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A first coffee & science evening at Amoret

intro board for Amoret evening
An evening of coffee and science at Amoret Coffee in Notting Hill

A couple of weeks ago we hosted a first “coffee and science” evening at Amoret Coffee in Notting Hill. Designed to explore a physics concept that you could notice in your coffee cup with people from a diverse range of backgrounds, in some ways, the evening itself was an experiment. Would anyone turn up? Would the experiments be interesting? Was I just making my coffee badly?

This last question referred to the fact that the connection for that particular evening had been the dancing drops that skirt across the surface of a V60 (or other pour over) as you prepare your coffee. I had noticed these a couple of years ago but at that point had not appreciated their significance. To answer that question, we were prepared two excellent pour overs by people who really knew what they were doing. And we were spoiled for the coffee which was a recently roasted Nicaraguan washed coffee grown by the Baltodano family who also came along for the evening. The two pour overs were prepared very slightly differently and produced drinks that highlighted different aspects of the flavour of the coffee (though sadly I only managed to try one). This led to a fair amount of discussion amongst those present, not just about which they preferred, but how the preparation affected and highlighted different flavour notes.

Pour overs at Amoret
Preparing pour overs by two (slightly) different techniques. But would we see the dancing drops? (Yes x 2)

The pour overs showed that the dancing drops were there (in both techniques) when coffee was made properly. This was a relief for me! But did they also supply a clue as to how these drops were able to survive, as liquid drops, on the surface of the coffee?

Ordinarily, when a drop drips into a bath of liquid, you would expect it to quickly coalesce with the liquid bath. Once the drop gets close enough to the surface, the van der Waals forces in the drop and the liquid bath will overcome the surface tension effects and the drop will be subsumed into the liquid. If the drop does not coalesce, but instead appears to ‘float’ on the surface there must be a reason.

The first reason that the drop may survive for a while on the surface is because there is a temperature difference between the drop and the bath. This sets up stresses within the drop that pull air into the region between the drop and the bath and keep the drop ‘floating’ for a little while.

Secondly, if you increase the surface elasticity of the droplet, you can stabilise it on the liquid bath for longer. This is usually done by adding soap to the water, not something we did with the V60. But could there be an effect of the coffee oils or some other aspect of coffee chemistry that is keeping these droplets afloat?

Experiments at Amoret
You can see a drop almost ‘sitting’ on the surface of the water here (circled). This particular drop was stabilised for about 15 minutes. I think if you look carefully you can see a ripple pattern around the droplet in addition to the standing wave pattern on the surface of the water caused by the loud speaker underneath (indicated by the red arrow).

Lastly, if you vibrate the surface of the liquid bath, you can create conditions whereby the droplet ‘bounces’ on a cushion of air on the bath. It was interesting, that in the preparation of both pour overs at Amoret that evening, the times that we observed the dancing drops coincided with those times that the pour over was dripping into the coffee bath, causing a noticeable ripple on the surface.

This last condition was the subject of an experiment in the corner of the upper room at Amoret where we used a loud speaker to generate vibrations to two different liquid baths (water and soap water for example) to see if we could obtain stable drops on the surface. Astonishingly, some of the participants on that Tuesday managed to keep a droplet stable for about 15 minutes, you can see their droplet in the photo. The photo is interesting because if you look closely, not only can you see the wave on the bath of water caused by the vibration of the speaker, but you can also see a circular ripple pattern around the droplet. Is that the ripple caused by the droplet’s bounce?

Conversations led on to the fact that these drops were not just seen in pour overs but could occasionally be seen in espressos too. I’m definitely looking forward to the video of that one. While we also got to discuss the importance of different parameters on the stability of the drops – it turns out droplet diameter, as well as the forcing amplitude (which translates to, how loud you have the volume on the loud speakers) are key parameters that affect the behaviour of the drop, something that has been pointed out elsewhere.

V60 droplet floating bouncing sitting on coffee
A drop in-situ

The evening also emphasised just how much we have to talk to each other about! One topic that kept coming up was fermentation, specifically with how the coffee cherries are processed. Hopefully this could become the subject of a future conversation.

Future events are planned (in theory but not yet in practise) and so if you’d like to make sure you hear about them, you can sign up to the Bean Thinking events list here. Also, if you didn’t get chance to take part in the evening but would like to continue the discussion and maybe add your videos & comments about the droplets, you can sign up to the Virtual Coffee House which will be discussing this topic (until the next coffee & science evening).

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Digital information at Kape and Pan, St Giles

coffee Kape and Pan
My pour over coffee at Kape and Pan.

The area around St Giles has changed significantly in recent years. As old haunts have disappeared, new ones spring up in that regenerative evolution that seems to characterise parts of London. Kape and Pan is in the foyer of one of these new buildings (that apparently also homes Google). In some ways it is very much in tune with the new buildings surrounding it and yet there are a number of touches if you take the time to sit and wait attentively for your coffee to arrive (and more that I discovered on researching the cafe later). I was alerted to the opening of Kape and Pan by its mention in Caffeine Magazine and, as I was in the area, it seemed an opportune moment to try it (once I had found which of the glass buildings was number 1, this does not seem to be an area where an address helps!).

Some interestingly titled non-coffee drinks shared the menu with the usual combination of espresso based drinks. Kape and Pan focus on coffees from South East Asia and edibles that are influenced by the flavours of that region. Three coffees were highlighted for the pour over menu. The ‘house’ coffee from Myanmar, an Ethiopian and one that I didn’t make a note of (but as these last two are on rotation, it will have changed by the time you try it). I ordered an Ethiopian pour over, with notes of peach and tea, found a table, and then waited for my coffee.

Light was pouring through the glass windows of the front of the building and so I sat towards the back of the space, with a good view of the pour over being made. That day, a technical problem had meant that payment was by cash only and watching each new customer arrive and encounter this problem, it was great to watch the relationships that had already built among regulars and the baristas and how that contributed to solving it in each case. It is interesting to see how new places develop a sense of community.

Clearly not stainless steel. Does the spoon affect the taste of the coffee? (Answer: yes it can and it has a chemical basis but you have to be drinking your coffee from your spoon)

One of the themes of the cafe appears to be a coffee drinking Buddha. Coffee in one hand, something to eat in the other, he is clearly enjoying his coffee while he contemplates it. And perhaps you may have imagined that the ‘digital’ referred to in the title would be connected with the fact that Kape and Pan is in a Google building, with associated physics connections of data processing and analysis, AI, etc. But no. The digital information comes from the representation of this sage.

As I enjoyed my (definitely tea like, though I did not ‘get’ peach) pour over, my non-coffee drinking companion studied the sugar jar. And gazing at the Buddha’s hand, commented about the perspective that the artist had taken of the fingers and then considered the evolution of the fully opposable thumb in tool use. This is a connection that would not have occurred to me as I don’t spend time drawing and emphasised the importance of listening to the experience of others (something that also was apparent in the evening of coffee & science held at Amoret on 11th June, more details about how this went next week).

It turns out that our idea of the evolution of the thumb and the human hand is still a debated issue and the story of our understanding is, in some ways, an interesting illustration of how science is done, together with the human prejudices and ideas that go together with that.

The idea had been that humans had split from our ape relatives fairly early on in the evolutionary time scale. And that, after that point, we had developed thumb use and other advantages that allowed us to develop tools and to become, as a species, fairly successful. But a spanner in the works came in the 1980s and 1990s when biologists were able to show by looking at the DNA of living primates, that humans were actually a lot closer to the chimpanzee than had been realised. So the model was adapted, humans and chimpanzees had a common ancestor but then the human hand structure evolved a longer thumb relative to the chimpanzees and so we acquired tool use etc. So far, so easy but how can we be more quantitative?

Buddha Kape and Pan coffee drinking
Happy coffee drinking contemplation.

The relative length of the thumb of a primate can be quantified by measuring the length of the thumb (down to the base of the hand) compared with that of the ring finger. Humans have a ratio of thumb length to ring finger length of about 0.75 compared with about 0.35-0.6 in other living apes (I challenge you not to measure yours!). And it is here that there is an issue when people look at the fossil record. There we find ratios that are similar to that of modern humans suggesting that while human hands differ significantly from currently living apes, our hands are not so different from some primates that lived long ago. This would mean that our hand evolution was not there for our specific evolutionary advantage.

But there is more, even the phrasing of the first question is telling as it implies an assumption of further evolution on the part of human beings “human thumbs are longer“. But are they really longer or is it that our other digits are smaller relative to other living primates? And the answer to that got increasingly complex as that depends also on body mass, how can this be estimated in the case of fossil creatures for which we only have skeletons?

As I finished my coffee, I thought again about how Crossrail has changed this area and how new spaces are developing. Perhaps it was apt, given the direction of the thought-flow that I had chosen an Ethiopian coffee to sample that day. Thought to be the origin of coffee, the coffee plant can still be found growing in the wild in parts of Ethiopia. Other coffees, other varietals, have developed (or been developed) since the coffee plant was taken from there to grow elsewhere, giving us an enormous range of flavours to try. The human thumb may not have evolved specifically to enable us to develop tool use, but it does enable us to pick up our cups of coffee.

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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.

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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

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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

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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?

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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

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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

Categories
Uncategorized

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

Categories
General Home experiments Observations Uncategorized

Exploring the sound of coffee

coffee at Watch House
We’re used to thinking about the aroma of coffee and how it looks, tastes, even how it feels, but what about how it sounds?

How much attention do you pay to your brewing coffee? You know the aroma, how the coffee blooms, you anticipate the taste and feel the warmth of the steam rising off the brew. But what about the sound? Admittedly this depends on your brew method, but what about the sounds as you filled the kettle or prepared a pour over brew? It turns out that the sound of dripping water was the subject of a recent paper in Scientific Reports.

Perhaps take time to watch a tap dripping into a bowl of water. Or maybe use this as an excuse to make another coffee by drip brewing. Each drop falling onto the water (or coffee) below first deforms the water’s surface then, as far as we can see, rebounds up with a splash of a returning drop or droplets. The phenomenon of what causes the characteristic sound of the drip has been investigated for over 100 years but in 1959 it was established using high speed photography that there were four key phases to any drip sound. First, the drop fell on the liquid, then a cavity formed just under the water surface and an air bubble formed just under that. Finally the water surface recoiled leading to a jet of droplets returning from the surface. It has been thought that the sound, that ‘plink’ of the dripping tap, was caused by that trapped air bubble expanding and contracting as it moved through the water under the water’s surface¹. But this has now been confirmed, along with some other interesting, coffee related, observations using ultrafast video recording (30 000 fps for most of the work, 75 000 fps for some of the extra details).

lilies on water, rain on a pond, droplets
Like the sound of falling rain? What causes the dripping sound of a tap?

The authors of this recent paper describe what must have been a fun experiment to do, dripping water into a tank below. You can see some of the videos of the droplet entering the water by scrolling down to the “supplementary information” in the paper. Two microphones (one above, one below the water surface) recorded the sound waves coming from the dripping ‘tap’ simultaneously with the video recording so as to match the timing of the sound with what was happening in the video. The microphone above the water surface largely recorded the same sound waveform as the microphone under the water with one crucial exception. When the authors lined the tank with MDF wood, the underwater sound was ‘damped’ quite quickly, in comparison the bare tank amplified the sound and so the sound wave took much longer to decay. Above the surface however, it didn’t matter whether the tank was lined or not, the sound signal remained the same. This may sound somewhat insignificant, but it means that it cannot be the sound created by the wobbly bubble itself merely propagating through the surface of the water. If this were the case, the microphone above the water surface should show the same signal as the microphone under the water’s surface. Instead the authors suggest that the oscillating bubble causes the surface of the water immediately above it to vibrate (in the bit that is depressed owing to the droplet having fallen into it) and it is this that we hear above the water surface.

science in a V60
Droplets on the surface of a brewing V60 may also form owing to a temperature difference between the dripping drops of coffee and the coffee ‘bath’ underneath.

It is a beautiful set of experiments but how can it link to coffee (apart from with the dripping)? It is in the way that it gives us the chance to experience our coffee with experiments involving more of our senses than just smell, touch and taste. Firstly, the study emphasises the connection between the drop’s diameter and speed to the sound of the drip (the best sounds are for drops between 1mm and 5mm diameter). This suggests that by changing the brewing parameters (whether you prepare your V60 in a jug or a mug or change the filter paper to a metal kone for example), you may hear a change in the sound of the drips. Do you? Secondly, it has been suggested that the sound that is formed is dependent on the temperature difference between the dripping drop and the water bath underneath. A temperature difference between drop and bath would also explain an odd phenomenon I noticed in the V60 a while back. Do you notice a difference in the sound of the brewing coffee when you prepare cold brew pour over as opposed to a standard breakfast brew? Lastly, the authors of this study found that they could suppress the sound of the plink by reducing the surface tension of the water bath that they were dripping water into. In their case they added washing up detergent to the bath. This seems an awful waste of coffee but is it possible that something intrinsic to our coffee brew could do the same thing? Oil will also change the effective surface tension of the water and different coffees (and different roast strengths) change the oil content of the brewed coffee. Have you noticed any change in the sound of the drips of the coffee depending on how dark a roast coffee you use?

It may not make ground-breaking science but it does offer us an opportunity to pay even more attention to our coffee. Does the sound of your coffee reveal the beauty of the physics at work just under its surface?

¹ Some history of the investigation into the dripping sound as well as the experiments can be found in: Phillips et al., “The sound produced by a dripping tap is driven by resonant oscillation of an entrapped air bubble”, Scientific Reports, 8, 9515 (2018)