. This all-glass brewing device makes approximately 2-300ml of filter coffee entirely without the need for any filters. It is my go-to brewing device for a decent sized cup of coffee for one person or a “small” and “tiny” cup for two people. The PureOver was designed by a group of glass-blowers in Portland (USA) who wanted to be able to brew drip coffee without waste filters. It is now made commercially in China and shipped around the world for people who want to brew likewise.

The PureOver works by creating a filter bed out of the coffee grounds themselves. The design of the brewer ensures that the coffee is fairly well packed at the bottom of the pot allowing the water to filter through but without (much) sediment falling into the cup underneath the brewer. I have written about the PureOver, including a “how-to” brew guide, elsewhere. The PureOver works well, brews a lovely cup of coffee and looks great. Which shows how well the hard bits have been hidden; much of life is an art where the performance hides the work behind it. In some parts of our lives this is obvious. Acting, for example needs to appear natural and not reveal the work that has gone into developing the character the actor plays. I think the same is true of teaching/tutoring* physics. Such teaching should be a seamless conversation and discussion between students and tutor, in some way hiding the work that has gone into the preparation of that conversation. The PureOver is exactly the same. There is a lot of physics that is within the filtration bed and the diffuser design, but the bit that I would like to focus on is the bit that we look straight through without noticing. It is the role of the glass.

The diffuser sitting on top of the Pour Over coffee brewer. The holes are to ensure that the water falls evenly and slowly onto the grounds below.

The PureOver is made of borosilicate glass which was first invented by Otto Schott (1851-1935) in the nineteenth century. It is made by combining silica with boron trioxide (B2O3). One of the things that makes borosilicate glass so special is that it has a really low thermal expansion coefficient. From a practical point of view, and why this matters in the PureOver, is that it means that it is not likely to shatter as you add boiling (or just off the boil) water to the glass. You can brew coffee without the brewer breaking. We just want to be able to use the coffee brewer without thinking too much about it, using borosilicate glass allows us to do this.

If we do think about it a bit more though, the thermal expansion coefficient reveals something to us of the atomic structure of the material. All atoms in a solid vibrate, as they gain more energy (in the form of heat), the amplitude of that vibration increases, so they vibrate more. But atoms within a solid structure do not vibrate symmetrically. It is much harder (it takes more energy) to push them together than it is to pull them apart. This means that as the temperature increases they can vibrate ever so slightly further away from each other than they can towards each other and the net effect is that the atoms get further away from each other and the material expands**. The thermal expansion coefficient can therefore reveal clues as to the internal energies and structure of different solids. Applying this to borosilicate glass itself gets problematic as glass is a disordered rather than a well defined crystalline structure, but the principle is there.

We often come across borosilicate glass in “Pyrex” glassware, although since the 1930s/40s “pyrex” has been made of soda-lime glass rather than the original borosilicate. Nonetheless, it is a story involving pyrex that provides the title of this post. In 1953, a chemist working at Corning Glass Works in New York State, got a surprise as he dropped a piece of experimental glass he had been working on when he removed it from the furnace. Donald Stookey had serendipitously discovered “Pyroceram” a type of glass that was not only extremely heat resistant, it had also bounced, not smashed, when he dropped it. However despite being commercialised for other specialist products, Pyroceram was not, initially, used for kitchen items because the parent company Corning, also sold Pyrex and did not want any competition with that other successful product. So more research was done on Pyroceram which did lead to new commercial opportunities, including one that we probably have with us right now. Because the toughness aspect of the Pyroceram type glasses developed into what we now know as “Gorilla Glass” which is probably the screen on your smartphone.

Perhaps not quite how the designers imagined brewing a coffee. I brew the PureOver into my V60 jug in order to avoid the few grains of coffee that get through the filter from going into the final mug of coffee.

You can read more about the story of this discovery (and how it got used in the Apple iPhones) in the June 2022 issue of Physics World. Stookey went on to be awarded the US National Medal of Technology by President Reagan and of course, Gorilla Glass is now found in many products. So you would be forgiven for thinking that this marvel of technology is a recent phenomenon as an unbreakable glass would surely have been highly valued if it had been invented earlier. The story of a Roman craftsman may provide a contrasting pause for thought. As described by Petronius (quoted in the book “The Alchemy of Glass”***):

There was once a workman who made a glass cup that was un-breakable. So he was given an audience of the Emperor with his invention; he made Caesar give it back to him and then threw it on the floor. Caesar was as frightened as he could be. But the man picked up his cup from the ground: it was dented like a bronze bowl; then he took a little hammer out of his pocket and made the cup quite sound again without any trouble. After doing this he thought he had himself seated on the throne of Jupiter especially when Caesar said to him: ‘Does anyone else know how to blow glass like this?’ Just see what happened. He said not, and then Caesar had him beheaded. Why? Because if his invention were generally known we should treat gold as dirt.

*I am careful to keep my comment here to tutoring as that is what I have most experience of. If you teach larger groups or in a school, please do let me know what you think, whether this applies to teaching too, in the comments.

**See for example “Thermal Physics, CJ Adkins, Hodder and Stoughton, 1976

*** “The Alchemy of Glass; counterfeit, imitation, and transmutation in Ancient Glassmaking”, Marco Beretta, Watson Publishing International, 2009

For more about glass including the question of how transparent glass is, please see this post by Bobreflected.

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.

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)

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.

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.

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.

1 2 3 30