Pour over science

floating, bouncing drops
How do you stabilise droplets of liquid water (or coffee) on a bath of water? And how long can you keep them on the surface?

On the 11th June, 2019 that is, in just under two weeks, we are going to try something exciting. Amoret Coffee in Notting Hill has offered to host the first ever “Bean Thinking’s evening of coffee & science”.

The idea behind what will (hopefully) become a series of evenings is to host a space for discussion and observation, exploring the physics within a coffee cup. On the 11th, we’ll be looking particularly at the phenomenon of “walking” droplets of coffee. These droplets can move across the surface of a cup of coffee and exist for many minutes (even days) if the conditions are right. And the conditions are fairly easy to create: we’ll be creating several such ‘walkers’ in the spacious upstairs area of Amoret’s Notting Hill branch that people can play with.

You can see such drops in your coffee in the morning. But what connects them to an early idea in quantum mechanics?

Although it takes the creation of certain conditions to achieve long-lasting droplets, you can often see them as you prepare a V60 pour over or even when dragging your (single-use) take-away cup over the surface of a table to create resonances on the surface of the cup. They crop up quite frequently, but why are they there at all and why do some last longer than others?

In addition to exploring these questions experimentally, we’ll also be discussing why these droplets sometimes ‘walk’ across the surface of the coffee and how this relates to an early interpretation of the phenomenon of wave-particle duality in quantum physics. How does something that you can sometimes see while brewing your coffee in the morning relate to the idea that fundamental particles such as the electron behave both as particle and wave? And what does this mean anyway?

It is hoped that future evenings will cover other topics such as climate change and coffee stains, I also hope that we will be lucky enough to have some of the coffee farmers that Amoret has direct-trade relationships with in order to explore these connections further. But, that is in the future, this time we are sticking with the fundamentals!

coffee at Watch House
There’s a lot of physics in a coffee. What do you see? Find out more at Amoret Coffee, 11th June 2019 or sign up to our events list.

So, if you are in London on the 11th June and would like to explore some physics in your coffee (or some coffee with your physics), please do come along to Amoret, from 5pm, for an evening of conversation and poring over science. We will be keeping people informed of plans for the evening (and for future evenings) via our events mailing list which you can sign up to here, or you can follow our progress on our Facebook events page here. Meanwhile, it would be helpful for planning reasons if you could let us know if you are coming either by signing up on Facebook or by emailing us. Looking forward to meeting some of you on the 11th.

Squaring the circle at Omotesando Koffee, Fitzrovia

Omotesando Koffee, Fitzrovia
The name “Omotesando” is represented solely by a square on a sign outside the shop. Is this a practical realisation of squaring the circle?

There was a lot of excitement late last year (2018) as the London branch of Omotesando Koffee opened just off Oxford Street. I watched as there were visits by Brian’s Coffee Spot, and Bex’s Double Skinny Macchiato and others, thinking that soon, I too would pop along. After all, it is a place that celebrates pour overs in central London. And yet, I went for the first time two weeks ago while meeting Sadiq of Amoret Coffee to discuss details of the first coffee and science evening being hosted in the Notting Hill branch of Amoret on the 11 June (more details and sign up page here).

On that first occasion, I had enjoyed a Rwandan by pourover and took in the minimalism and cubist geometry of the cafe but largely was too involved in discussing details of the event to think about the connections that the space prompted. And so a second visit was arranged. Again I found that the fold out chairs underneath the bench tables were a little too tall for me (though on the second occasion I didn’t fall off) but it did mean that, although I had a prime seat in front of the bar where they were preparing my pour over (a Burundi from guest roaster La Cabra), it was not easy to turn around to watch. It was however great to find that the cake menu at the order point at the front of the cafe clearly listed all the allergens in each of the cakes and so I was able to confidently enjoy a vegan banana cake with the coffee.

Omotesando Koffee, brownie with square revealed
Cubes and squares were a recurring theme inside the cafe

Omotesando offers a challenging space for a website built on the premise that any cafe offers an opportunity to explore connections to the wider world of physics if you just slow down, take in your surroundings and notice them. It is a space that seems to revel in minimalism. Most of the space is a fairly light coloured, mostly uniform wood. The bar is framed with a cube, a shape that seems to crop up all around Omotesando, even in some of the cakes. The fold out stools (circular) are made of the same colour of wood as the rest of the majority of the cafe (though there are a couple of exceptions to this which hint at the carpentry). Perhaps the idea is that we should focus on the coffee rather than the environment. And maybe that is where your mind enjoys wandering, but another thought suggested itself to my mind.

Sitting on the stool facing the window, wishing that I could turn around to watch the pour over being poured while remaining comfortable (there is a foot rest when facing forward), it struck me that sitting right in front of the bar did not help me when I wanted to use the glass of the window as a mirror to the inside of the cafe. The glass was perfectly transparent to my eye and reflected very little of the light behind me. The fact that the side of the (La Marzocco) espresso machine was transparent rather than metallic and revealed the pipework and wiring that enabled great espressos to be prepared (we also enjoyed an iced latte) briefly led me to consider why some materials are transparent and others not (and also how transparency varies as the frequency of light changes).

Banana bread and coffee with IoP bag
My pour over coffee, a banana bread and my IoP re-useable bag sitting on the table at Omotesando, Fitzrovia, London

But as I reflected further, I could see in my mind’s eye, the viewpoint of a deep sea diver looking up from the sea bed towards the sky. A circle of light, “Snell’s Window” opening above them. You can see images of Snell’s window where divers are framed by the effect in the photograph here. The effect is caused by the refraction of the light as it enters the water. Just as a straw (paper of course) appears bent as you view it through the glass of water, so light entering the sea will be bent by an amount given by Snell’s law. Even light entering at a grazing incidence will be refracted towards the ‘normal’ (the line perpendicular to the sea-air interface) and so if you work through the maths (there’s a good description here), you find that you will only see light from a cone of about 100 degrees around your view point.

Coffee reflections
What would you reflect on?

But although Otomesando has an entirely glass frontage, you do not feel you are in a gold fish bowl, nor can you only see a small window outside. The wide window instead offering plenty of opportunity for watching the office workers and builders scurry about outside. And, on writing this and looking through my photos of the cafe, I noticed that my photographs of the front of the cafe and of a coffee inside were both taken at shallow angles showing the reflections from the surface of the window and the coffee rather than the interior. An effect almost opposite to that of the deep sea diver. Omotesando Koffee offers a space where each cup offers further opportunities for reflection: more time for noticing the physics of the everyday. A great place therefore to spend some time thinking about, as well as enjoying, your coffee.

Omotesando is at 8 Newman Street, W1T 1PB

Is it summer yet? The Swallow Coffee shop, Shepherd’s Bush

Coffee Shepherd's Bush
Outside the Swallow Coffee Shop on Goldhawk Road

It was a spring day as we walked along Goldhawk Road towards the Swallow Coffee Shop. A sign, hanging above the door alerted us to the location of the cafe: an image of a swallow in flight, no name, just the image. A nod to the coffee houses of old perhaps that would advertise themselves with a picture above their doorway. The cafe is on the corner of Goldhawk Road and Richford Street and immediately strikes you as being more open and airy than some of the shopfronts we’d passed along the way. The counter is on the right as you go in and coffee is by Ozone.

The cakes looked good but sadly the tempting brownie was decorated with pistachio. Often I find that my nut allergy does have the incidental effect of keeping my waistline down. So sadly, once again it was just the long black that day. There is plenty of seating inside the Swallow cafe and we chose a table up the stairs, on a type of mezzanine level towards the back of the cafe. A map of London was on the wall next to us, which we studied a little in order to discover that a bit of artistic license had been taken with the geography. On the wall opposite, another map showed the region of Hammersmith. There is something interesting in the way that these maps were rendered. What was it that the cartographer intended to convey?

Lubrication station or plant stand
From mirrors to fireplaces and the nature of heat, what do you see in a coffee shop?

A sign above some plants indicated a “Lubrication Station”, perhaps needed by the Swallows on their hazardous migration to and from South Africa. Looking down towards the front of the shop it appeared that there was a mirror that I hadn’t previously noticed on the wall. It was a large, circular mirror. How come I had not seen it earlier as I walked in? And then it struck me, when I walked in, it was not a mirror but another framed map. It seemed as if it had changed its appearance because, from my location sitting towards the back of the cafe, the light was being reflected at a very shallow angle and so I was not able to see any of the ‘information’ behind the glass, only the reflections from the street. What appeared to my eyes as a mirror was in fact a map.

What do maps need to convey? A visual idea of the geography? Or perhaps, the way of getting from A to B. If it is the latter, there is no reason that the map should be geographically accurate and moreover, it could appear as a cartoon like strip of information so that you can ‘read’ your directions as you go along. We came across one such map a few years ago at St Michael’s Mount in Cornwall (see picture). Or, perhaps a more famous example of a geographically inaccurate but perfectly useful map is that of the London Underground. But then, the map may not be about getting from A to B at all but instead, should give an idea of the physical surroundings of a place or indeed could be intended to convey deeper  information such as the poverty maps of Charles Booth. Towards the end of the nineteenth century, Booth walked London mapping the levels of poverty (or affluence) in an area. You can access the maps here. In addition to seeing how some things have changed (or have not), the maps reveal how in London areas of relative wealth so often exist side by side with areas of relative poverty.
what information do we want a map to provide
A map at St Michael’s Mount in Cornwall offers an alternative depiction of the journey from A to B.

Booth apparently come up with the idea of the maps as he had disagreed with the suggestion that 25% of Londoners lived in poverty. And so he’d set out to conduct a statistical survey for himself. Categorising neighbourhoods into different groups according to relative income or wealth, he discovered that, in fact, 35% of Londoners were living in abject poverty, a worse result than he’d anticipated. His findings led to reforms such as the implementation of noncontributory state pensions and to the development of social surveys.

The work of Charles Booth somehow fits together with the research of John Snow who had similarly mapped the cases in the cholera outbreak of 1854 and so traced the source of the problem to the Broad Street pump. New fields of social research were being developed that relied on maps as a base for seeing the world. How we choose which information to include in the map (and by implication which to omit) and the way we choose to display that information, will affect how quickly our audience, or indeed ourselves, can understand the data presented. Some of our decisions are hidden but may affect how the data is later reported in the media. For example, did Booth define “abject poverty” for his maps in the same way as the previous efforts had shown “poverty” levels of around 25%?

In a world with ever growing amounts of data and eye-catching (click-bait) headlines, it is a problem that affects us still. What are the graphs and maps really telling us? Does the data really confirm our existing beliefs or is the devil in the detail of the display? If we find that our preconceived ideas are refuted (or perhaps worse confirmed), do we have the intellectual honesty to sit back, perhaps with a coffee, and question once more both our beliefs and the data that has challenged them? A cafe such as the Swallow, full of maps and prints, with plenty of seating and a light and friendly environment would be a great place to start.

The Swallow Coffee Shop is at 75 Goldhawk Road, W12 8EH

Latte Art

Latte art scutoid tulip
The physics of bubbles. What links latte art to the shape of cells as an embryo develops?

An odd one out competition: which of the following is not a type of latte art? Tulip, heart, swan or scutoid? You may well ask, “what on earth is a scutoid?” and so identify this as the odd one out and, to some extent you would be right. Scutoids are not a type of latte art. But I would wager that you can still occasionally see them in your coffee.

Twitter can be a great thing and I was recently alerted there to a New York Times article about Karen Uhlenbeck by @Bob_Mat_Phys. Uhlenbeck is a mathematician at the University of Texas who has just won the Abel Prize in mathematics for her work on the maths of bubbles. The article was fascinating in itself but also mentioned in the article was the fact that there may be, on occasion, a connection between a cup of coffee and the cell structures seen in foetal development. And while I’m very well aware of the extraordinary number of connections that can be made between coffee and the science of the everyday world, I’ll admit, that one surprised me.

Metal jug and transparent glass
More bubbles in your coffee. But what determines their shape? And what shape are they?

By this point you may be unsurprised to hear that the connection is made via the scutoids, but what are they? A new type of shape, they were first described in a Nature Communications article about the development of cells as organisms such as fruit flies grew. Scutoids formed as the embryonic cells grew to form tubes or egg shapes. On one surface of the tube the cell was contacting a different number of cells to that which it contacted on the other surface (so perhaps the cell looked like a pentagon on the top and a hexagon on the bottom). In order for the cell to do this, it formed a further triangular face along one side of the cell and it is this cellular shape that is the scutoid.

Where is the connection with a coffee? Well, the amazing thing is that this shape can be the result of the physics that determines the shape of bubbles, in this case when they are confined between two curved surfaces, such as two cylinders. The shape of a bubble is the result of the minimisation of the surface energy of the bubble. So, in free space, the bubble will be spherical but somehow squash bubbles into a box and you can form a cube shaped bubble in the middle of the box. The shapes that form are the result of the minimum surface energy of the bubble surface. Now, if we return to the curved surfaces and the scutoids. The idea is that if there is a single layer of bubbles between two curved surfaces and that these surfaces are then moved away from each other, the bubbles will first resemble prisms and then, as the surfaces are stretched further, some bubbles will form a prism shape but with a triangular surface at one of the bounding walls: a scutoid.

latte art by Mace, Eiffel Tower and hot air balloon
It is astonishing what you can see in a coffee when you look closely enough.

The paper that showed this (published in Philosophical Transactions but you can read the full version here) combined mathematical modelling of the minimisation of surface energy with experiments involving two cylinders and some soap suds. They then photographed the resultant bubble structures. The results suggest that the minimisation of energy (ie. the physics of the bubble shape) could be a first approximation for explaining the cell structures that form in foetal development. But can you see them in your coffee?

You would need a coffee mug or French press and a smaller cylinder that fits neatly inside it. You would then need to form a foam somehow. Soap suds are obvious, some form of milk texturing would be more interesting. You can then look closely and see, can you in fact see scutoids in your latte art?

Opposition at Antipode, Hammersmith

Antipode coffee Hammersmith
Outside Antipode, Hammersmith.

At the end of Fulham Palace Road, just next to the Hammersmith gyratory is Antipode, an Australian influenced (the clues to this are not so subtle) cafe. In truth, I have been to Antipode a few times now but not to take time to properly take it in. Once was after a tricky teaching session where it was difficult to sit back and reflect on anything but what had happened in the previous few hours, another time I was talking to someone rather than taking time to think about the location. There seems an urgent need for us now to take some time out and think about where we are and what we think. Indeed, part of the point of Bean Thinking is to explore how this space to ponder can be found in any cafe, if we but pause to look. Would this visit to Antipode be different?

Outside the cafe, a few tables were arranged so that you can enjoy your coffee in the open, next to the glass front window. Strangely the chairs/stools for these tables were stacked inside, possibly because it had become chilly again after a brief warm spell earlier in the year. A picture of a takeaway cup was drawn on the window as if to emphasise what you may expect to find inside, reminiscent of the old signs advertising coffee houses of the past. Going in, the counter is on the right and, while there was a selection of cakes etc. I opted to stick with the long black that afternoon. A seating area is at the back of the cafe where there are about 4 separated tables with a bench seat running along behind them with a fifth table along the rear wall.

Coffee at Antipode, pink salt, brown sugar, reflections and shadows. And a hint as to Bean Thinking
Coffee, salt and sugar. What do you see?
The coffee was drinkably fruity. More apples and redcurrants to my palette. On the table behind my coffee was a jar of pink salt and another of brown sugar. Which got me thinking about crystal structures and how it is often impurity, rather than purity that gives precious stones their colour. Is there a metaphor there?
 
But a second effect jolted to my attention. Someone sat down on the bench seat just along from me and as she sat down so I went up: a little see-saw. Across the room from me was a picture which, somewhat strangely, had two picture hooks either side of it, almost balancing each other on an imaginary line across the frame. Behind the table adjacent to me was a picture with a caption, to the effect of there being a very thin line between love and hate. Was this another instance of balance and equivalence?

Balance is something that we use in physics a lot, from the balance of forces to the use of balances in experiments. The imminent redefinition of the unit of the kilogram is based on a balance of forces. In the new definition, a balance is used so that the gravitational force pulling a mass down will be perfectly balanced by an electrically induced magnetic field pushing the mass up. The redefinition means that to calibrate 1Kg, scientists will no longer have to compare their 1Kg mass to the mass of a lump of platinum-iridium kept in Paris. The redefined kilogram will instead be calibrated based on its relation to Planck’s constant. This means that any lab around the world can calibrate the kg, they do not have to rely on copies of the mass kept in Paris.

Victoria Regina: What changes have happened since this post box was installed here in Hammersmith? What changes will do so before it is finally retired?
Victoria Regina: What changes have happened since this post box was installed here in Hammersmith? What changes will do so before it is finally retired?

The redefinition of the kg is going to happen on 20 May, 2019 (world metrology day). On a day to day basis, it probably will not affect many of us that much. Our 20g of coffee measured out to brew our morning coffee is going to be, to all intents and purposes, the same 20g as we would have measured on the 19 May 2019. Nonetheless, the changes are important not just for the metrology community but also for the way that we do science. In the past, all of our units were related to fixed, physical objects. The metre was defined by the length of a metal rod, the second was originally defined as being 1/86400 of the mean solar day and the kilogram by the aforementioned lump of PtIr in Paris. The kg was the last of the units to still be defined by a unique physical object. As of 20 May 2019, each of these units will be related to physical constants meaning that at no point will we have to go to a lab elsewhere and check that my kg is the same as your kg.

As I left Antipode, I noticed the post box just outside with “VR” on it. The post box has been there since the time of Queen Victoria. How things have changed since scientists wrote to each other with news of their latest experiments, scientific papers were posted to journals and measured lengths were compared to a physical ‘metre’ long metal rod! How things change as we move ever faster emailing results around and tweeting our latest news. We are, in 2019, moving from calibrations based on weighing physical objects to measuring the balance relative to physical constants that were just being discovered at the point that post box first came into service. And yet we humans don’t change much. We still need time to ponder balance from false balance, equivalence from false equivalence. It is not a contradiction to say that it is urgent that we find a way of pausing and reflecting on some very weighty issues.

Antipode is at 28 Fulham Palace Road, W6 9PH

Questioning my assumptions at Everbean, Marylebone

Coffee cake Everbean
Coffee and cake at Everbean.

Alerted by Caffeine Magazine (on Twitter) to the opening of the second branch of Everbean, we arranged a quick visit into central London. A fair few had beaten us there. Initially, it seemed that the cafe was quite small with limited seating but a sign on the staircase pointed us to an entire area downstairs. Although there are tall stool-type seats upstairs fronting the window, there are more chairs and cushion backed bench seats downstairs (together with a comfy arm chair but more on that later). Downstairs was clearly the place to be on that day and is certainly a comfortable space for enjoying your coffee. As you enter, the counter upstairs is quite large and features a number of tempting cakes. Too tempting. Together with my Americano, I enjoyed a delicious vegan mandarin and chocolate cake.

Downstairs could be described as cosy. Cushions with birds embroidered onto them line the bench running down one side wall. A bookshelf with an eclectic collection of books is in the corner of the room next to the arm chair, suggesting a great (phone-free) way of spending an afternoon. I would share with you some of the titles but in some ways, that would be to judge the books by their cover (titles). Which in some way connects with the thought train that we encountered here at Everbean.

Mirror at Everbean, coffee Marylebone
Mirror, mirror on the wall: We can see ageing effects in metals but taste them in coffee.

On the wall behind us a lattice effect mirror reflected the room to itself. The lattice was painted but bits of paint had aged leading to rust and corrosion effects on the metal lattice work. Age, in the form of oxygen and moisture, affecting metal work in a similar way to how age affects the flavour of coffee. At this point, my thought train at the time went towards the ways in which different materials oxidise and the use that this can be put to. But a different thought train occurred to me when I started to think about this cafe later as I came across Brian’s Coffee Spot’s thoughts on coffee bean storage and specifically, should you ever store your beans in the freezer.

In addition to showing that, depending on your defrosting conditions, it was perfectly fine to store your coffee beans in the freezer, Brian’s Coffee Spot had highlighted a Twitter poll concerning coffee storage. The results of the poll had been definitive. Of 118 voters, 99 had ticked the “never store coffee in the freezer” option. I admit I was one of them. In hindsight, I can self-justify: I could say I was thinking about the (very real) problems with moisture affecting the ageing of coffee and the possibility of water already in the bean causing structural issues for the bean. However these are also problems that are avoidable, as Brian’s Coffee Spot outlined. If I am honest, in reality, I saw the poll, had a negative view towards the freezer option and so clicked “never”.

After reading Brian’s Coffee Spot, doing a little bit more reading about it online and then sitting back and actually thinking about it, I realised that I had perhaps been hasty. Is there still time to change my mind now I know more about the issue? We need a second vote!

But reading about the issues of freezing coffee beans also alerted me to a study that had been done a couple of years ago about the effect of the temperature of the coffee bean on grind size. The question was, when we grind coffee, does the temperature of the bean matter?

books at Everbean
You could sit here all day. Imagine what you would learn.

To test this question the authors subjected batches of 20g of coffee beans to two hours of four different temperatures: liquid nitrogen (-196C), dry ice (-79 C), freezer temperature (-19C) and room temperature (20C). Following this, the beans were immediately ground using a Mahlkonig EK43 grinder. They found that, under otherwise identical grinding conditions, the colder beans showed a smaller grind size and a reduced particle size distribution.

The authors of the study suggested their results as a possible explanation for the need in many coffee shops to tune the grinder to a closer grind size as the day progresses: they argued that the beans are warming up while sitting in the hopper on the grinder and that this results in a change in the way that they grind. They also suggested a possible long term solution for the storage of coffee beans: liquid nitrogen. Just a little bit colder than a freezer.

Which takes us a long way from the basement at Everbean on Seymour Place. Or does it? Perhaps you need to take some time out and sit in the armchair, questioning and investigating your perspective.

Everbean (2) is at 21 Seymour Place, W1H 5BH

On mountains, molecules and coffee

A tea plantation in the mountains of the Cameron Highlands, Malaysia. But how high would you need to climb in order to boil water at the perfect temperature to prepare your brew?
A tea plantation in the mountains of the Cameron Highlands, Malaysia. But how high would you need to climb in order to boil water at the perfect temperature to prepare your brew?

Walking in the hills or, if you are lucky, the mountains, we can easily be reminded that atmospheric pressure decreases with height. We just have to look at the way that the plastic water bottles we may be carrying have been crushed, or open a yoghurt pot slightly too close to our face. We may remember that the boiling point of water decreases with decreasing atmospheric pressure and so that a kettle boils more quickly at the top of a mountain than at the bottom. But how high would we need to climb to make a perfect cup of coffee with just-boiled water? And what has this to do with the reality, or not, of molecules?

Although the effect appears obvious to us, it is not trivial to calculate exactly how the atmospheric pressure varies with height. To see why, we could think about what pressure is. The pressure exerted by a gas on an object is proportional to the number of gas molecules colliding with and recoiling from the object concerned. These collisions create a force on the object and pressure is just force ÷ area. So why would this change with height?

Small waves seen from Lindisfarne
Think about a layer of air with air pressure above and below it but further acted on by gravity pulling it down. What happens?

Think about a layer of air. Above it, the molecules in the gas are exerting a pressure, pushing down on the air. Below it, there are molecules pushing upwards and keeping it up. But there is one more force that we need to consider: gravity. In physics, we like to think of things in equilibrium, perfectly balanced. So when we think about our layer of air, the forces acting down on the layer of air (the pressure from above and the gravity of the earth) have to be perfectly compensated by the force acting up, i.e. the pressure from below. If this were not the case, the layer of air would sink. Perhaps it is starting to become clearer, why the density of the atmosphere decreases with height. The only thing that remains is to work out exactly how it does it.

And while we could do the calculation here, it has (fortunately) already been done for us by a remarkable physicist called Jean Perrin back in 1910. He was remarkable not just because of the detail of his experiments but also because of the connections he made.

“It appeared to me at first intuitively [that]…. Just as the air is more dense at sea-level than on a mountain top, so the granules of an emulsion, whatever may be their initial distribution, will attain a permanent state where the concentration will go on diminishing as a function of the height from the lower layers and the law of rarefaction will be the same as for the air.”

Jean Perrin, Brownian Movement and Molecular Reality, 1910
coffee at Watch House
In search of the perfect coffee. How far would you travel?

Perrin realised that to calculate the balance of forces acting on our imagined layer of air, one has to assume molecules exist, just as we have done above but something that was not obvious at the turn of the 20th century. But he also realised that this calculation would be the same for any fluid containing a suspension of particles whether that was the atmosphere or a drop of water colour paint. Assuming that the molecules exist allows us, and allowed Perrin, to make quantitative predictions for the variation of pressure with height or, in Perrin’s case, the variation of the number of granules in an emulsion with depth. Perrin considered a paint pigment suspended in water under the microscope, but his theory is also valid for the (non-soluble) matter in coffee. The fact that these quantitative predictions matched so extraordinarily well with the experimental observations of thousands of water droplets containing suspended paint pigment (the poor PhD students of Jean Perrin!) went a long way to proving the existence of molecules. Hence Perrin’s book “Molecular Reality” and the ceasefire in a philosophical disagreement about whether physics should seek to understand what was happening or merely describe phenomena such as pressure (but that’s another story).

Which takes us back to how to brew coffee properly. Calculating the variation of pressure with height is the first part of the problem. The second is calculating what that means for the boiling point of water, which actually is done by extrapolating from experimental data. But it does mean that we can calculate, for a small range of temperatures near 100C, the altitude at which you would need to boil a kettle for the boiling temperature to be identical with the optimum brewing temperature for your drink. Listed below are a few recommended mountains on which you can prepare your drink of choice. I will leave it to someone else to calculate the energy saving (and hence the saving in CO2 equivalent emissions) of boiling your kettle on top of a mountain rather than in your kitchen. We’ll assume that there’s electricity on top of Mont Blanc.
 

Drink – Recommended brew temperature – Equivalent Altitude – Suggested mountain

Coffee – 93.3 C* – 2000 m – Kebnekaise (Sweden),

Coffee – 96 C** – 1000 m – Any of the Scottish Munroes

Oolong tea – 87.8 – 93.3 C*** – upwards of 2000m – Mont Blanc (France) could be good

Pu’er tea – 93 – 100C| – why leave your living room?

*Coffee Detective

**The Kitchn/Blackbear coffee

***The Spruceeats

|The tea leaf journal

Rosie and Joe, St Giles churchyard

Coffee in a Wake Cup at Rosie & Joe in St Giles. The space rewards those who notice.

There is a long history of hospitality on the site of St Giles in the Fields stretching back far earlier than the Notes coffee barrow. But Rosie & Joe is a lovely iteration to that tradition. There’s a definite focus on tea at Rosie & Joe but the coffee is roasted by Square Mile and prepared on a La Marzocco machine. There is also a good selection of food to nibble on (as well as more food stalls nearby on weekday lunchtimes). And although it is a cart, there are a few seats and tables dotted around so it is easy to sit back and enjoy your coffee while the world races by.

St Giles High St is a very busy road and yet, sitting in the churchyard of St Giles is strangely peaceful. Despite the traffic and the occasional siren, it is one of those rare places in London that you can find the stillness to listen. A beautiful place to enjoy a coffee from an independent stall in fact! And if you have your own cup with you, there is even 10p off your coffee. The coffee was smooth and sweet, fruity but definitely a sweet and full bodied type of fruity cup. But why was it so peaceful? Was it merely that it was a lovely (but breezy) spring morning when I tried Rosie & Joe? Or was it that it is a small bit of nature in a built up environment? Both of these helped but I think it is also the way that the place rewards those who notice by offering more each time you look.
The ghost sign hidden behind the tree just outside the churchyard.

There’s the, perhaps slightly grim, history suggested by the fact that the ‘garden’ is significantly raised above the level of the pavement in parts. There’s the brickwork and stone walls of the church itself of course. The ‘ghost sign’ on a nearby building that is revealed to the coffee drinker by the fact that the tree between us and it has not yet got its summer leaves. And then the nod to the history of the site hinted at by the coffee cart itself: Since Matilda, wife of Henry I founded St Giles’ leprosy hospital on the site, a “cup of charity” was given to condemned prisoners as they made their way past St Giles on their way to their execution at Tyburn*. Very different now, but the tradition of refreshment for the traveller is continued.

But then a fire engine’s siren reminds you that you are in a cosmos, a universe filled with beautiful physics. You know whether the fire engine is approaching or has passed away from you from how the pitch of the sound changes as it goes past. The Doppler shift meaning that sound waves travelling towards you have a shorter wavelength (higher frequency, higher pitch) than those travelling away (longer wavelength, lower frequency, lower pitch). And part of the beauty of physics is that it is so universal; what works for sound also works for light. If an object emitting light is moving away from us, the light appears to have a longer wavelength (lower frequency, it is red-shifted) than if the same object were stationary or moving towards us where it would appear as if it emitted light with a shorter wavelength (higher frequency, blue shifted).

signboard at Rosie and Joe
Doesn’t a right imply a duty? There’s a lot that could be said about #supportindependent
So, similarly, if we were to look at the surface of a rotating planet and saw how the light reflected off that planet’s surface, the side of the planet that was rotating towards us would look ever so slightly bluer than the side rotating away from us which would look slightly redder. And if the planet’s surface was like Venus and obscured by clouds (rather like the ghost sign at Rosie & Joe will be obscured by leaves in a couple of months time) we could use the reflection of radio waves from the surface rather than visible light to see the same red-shift/blue-shift in the radio waves as the planet rotates**. In this way we could determine the direction of rotation of the planet and how fast it was rotating just as we get an indication of the speed of the fire engine from listening to the sound of the siren.

The siren takes us from a consideration of inner stillness to a recognition of the scale of the universe. Which is rather apt for a cafe in a churchyard, where the architecture of a church is often designed to be read symbolically, from the person to their place in the grand scheme of things***. One great thing about this particular cafe though was how much there was to see that cannot be included in this cafe-physics review for reasons of space. The location truly rewards those who pay attention to what they notice here. I can only recommend that you take some time out, take your re-usable cup and go to find some time to enjoy your coffee (or tea) in this quiet space in central London.

*The London Encyclopaedia, 3rd Edition, Weinreb et al., 2008

**Astronomy, the evolving universe, 6th Edition, Zeilik, 1991

***How to read a church, Taylor, 2003

Rosie and Joe can be found in St Giles in the Fields churchyard, Monday-Friday.

An effective medium for coffee roasting?

coffee bowl pour over

How would you measure the moisture content of a coffee bean?

Recently I had the pleasure of a tour of Amoret coffee in Notting Hill. In addition to discussing an upcoming event that Amoret are kindly hosting (an evening of coffee physics, sign up to the events list to find out more), it was great to see the coffee roaster that is installed there. Fascinating, with what looks to be a really interesting series of coffees lined up ready to roast. And in the course of all this, we came upon the moisture meter, which got me thinking.

Measuring the water content of green (and then roasted) coffee beans is quite critical to gaining an understanding of your roasting process apparently. Sitting on the shelf next to the roaster at Amoret, a small box contained an instrument designed for measuring exactly this. Although it looks as if it is a giant ice cream scoop with a central pillar in the middle, it is actually designed to measure the water content of the coffee beans capacitively. How does it work and, knowing how it works, can we make any predictions as to anomalous results that it may occasionally provide?

The simplest style of capacitor consists of two metallic plates with a gap between them. The capacitance changes depending on the size of the metallic plates, the distance between them and, crucially for this subject, the material that fills the space between the plates. When you apply an electric field between the two plates, the electric moments of the material within the capacitor will tend to align with the electric field. Different materials will react differently depending on their “polarisability”. You only have to think about how a stream of water reacts to a statically charged balloon to see why.

Pulp, Papa Palheta KL

Electrical boxes in Pulp by Papa Palheta KL. The moisture meter at Amoret is much smaller than these old boxes at this ex-printing works.

What this means in practise is that a capacitor formed of plates filled with water will have a different capacitance to the same capacitor filled with air. We say that the ‘permittivity’ of the air is different from the ‘permittivity’ of the water. Measuring the capacitance tells us the permittivity of the material between the plates and so whether the capacitor is filled with air or water. Now fairly obviously, it’s not quite as simple as this because a coffee bean is neither air nor fully water and the moisture meter is not two parallel plates. But in terms of the physics of the measurement, the shape doesn’t really matter here while another bit of physics called “effective medium” theory helps us with the fact that the bean is neither fully air nor fully water. Effective medium theory tells us that the relative permittivity of the mixture is simply proportional to the sum of the individual contributions from the polarisability of each set of molecules. So, merely changing the number of water molecules between the plates will change the capacitance. By knowing what the contribution of the dry beans are, we can calculate the moisture content of the coffee beans as a percentage. Or at least, the instrument can do this calculation internally and provide you with a number on the display.

But. This is what got me thinking about the measurements of the coffee at Amoret. Coffee beans come in a range of sizes and shapes, as you can see by taking a look at the online selection at Amoret (here). Some of these coffees are small, tending towards a more spherical shape while some are significantly larger and more conventionally bean shaped. Is it obvious that the moisture content measured for different coffees is directly comparable? This is not to diminish the use of the moisture meter. As a comparative tool to measure before and after roasting for example, it should be a fairly good indicator. But what should we expect for the absolute accuracy of the instrument? Is a 16% moisture content measured in a small bean really equal to a 16% moisture content measured in a big bean?

At first sight it may seem a silly question, after all, the moisture content is expressed as a percentage; why should size matter? But perhaps we could have a little further think about this. The moisture meter will be optimised for a dense packing of coffee beans. So if we filled it with small beans such that there were very few air gaps between the beans, we would expect a fairly accurate moisture content measurement. If on the other hand, the beans were larger such that there were quite a lot of air gaps between the beans, the actual volume fraction of water molecules in the meter would be reduced (16% of 100% full is greater than 16% of 90% full). And as the capacitance is directly related to the number of water molecules in the sample, the water content that was measured would be less than the true value in each individual bean. So this leads to my first question for roasters using capacitive moisture meters:

  • Do your large beans, that don’t pack well into the moisture meter, often show lower moisture contents than your smaller beans?

variables grind size, pour rate, pour vorticity

Coffee roasting is part-science, part-art and requires great skill and attention. But can thinking about a little extra physics help to understand some of what goes on with the process?

A second point is slightly more subtle. Consider that I had two beans of equal moisture content (%). But one of those beans packs more fully into the moisture meter than the other larger, more irregularly shaped bean. On roasting these beans, they both lost the same volume fraction of water so, say, they went from 16% to 12% water content on roasting. Would both beans show that they had lost the same amount of water?

We could start by thinking about packing these beans into the meter. The one that was densely packed would show a moisture content that was close to the real value (in our example 16%).  The one that was less densely packed however would have a lower volume fraction of water and so show a lower water content. If we assume that the beans filled 90% of the space, the percentage that we measure would be 16% of 90% = 14.4%. On roasting, the two beans are again loaded into the meter and again the densely packed one will show a moisture content close to the real value (in our example 12%). The loosely packed one will show a moisture reading of 12% of 90% of the volume which is 10.8%. Crucially, if we are looking at moisture difference, the densely packed bean will appear to have lost more water (16% – 12% = 4%) than the loosely packed bean (14.4% – 10.8% = 3.6%). Which leads to my second question for roasters:

  • Do small beans that pack well into the moisture meter appear to lose more water for an optimised roasting profile than your larger, less densely packed beans?

Clearly, different beans will have different moisture contents anyway and so it may be difficult to discern any pattern between two specific coffees. The moisture readings may genuinely reflect the fact that the smaller beans have higher water content or vice versa. And also obviously, the measured moisture content is only one part of determining a successful roast profile. However the question is one of statistics. On average, do your larger, less well packed beans have a moisture level lower than you expect? And on average, do they seem to lose less water (measured capacitively) on roasting?

I’d be fascinated to hear your thoughts, here, on Twitter or Facebook.

Semi skimmed at Full Fat, Balham

exterior of Full Fat Balham

The shopfront almost seemed to skinny for the name of this cafe.

Just around the corner from Balham tube station, on Chestnut Grove is a new (old) café, Full Fat. Although it has only been in its current location since September 2018, it was previously known as the Balham Kitchen and was apparently quite popular, not just because it was a friendly local café, but also because of the coffee and the chapatis.

We had chanced upon Full Fat while in Balham and took the opportunity for a chapati brunch. During our coffee break several customers came in and had a chat with the two people behind the counter. With coffee roasted by Workshop and an excellent range of homemade chapatis, along with friendly people, what is not to like? We enjoyed an Americano and an oat milk hot chocolate together with two chapatis. My egg chapati was perfectly done, just the right amount of pepper-to-egg balance. And the coffee was also very well made with plenty to think about just by gazing into it.

Although the café itself is quite small, there are three tables and several seats, enough to be able to ensure that you can probably perch somewhere to enjoy your coffee and chapati. There are even entertaining drawings on the walls of the café that make me wonder whether these are previous customers, immortalised on the walls enjoying a cup of coffee with their morning paper? Certainly they are a good reason to put down your smart phone and just soak in the atmosphere here.

pencil drawings in Full Fat

Pencil drawings at Full Fat Balham

Apart from the drawings, the other decoration in the café that caught my attention was behind another table: a striped piece of woodwork that was reminiscent of a type of cake served in Malaysia and Singapore, kueh lapis. It is a layered cake, the idea being that you peel each layer off to eat it, extending the enjoyment that you can get out of a cake. Although the layers peel easily, ordinarily the cake holds together as a cake, the layers do not glide over each other as if they are wholly separate, perhaps like chapatis stacked on top of each other. No, whether the cake works or not, as a kueh lapis, is down not just to the flavour, but also to the texture and to the way that each layer peels from the next, the subtle tug of one layer held, but not quite, by the last.

These interlayer or interface effects are often, ultimately, an atomic phenomenon. Just as when you drop a drip of coffee on the table, the thing that ultimately determines the shape of the droplet is the attraction between the molecules in the water and the atoms on the table surface*, so the stickiness of the cake has to be occurring, ultimately, at the atomic level.

Interface effects can be crucial in other solids too, not just coffee and cake. Consider sapphire. Sapphire has the molecular formula Al2O3. Ordinarily it is colourless and transparent, it becomes blue when impurities are added to it. But sapphire has another property which is that it is electrically insulating: a sapphire would not conduct any electricity if you tried to use it to connect to your light bulbs. This insulating behaviour is shared with another oxide, strontium titanate, SrTiO3, which is also a colourless and transparent material. Nothing of any interest there then. But, and this is the key thing, if you took a piece of strontium titanate and grew the sapphire on top of it, under certain growth conditions, this bilayer becomes extraordinarily electrically conducting, far better than many metals that you can think of. And it is because of the atomic effects that occur at the interface between the strontium titanate and the Al2O3.

Layering of wood at Full Fat

Can you see the layering on the wooden panel behind the table? Thought not, you’ll just have to visit Full Fat and see for yourself.

But then another example, a new finding that is perhaps more closely associated with coffee. When you take the semiconductor molybdenum ditelluride (MoTe2) and sandwich a thin layer of it between two sheets of graphene, some atomic-level interactions occur. In the case of MoTe2, the “band gap” of the semiconductor (which determines how it conducts electricity when light is shone on it) is altered by the van der Waals forces associated with the graphene layers: the interface effects are changing the way that the semiconductor reacts to light. Ordinarily if you shine a pulsed laser at such a semiconductor, the electrons† that get excited across the band gap are few enough that they behave as a gas within the semiconductor. This means that if you add more electrons (by pulsing the light on the semiconductor again), the density of the electron ‘gas’ will increase. What researchers have just shown is that for this 2D layer, if you shine very high powered lasers at the MoTe2 layer for a short length of time, so many electrons get excited that the semiconductor no longer behaves as if it contains an electron gas, but an electron ‘liquid’. A sort of droplet of electrons in the locality of the laser beam. And because of the way that the band structure had been affected by the interface layers, they managed to obtain this behaviour at room temperature. Although such liquids had been created before, the new thing about this result was that previously these experiments were usually done at very low (-269C) temperatures. Quite a temperature increase on previous results!

Droplets of an electron-hole excitation may seem a far step from the droplets of coffee that you could be enjoying at Full Fat. However, it is just a short hop for the imagination if you stopped to give it time to think. What will you notice next time you are in a café?

Full Fat is on Chestnut Grove, literally just around the corner from Balham tube station.

*Assuming that the table is flat. If the table is nanostructured, the structure itself can influence the ‘wettability’ of the table.

†Yes, and holes.

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