Black holes in your kitchen

LIGO photo
Evidence for the collision of two black holes was found a few years ago at the LIGO detectors. But where can you find a link between your coffee and a black hole? An aerial photo of the LIGO detector at Hanford. Image courtesy of Caltech/MIT/LIGO Laboratory

Where, in your kitchen, would you find a link between the physics of the everyday and the physics of black holes? It’s a question with many answers, and maybe you could think of a few. But one involves a process you may see while brewing your coffee, though you may have to slow down to see it.

The connection is in the way a gentle stream of water breaks up into droplets as it falls. Brewing coffee using a swan necked kettle in a V60, it is something that I see as I slow the rate of pour. Is this a good way of preparing a coffee? Possibly not, but it does allow me to experiment with the physics. You could also see the effect from a slowly dripping tap or in a few other places around the home. It occurs when the cylinder of flow is much longer than the radius of the flowing water.

The question is really, why would a cylinder of flowing water seemingly spontaneously break up into a broken stream of raining droplets? The answer is in a phenomenon now known as the Plateau-Rayleigh instability.

To see why it may occur, we can think about how water flows out of a kettle or a tap. In any cylinder of fluid there will be regions of the flow that are a bit fatter and regions that are a bit thinner. These can be imagined as a series of waves on the surface of the cylinder (you can see a schematic of this effect here). At small wavelengths, the water cylinder remains stable, so for very rapid (but small) fluctuations in the diameter of the flow, you will not notice any difference to the way you pour. But as the wavelengths become larger, and beyond a critical wavelength, the amplitude of these oscillations increase rapidly with time (the maths describing the ‘why’ is here).

kettle, V60, spout, pourover, v60 preparation
Pouring water from a swan necked kettle offers a perfect opportunity for observing Plateau-Rayleigh instabilities.

As the amplitude of the oscillations grows, there will come a point at which the bulges are so large and the necks of the stream so thin (relative to the stream’s diameter) that surface tension effects will cause the necks of the cylinder to break resulting in the stream of droplets that you see. When Plateau first observed this in 1873, he thought that the continuous stream became a flow of droplets when the length of flow was just over 3 (around π) x the radius of the flow. In fact, the break up seems a little more complex, and from my V60 kettle I’d estimate that the length at which it occurs is greater than 3x the radius of the pour, but the experiments of Plateau and the theory of Rayleigh did rather explain what was going on with the stream.

How is this related to black holes? Black holes are massive objects that exist within a very small region of space. Many black holes are thought to be the result of the collapse of a massive star at the end of its life, although there are examples of smaller and more massive black holes. The sort that result from a collapsed star can have a mass around 20x that of the Sun but fit into a space with a diameter of just 10 miles, which is about the distance from Heathrow to Hammersmith (still not central London!). Every planet, moon, star or black hole has an “escape velocity” associated with it that is a function of the object’s mass. The escape velocity is the speed at which you would need to move away from the object in order to avoid being pulled back to the object’s surface. For the earth you need to travel at more than about 11 km per second in order to escape the earth and enter into orbit around it (or move beyond that). For the moon, because it has much less mass, the escape velocity is far lower. For a black hole, the escape velocity is much higher and actually exceeds the speed of light.

The “event horizon” of a black hole is the point at which the escape velocity from the black hole is so high that it exceeds the speed of light. We cannot see into the black hole, because the light cannot escape from within the event horizon.

a heat sensitive coffee mug
What other astronomical connections can you find in your coffee cup? Do let me know what you think.

It turns out that for certain mathematical reasons, it can be useful to consider the event horizon as a stretched fluid membrane with elastic like properties much the same as the surface tension causes to water. At this point it gets a little complicated because not all black holes are spherical*, some indeed can be cylindrical. So we have a cylindrical object with an event horizon with properties that cause it to behave in a manner similar to a fluid with surface tension.

You may well have seen where this is going already. Because yes, it turns out that such cylindrical “black branes” are susceptible to breaking up into many smaller objects exactly analogously to the Plateau-Rayleigh instability in a stream of water. Exactly how they broke up (eg. did they break into spherical objects) was left to further investigation, but the maths was developed in a 2006 study to explore this phenomenon further, you can read more about it here.

It is a bit of a bizarre connection to realise in your kitchen. But the world is often weirder, more beautiful, and more connected than we are sometimes tempted to think. Do let me know of other astronomical connections to your kitchen that you can see. I can think about one or two more related to black holes, but I’m sure you can think of many more. Please just leave a comment below, on Twitter or on Facebook.

*This is certainly true in the maths of black holes, it’s too far outside my subject field to know if such objects have been observed or thought to have been observed in reality.

(Im)perfect reflections on coffee

science in a V60

Have you noticed droplets like these dancing on your drip-brewed coffee?

With the recent coffees from Hundred House and Quarter Horse, there have been many opportunities to observe the coffee brewing in the V60 in the mornings. The steam rising from the filter paper, the different ways different coffees bloom and out-gas, the droplets that skim the surface of the coffee and bounce off the walls of the jug and then, of course, the many different effects with light. Watching the dancing droplets (an explanation of why they may dance is here), it is perhaps not immediately obvious that you could form a connection between these, the light reflections and an insight into something you may have noticed while passing through customs. And yet the connection is definitely there.

The connection is formed through a technique called Raman spectroscopy. Named after Chandrasekhara Venkata Raman (1888-1970) who discovered the Raman effect in 1928. As the ‘spectroscopy’ part of the name suggests, it is a technique that offers a way to identify different chemicals, or components, in a substance. For coffee it has been used both as a non-destructive technique to determine the kahweol content of coffee beans and hence help as a test for identifying rogue robusta in arabica beans and as a way of analysing the brewed coffee. But what is it, how does watching a brewing V60 help to understand it and why would you want to know about Raman spectroscopy while travelling through an airport?

beauty in a coffee, coffee beauty

A collection of bubbles on the side of the coffee. What happens when one of the dancing droplets collides with a group of bubbles?

Generally, it helps to begin with coffee and the link is the way in which the droplets bounce off the side of the jug. Brew a coffee and watch them (if you are a non-coffee drinker, you could try dripping hot water through a filter paper into a jug). When one of these droplets hits the wall of the V60 container, it generally bounces back with a trajectory expected for an elastic collision. Given the relative masses of the droplet and the jug, the speed of the reflected droplet is essentially unchanged (even if its direction is reversed). This is similar to what we would normally expect for light. We are used to considering light as waves but because of the wave-particle duality of quantum mechanics it is equally valid to consider light as a stream of particles called photons. As the photons hit a surface and are reflected off, they recoil with the same energy that they initially had, just like the droplets in the coffee. But now look more closely at the dancing droplets. Normally they hit the walls and not each other but just occasionally, they can hit either another droplet or a group of bubbles that have formed on the coffee surface. In these cases, rather than get reflected as before, the droplets transfer some of their energy to the collection of bubbles causing them to move and to wobble. And when the droplet is reflected back, it has a noticeably slower speed (and so we could say a lower kinetic energy) than when it initially danced into its collision. Where is the analogue with light?

When we think about a coffee bean, we probably think about something that is about 1cm oval, brown and quite solid. But if we zoom in, we find that it is made up of a collection of atoms bound together in molecules or, if we are thinking about a solid like salt, in a crystal structure. These atoms act as if they are balls connected by springs and so they wobble as would any structure of masses connected by springs. This is true whether the crystal is diamond or the molecule is caffeine, kahweol, cocaine or semtex (do you see where the customs part is going to come in yet?). Different crystal structures have different atomic arrangements meaning that they are effectively connected by springs of differing strength. If you build a mental model of masses connected with springs, you can see that changing the spring strength will change the vibration energy of the structure. So if now we think about the photons hitting such a structure, while most will bounce off as we saw with the droplet hitting the V60 wall, some photons will trigger a wobble in the crystal structure and bounce off with lower energy. It is a process analogous to the droplet hitting and bouncing off the collection of bubbles on the coffee surface.

Sun-dog, Sun dog

Sun dogs are caused by a different interaction between light and crystals. Rather than the inelastic scattering of Raman spectroscopy, Sun dogs are caused by the refraction of light by hexagonal platelets of ice crystals.

When a photon of light loses energy, it is equivalent to saying that the frequency of the light has changed (which is very closely related to what Albert Einstein got his Nobel prize for in 1921). So a photon that creates a crystal vibration and is scattered off with lower energy has a lower frequency (or longer wavelength) than it had when it first hit the crystal. Importantly, the energy lost by the photon is identical to the energy gained by the vibrating crystal and so by measuring the frequency change of the scattered light we have a way of determining the energy of the crystal (or molecule) vibration. As this energy depends on the way that the atoms are arranged in the crystal or molecule, measuring the frequency shift offers us a way of identifying the chemical under the laser light: kahweol or cocaine.

It is not an easy technique as you can guess from the V60 analogy. Only around one in a million photons incident on a solid will be Raman scattered. You need some pretty decent optics to detect it. Nonetheless, it is a powerful technique because no two chemical structures are the same and so it can be used to identify tiny amounts of smuggled material completely non-destructively. It becomes easier to understand how this elegant technique has become useful for many areas of our lives from customs, through to pharmaceutical development and even into understanding how fuel cells work.

Although it is stretching the analogy too far to say that you can see Raman scattering by watching the droplets on your V60, it is certainly fair to say that watching them allows you the space to think about what is happening on a more microscopic level as your bag is hand-scanned at customs. What do you see when you look closely at your brewing coffee?


Time standing still at VCR, Kuala Lumpur

VCR chalkboard

A trip down memory lane via a new cafe. VCR in Bangsar, KL

One of the first science-based talks I gave was about how VCR tapes worked. Depending on how you viewed it (and whether you had to listen), this was either an achievement given that I was at school and didn’t really understand magnetism nor magnetoresistive devices, or a thing to be suffered through (for much the same reasons). So when I learned that a new café called VCR had opened in Bangsar in Kuala Lumpur, it prompted a series of fond (and a few embarrassing) memories.

Moving on, it is clear that this second branch of VCR (the first is in Pudu, in the main part of KL), aims to provoke such memories of times past. From the name of the wifi to the pulleys behind the counter and the wooden screen at the back of the café, various details around the café pull your memory in different directions. However the coffee is very much in the present. With three types of coffee available to try as a pour over as well as the standard espresso based drinks, this café has a lot to offer. The coffee is roasted by VCR themselves in their Pudu branch. There is also an extensive food menu with an interesting Chawan mushi as well as an intricate avocado toast (topped with pomegranate seeds, toasted quinoa and feta).

coffee at VCR Bangsar

Coffee and pour over jug. But is the number 68 or 89?

The friendly baristas were happy to advise on which coffee to match with which brewing device (though there seemed a marked preference for V60s on the days I visited). In total I tried 4 pour-overs, one with the Kalita Wave and the others by V60. These coffees were all excellent but very different. A couple were fruity, one was sweet and full bodied, one reminded me a bit of the local fruit durian, not I hasten to add because of its taste, but because the aroma from the cup was so different from the flavour of the drink. It was a great privilege to be able to try these different coffees consecutively and to really experience the variety of flavours in coffee. Great care was taken while making the pour over before it was brought over to the table, together with a jug of water, it also seemed to me that the baristas kept a discreet eye on me afterwards to ensure I enjoyed the coffee. So it was a good experience to have had the opportunity both to enjoy one of those pour overs and to observe the people and the surroundings of VCR when I had to wait for 1 hour for someone with no phone and no book. If you get the opportunity to do this I would very much recommend it. Find a comfortable café, order a coffee and then sit, without distractions, and watch what your mind notices and where it wanders for an hour.

An obvious place for a mind to wander would be to the mechanism of tape recording (and why mini-disks are the superior recording medium for the elegance of the physics involved). However, in an hour a mind wanders far further than the name. Supporting the cakes (and a display case for the 2nd place award of the brewers cup), was a table with a concertina type decoration around its edge. Was this a nod to the Kalita Wave brewing device? This is a significant difference between the V60 and the Kalita Wave: the ridges (or wave pattern) on the filter of the latter. How does coffee flow past these ridges? Does this difference in flow dynamics make a difference to the taste of the coffee?

variables grind size, pour rate, pour vorticity

It seems that there would be a lot of physics to observe in the fluid flow in a Kalita Wave filter.

A few weeks previously a friend had made a (lovely) coffee with her Kalita Wave. It was interesting to note the different dose of coffee she used and the way the grinds built up in the ridges (compared with my ‘normal’ V60). Why do the grinds end up in the ridges? Why is there a layer of dust on the blades of a fan? Why do some corners of a building collect more dust or leaves than others? Are these questions related and does it change the flavour of the coffee in the Kalita?

In fact, there are many subtleties in understanding how fluids move around solid objects. One of these is that at the interface of the fluid with the solid, the fluid does not flow at all, there is a stationary layer. Known as a boundary layer or Prandtl boundary layer (after the person who first suggested their existence, Ludwig Prandtl), realising these layers existed revolutionised the field of aerodynamics. The problem had been how to model the drag experienced by a solid object in a fluid flow. Although perhaps only of academic interest in terms of the flow of coffee around a Kalita filter or a spoon, by the end of the nineteenth century and particularly, with the invention of airplanes, how to calculate fluid (i.e air) flow around a solid (i.e. wing) object became very important for practical reasons.

vortices, turbulence, coffee cup physics, coffee cup science

Another cool consequence of boundary layers:
Vortices created at the walls of a mug when the whole cup of coffee is placed on a rotating object (such as a record player).

Prandtl introduced the concept of a boundary layer in 1904. The idea allowed physicists to treat the main body of the moving fluid separately to the layer, very close to the solid, that was dominated by friction with the solid. This meant that the Navier-Stokes equations (that are used to describe fluid flow and ordinarily do not have an analytical solution) are simplified for this boundary layer and can be quantitatively solved. Although simple, by the 1920s Prandtl’s layer (and consequently the solvable equations) were being used to quantitatively predict the skin friction drag produced by airplanes and airships.

The boundary layer allows us to understand how vortices form behind cylinders or around the corners of buildings. I suspect a mix of the boundary layer, turbulence caused by the coffee going over many of the ridges and the brick like stacking/jamming of the coffee grains would combine to explain the difference in the grind shape around the Kalita Wave and the V60 filters. What this does to the flavour of the coffee and whether better brewing would involve more agitation, I will leave to Kalita Wave coffee lovers to investigate. And when you do, I would love to hear of your results, either here on Facebook or Twitter.


Waiting for the drop at Kurasu, Kyoto (Singapore)

Kurasu Kyoto Singapore, coffee Raffles City

The sign towards the entrance at Kurasu Kyoto, Singapore

Kurasu Kyoto, in Singapore, was recommended to me as a great place to experience pour-over coffee. Although they will serve espresso based drinks too, it is the pour over coffee for which they are famous. The Singapore branch is at the front of a shared working space in an office block. Entering from the street, you have to go up one level before the smell of the coffee will guide you to the café.

Ordinarily, coffee chains would not be featured on Bean Thinking. However, despite it’s name, this is a ‘chain’ of only two outlets, the original branch in Kyoto, Japan and this one in Singapore. The menu featured several coffees with their differing tasting notes together with a few other drinks. Coffee is shipped from Japan weekly as well as being locally roasted in Singapore. It is very much a place to enjoy your coffee while sitting on the comfortable chairs before getting back to work (or perhaps, a place to meet potential colleagues over a refreshing cup of coffee). And it is highly likely you will enjoy your coffee which is prepared for you as you wait.

coffee machine, V60 Kalita

The bar and some of the coffee equipment in the cafe space at Kurasu Kyoto Singapore

There is no hint of automation here. Each cup of coffee is prepared carefully and individually by the barista behind the bar. V60 or Kalita, it was somewhat mesmerising to watch the pour over being prepared, rhythmically, carefully, by hand. Indeed, automation seems almost alien to this place where the act of making coffee is truly artful. Once prepared, the coffee is brought to your table in a simple ceramic mug for you to taste for yourself and see how your tasting notes compare.

As I was watching, two thoughts occurred to me, the first of a directly scientific nature, the second more about our society. Firstly watching the barista slowly prepare the pour over, it is difficult not to be reminded of the pitch drop experiment.

You may remember the story from 2013 and then again in 2014. Two experiments that had been set up in 1944 and 1927 respectively finally showed results. The experiments were (indeed are, they are still going) very similar and concerned watching pitch (which is a derivative of tar) drop from a funnel. Pitch is used to waterproof boats and appears to us almost solid at room temperature although it is actually a liquid but with an extremely high viscosity. To put this into perspective, at room temperature coffee has a viscosity similar to water at about 0.001 Pa s, liquid honey has a viscosity of about 10 Pa s, but this tar has a viscosity of 20 000 000 Pa s. The experiments involved pouring this tar into a funnel and then waiting, and waiting, for it to drip. Both experiments seem to drip only approximately once a decade but until 2013 (and 2014 for the other experiment), the actual drop had never been seen. Both experiments are now building their droplets again and we await the next drop in the 2020s.

Imagine waiting that long for a drip coffee.

coffee Kurasu Kyoto Singapore

Apparent simplicity. The coffee at Kurasu Kyoto Singapore

But then a second thought, there is currently a lot of angst, particularly about automation and our environmental and/or political situations, as if they are something from outside ourselves being imposed upon us. To some extent it is true that we are not in control over many things happening around us. But in our feeling of powerlessness, are we resigning more than we ought to of our responsibility for the power that we do have? It was something that deeply concerned Romano Guardini in his essay “Power and Responsibility”¹. To use the example of automation and the pour over. Guardini argues that people become poorer as they become more distant from the results of their work (e.g. by automating the pour over coffee with a machine). And that the better the machine, the “fewer the possibilities for personal creativeness”¹ that the barista would have. For Guardini, this has consequences for the human being for both barista and customer. The barista clearly loses the element of their creativity when preparing a pour over with a machine but the customer too is affected by the loss of a personal contact, possible only through individually created things. Rather than celebrating each other as individuals we become consumers with tastes “dictated by mass production”¹ and people who produce only what the “machine allows”. To respond to the challenges of our contemporary society involves discovering where we each have responsibility and exercising it, no matter how small or large that responsibility seems (to us) to be.

Which is somehow resonant with the interview that one of the Kyoto based baristas at Kurasu Kyoto gave that was recently circulated by Perfect Daily Grind. Asked what was her preferred brewing method, she replied it was the V60 because of the control that the individual barista could gain over the flavour of the cup merely by tweaking some of the details of the pour. A knowledgable art rather than a technology. And it is precisely this knowledgable art that you can see carefully and excellently practised in the Singapore branch.

Kurasu Kyoto (Singapore) is at 331 North Bridge Road, Odeon Towers, #02-01

“Power and Responsibility” in “The End of the Modern World”, Romano Guardini. ISI books, (2001)


Coffee and cream baubles – not just for Christmas

floating, bouncing drops

Drops of water can be stable on the water’s surface for many minutes if you put the water on a loudspeaker, more info on how to create these at home here.

You may have noticed them before: balls of liquid dancing on the surface of your coffee (or tea) that seem to last for ages before being absorbed into the drink? Perhaps you have added milk to your coffee and noticed that it took some time before the milk entered into the brew?

It turns out, there’s some very interesting physics that is happening whenever you add milk to your tea or when you are preparing a pour-over. It can link coffee to wine and to quantum mechanics. It is worth taking a closer look at these drops.

You may remember that you could use a loud speaker to make droplets of coffee bounce on a cup of the same. The vibrations in the cup meant that the air between the droplet and the drink never got squeezed out of the space between them. So, rather than coalesce, the drop jumped up and down on the coffee surface before finally disappearing under. This type of bouncing bauble has been shown to behave in similar ways to quantum particles in wave-particle duality. An analogue of quantum physics in the macroscopic droplets on the surface of your drink.

But that type of bauble required the use of a loud speaker (or some similar way of generating vibrations on the surface of the coffee). What if you could ‘bounce’ a drop of coffee on a cup of coffee without any external props like speakers? Well, it turns out that you can. In November 2017 a group of researchers showed how a temperature difference between a drop falling into a drink and the drink itself could result in the drop appearing to float on the surface of the drink for many seconds. The obvious example was cold milk into a cup of coffee (or tea). But I think that it may also happen in a V60 when you prepare a pour over, more on that below.

science in a V60

Bubbles of liquid dancing on the surface of a brewing coffee.

The idea is quite simple. If there is a temperature difference between the drop and the coffee, when the drop approaches the coffee, there will be thermal gradients across the drop/cup system. Surface tension is temperature dependent: the higher the temperature, the weaker the surface tension. Differences in surface tension across the surface of a liquid result in compensating liquid flows (one of the best places to see this is in a glass of wine, but there’s also a great party-trick experiment you can do to demonstrate it which is here). So, because there is a temperature difference across the surface area of the droplet (owing to the difference between the droplet and the cup), there will be liquid flows set up within the drop. These flows are like circulating vortices which draw the surrounding air into the gap between the drop and the cup and so prevent the existing air between the drop and the cup from escaping. If the air has nowhere to escape to, the drop can’t merge with the drink, in fact it ‘levitates’ for a number of seconds.

The authors suggest that this is a reason that you can often see rain drops staying on the top of puddles or ponds before being subsumed into the water, or why you can see the cream (or milk) stay as globules on the surface of your coffee (or tea). And so I wonder, could this also be the explanation for an odd phenomenon that I sometimes notice while brewing coffee in my V60. Perhaps you have seen this too? After some time, the new drops of filtered coffee impacting on the surface skit along to the edge of the jug. They stay as balls of coffee on the coffee’s surface for quite some time before becoming part of the brew. You can see a photo of some of these droplets above. Initially I thought that this was because the surface of the coffee had started to vibrate with the impacting droplets. But it is also possible that it could be this temperature effect. As the (brewed) coffee in the jug would be cooler than the water dripping into it from the filter, there would be a temperature difference between the droplet and the coffee but the reverse of the milk-coffee situation. The drop would be warmer than the coffee it’s dripping into. The authors of the study suggested that it was the magnitude of the temperature difference that was the key, not the sign of the temperature difference. So that would fit with the V60 observations seen previously. However how would you show which effect (vibration or temperature difference) is responsible for the behaviour?

Enjoy playing with your tea, coffee and V60s. Do let me know the results of your experiments. Is it a vibration thing or does the temperature difference have to be there to begin with? Let me know what you think is going on.

I am also grateful to Amoret Coffee for alerting me to this story in the first place through Twitter. If you come across some interesting coffee-science, please let me know, either here in the comments section (moderated, please be patient), or on Twitter or Facebook.




Coffee under the microscope

Inside Coffee Affair

There are many great cafés in London serving excellent coffee but inevitably a few stand out. One such café is Coffee Affair in Queenstown Road railway station which ‘inhabits’ a space that really encourages you to slow down and enjoy your coffee while just noticing the environment. An ex-ticket office that whispers its history through subtle signs on the parquet floor and in the fixings. The sort of place where you have to stop, look around and listen in order to fully appreciate it. And with a variety of great coffees on hand to sample, this is a café that is a pleasure to return to whenever I get the opportunity.

So it was that a few weeks ago, I happened to wander into Queenstown Road station and into Coffee Affair. That day, two coffees were on offer for V60s. One, an Ethiopian with hints of mango, peach and honey, the other, a Kenyan with tasting notes of blackcurrant and cassis. But there was an issue with them when they were prepared for V60s. The Ethiopian, “Gelana Abaya”, caused a considerable bloom but then tended to clog the filter cone if due care was not taken during the pour. The other, the Kenyan “Kamwangi AA”, did not degas so much in the initial bloom but instead was easier to prepare in the V60; there was not such a tendency to clog.

What could be going on?

So we had a look under the microscope at these two coffees. Each coffee was ground as if it was to be prepared in a V60 and then examined under the microscope. Was there any difference between the appearance of the Gelana compared to the Kamwangi? A first look didn’t reveal much. Magnifying both coffees at 5x, it could be said that the Kamwangi had more ‘irregular protrusions’ on the ground coffee compared to the smoother Gelana, but it was hard to see much more:

coffee under the microscope

The samples of ground coffee imaged under an optical microscope at 5x magnification. Kamwangi is on the left, Gelana on the right. “500 um” means 500 micrometers which is 0.5 mm.

So, the microscope was swapped to image the coffee in fluorescence mode. It was then that the cell structure of the coffee became clear. Here are the two coffees magnified 10x:

Fluorescence microscopy 10x, Ethiopian, Kenyan, Kamwangi, Gelana

Fluorescence microscope image of the two coffees at 10x magnification. Note the open structure in the Kamwangi and the more closed structure in the Gelana.

and at 20x

Kamwangi and Gelana coffee under the microscope

A fluorescence microscope image magnified 20x – not ‘um’ means micrometers (1/1000 of a mm), so the scale bar represents 1/10 mm.

So there is perhaps a clue in the cell structure. It seems as if the Kamwangi structure is more open, that somehow the cells in the Kamwangi break open as they are ground but the Gelana somehow keeps its cells more intact. Could this be why the Gelana blooms so much more?

Which naturally leads to a second experiment. What happens when you look at these two coffees in water under the microscope? Here the fluorescence images didn’t help as all you could see were the bubbles of gas in each coffee but the optical microscope images were of more interest.

optical microscope image in water

The two coffees compared under the microscope while in (cold) water. Magnfied 5x

‘Bits’ broke off the Kamwangi as soon as water was added but in comparison, there were far fewer bits of coffee breaking off the Gelana grains.

So what do you think has happened? If you remember our question was: when these two coffees were prepared with a V60, the Gelana bloomed a lot but then clogged in the filter (without extreme care while pouring the filter). Meanwhile the Kamwangi did not bloom so much but also did not clog the filter, what could be happening?

From the microscope images, it appears that

  1. Before adding any water, the cell structure in the Kamwangi is more open, the Gelana appears ‘closed’.
  2. When water is added, there are many more ‘bits’ that come off the Kamwangi whereas the Gelana does not show so much disintegration on the addition of water.

If pushed for a hypothesis, I wonder whether these two observations are linked. What is happening is that the cell structure in the Kamwangi is, for whatever reason, fairly fragile. So as soon as it is ground, the cells break up and a lot of the carbon dioxide is released. Consequently when water is added to it, the bits of broken cell quickly disperse through the water and it doesn’t seem to ‘bubble’ that much. In comparison, the Gelana cell structure is tougher and the cells only open up when water is added. I wonder if this means that the ground Gelana coffee will swell rather than break up and so ‘jam together’ as each grain tries to expand rather like trying to inflate many balloons in a bucket. They will push against each other and prevent water from easily percolating through the ground coffee.

Sadly, many more experiments would be required before we could see if there’s any truth in this hypothesis however that does provide a great excuse, were one needed, for many return trips to Coffee Affair. Meanwhile, what do you think? Do any of the images stand out to you and why? What do you think could be the cause of our V60 coffee mystery? I’d love to hear your thoughts so please let me know either here in the comments section (moderated and experiencing a lot of spam at the moment so please be patient), on Facebook or on Twitter.

Phlogiston in the Watch House

Watch House coffee Bermondsey

The Watch House in Bermondsey

At the end of Bermondsey St, tucked away in an odd looking building on the corner, is a café known as the Watch House. Stepping inside you are met with a very strange impression: this is far from your normal rectangular room. Instead an octagonal space, complete with Victorian style tiling and wood burning stove greets you. There are about five small tables inside, which were all occupied (some shared) when we arrived late in the lunch hour. So we sat at a table outside, although there was also bench seating on the other side of the door and a lovely park just next door, the old St Mary Magdalen graveyard.

The building itself dates from the time when the “watch house” was the base for a makeshift local constabulary that would monitor the local area ensuring that no body-snatchers were operating in the graveyard next door. The body snatchers used to ‘acquire’ recently buried bodies for use in anatomy classes at the capital’s teaching hospitals. Nowadays, as with many other disused burial grounds in London, the graveyard next door has been transformed into a park. On the other side of the café, a drinking fountain (the gift of a Henry Sterry Esq.) is embedded into the wall. An interesting feature reminding us of the drive to provide drinking water to London’s population both then and now with the newly installed fountains at the nearby Borough Market.

coffee at Watch House

What fantastic colour in this filter.

As I placed my V60 on the table outside, the light shone through it making the coffee appear to glow with a deep red tinge. Temporarily ignoring my normal idea that such transient beauty can’t be captured, I tried to photograph it, an endeavour that predictably failed to capture the full radiance of the cup. Nonetheless, the clear red coffee did not have significant sediment at the bottom of the cup. Perhaps this is not surprising, it was a V60. But nevertheless this lack of sediment has a connection with the water fountains both at the Watch House and at Borough Market and the wood burning stove. You could even make a macabre link to the graveyard next door. But without pursuing that last one too much, the link is Antoine Lavoisier (1743-1794) and the transmutation, or not, of water into earth.

The problem was this: In the early seventeenth century Jon Baptist Van Helmont had planted a 5lb (2.3 kg) willow tree into a pot of soil of mass 200 lb (91 kg)¹. He covered the pot of soil and only allowed rainwater into the tree/pot system for 5 years. At the end of his experiment, the mass of soil was unchanged but the willow tree was now 169 lb 3 oz (76.8 kg). Clearly, the “element” water had transmuted into the “element” earth* and so added to the mass of the tree. A few years later and scientists boiling distilled water (which had of course been purified by previous boiling) noticed that there was always a solid residue left after the water had boiled away². Another piece of evidence for the transmutation of water into earth.

Lavoisier, who became known as the father of modern chemistry, thought differently. He had been interested in obtaining clean, safe drinking water for the inhabitants of Paris and had noticed that when rainwater was repeatedly distilled, the amount of solid residue left after boiling decreased with each distillation. How was this reconcilable with the idea that each time you boiled water part of it became the element earth? But if water wasn’t ‘transmuting’ into earth, what could explain the solid residues observed by the other scientists of his day?

Lavoisier suspected the potash or soda used in making the glass vessels used in the experiments. He thought that this could be dissolving out of the vessels when the water was boiled, leaving what looked like a solid residue at the bottom of the cup². To demonstrate that this could be the case, Lavoisier took a sealed container of water called a ‘Pelican’ (which has two arms to allow the water vapour to cool and drip back down to the base of the unit). He first weighed the water and the vessel, separately and together and then boiled the water in the sealed pelican for 100 days. After 100 days he weighed the container-water system again. The total mass had not changed. However, when they were weighed separately, something odd had happened. The glass vessel (the pelican) had lost some mass while solid salts had appeared in the vessel. Although these salts weighed slightly more than the mass lost by the pelican container, Lavoisier considered the discrepancy within error thereby showing that the ‘transmutation’ observed by other scientists was actually salt dissolving out of the glass vessel.

Lavoisier’s experiments were an important contribution to the development of experimental method as well as a refutation of the old idea of the transmutation of the elements earth-air-fire-water.

Lavoisier, drinking fountain, Bermondsey

The fountain on the side of the Watch House. How had a need for supplying the public with drinking water shaped our scientific thinking?

Which leaves one last connection: the wood stove. Since the dawn of humanity, there has been the question “what is fire?”. By the time of Lavoisier, fire was explained by the idea that matter contained more or less “phlogiston”. Something could catch fire if it contained a large amount of phlogiston, it would not ignite were it to have too little phlogiston³. One observation clearly explained by the phlogiston theory was the observation that a burning candle, covered by a glass bell jar, would extinguish itself. The idea was that the candle (which contained phlogiston) released that phlogiston into the air. If the candle burned within a jar, the air surrounding the candle would became saturated with phlogiston. Once saturated, the air could ‘hold’ no more phlogiston so none could escape the candle wick. This would mean that the flame would go out.

Lavoisier, now recognised as one of the three independent co-discoverers of oxygen, showed that oxygen, not phlogiston, was needed for burning to occur. The question is how did he do it? And a question for you, when you are enjoying your sediment free delicious coffee next to a warming wood fire: how would you?


*to be fair to Van Helmont, it is hard to blame him for arriving at this conclusion. It was still a few centuries before photosynthesis was discovered and the idea of the four elements of fire, earth, water and air was still active in his time.

The Watch House is at 199 Bermondsey St, SE1 3UW

¹”Lavoisier in the year one”, Madison Smartt Bell, Atlas Books (2005)

²”Lavoisier”, Jean-Pierre Poirier, University of Pennsylvania Press, (1996)

³”From phlogiston to oxygen”, John Cartwright, Hatfield (2000)


Theme on a V60

bloom on a v60

V60 bubbles. There is much to be gained by slowing down while brewing your coffee.

Preparing a coffee with a pour-over brewer such as a V60 is a fantastic way to slow down and appreciate the moment. Watching anti-bubbles dance across the surface as the coffee drips through, inhaling the aroma, hearing the water hit the grind and bloom; a perfect brewing method for appreciating both the coffee and the connectedness of our world. The other week, while brewing a delightful Mexican coffee from Roasting House¹, I noticed something somewhat odd in the V60. Having placed it on the kitchen scales and, following brewing advice, measured the amount of coffee, I poured the first water for the bloom and then slowly started dripping the coffee through. Nothing unusual so far and plenty of opportunity to inhale the moment. But then, as I poured the water through the grind, I noticed the scales losing mass. As 100g of water had gone through, so the scales decreased to 99g then 98g and so on. It appeared the scales were recording the water’s evaporation.

science in a V60

Bubbles of liquid dancing on the surface of a brewing coffee.

It is of course expected that, as the water evaporates, so the mass of the liquid water left behind is reduced. This was something that interested Edmond Halley (1656-1742). Halley, who regularly drank coffee at various coffee houses in London including the Grecian (now the Devereux pub), noted that it was probable that considerable weights of water evaporated from warm seas during summer. He started to investigate whether this evaporating vapour could cause not only the rains, but also feed the streams, rivers and springs. As he told a meeting of the Royal Society, these were:

“Ingredients of a real and Philosophical Meteorology; and as such, to deserve the consideration of this Honourable Society, I thought it might not be unacceptable, to attempt, by Experiment, to determine the quantity of the Evaporations of Water, as far as they arise from Heat; which, upon Tryal, succeeded as follows…”²

Was it possible that somehow Halley’s demonstration of some three hundred years ago was being replicated on my kitchen scales? Halley had measured a pan of water heated to the “heat of summer” (which is itself thought provoking because it shows just how recent our development of thermometers has been). The pan was placed on one side of a balance while weights were removed on the other side to compensate the mass lost by the evaporating water. Over the course of 2 hours, the society observed 233 grains of water evaporate, which works out to be 15g (15 ml) of water over 2 hours. How did the V60 compare?

Rather than waste coffee, I repeated this with freshly boiled water poured straight into the V60 that was placed on the scales. In keeping with it being 2017 rather than 1690, the scales I used were, not a balance, but an electronic set of kitchen scales from Salter. The first experiment combined Halley’s demonstration with my observation while brewing the Mexican coffee a couple of weeks back. The V60 was placed directly on the scales and 402g of water just off the boil was poured into it. You can see what happened in the graph below. Within 15 seconds, 2 g had evaporated. It took just a minute for the 15g of water that Halley lost over 2 hours (with water at approximately 30 C) to be lost in the V60. After six minutes the rate that the mass was being lost slowed considerably. The total amount lost over 12 minutes had been 70g (70ml).

evaporation V60 in contact with scales

A V60 filled with 400g of water just off the boil seemed to evaporate quite quickly when placed directly on the scales.

Of course, you may be asking, could it be that the scales were dodgy? 70g does seem quite a large amount and perhaps the weight indicated by the scales drifted over the course of 12 minutes. So the experiment could be repeated with room temperature water. Indeed there did appear to be a drift on the scales, but it seemed that the room temperature water got moderately heavier rather than significantly lighter. A problem with the scales perhaps but not one that explains the quantity of water that seems to have evaporated from the V60.


Hot water (red triangles) loses more mass than room temperature water (grey squares).

Could the 70g be real? Well, it was worth doing a couple more experiments before forming any definite conclusions. Could it be that the heat from the V60 was affecting the mass measured by the electronic scales? After all, the V60 had been placed directly on the measuring surface, perhaps the electronics were warming up and giving erroneous readings. The graph below shows the experiment repeated several times. In addition to the two previous experiments (V60 with hot water and V60 with room temperature water placed directly on the scales), the experiment was repeated three more times. Firstly the V60 was placed on a heat proof mat and then onto the scales and filled with 400g of water. Then the same thing but rather than on 1 heat proof mat, three were placed between the kitchen scales and the V60. This latter experiment was then repeated exactly to check reproducibility (experiment 4).

You can see that the apparent loss of water when the V60 was separated from direct contact with the scales was much reduced. But that three heat proof mats were needed to ensure that the scales did not warm up during the 12 minutes of measurement. Over 12 minutes, on three heat proof mats, 14g of water was lost in the first experiment and 17g in the repeat. This would seem a more reasonable value for the expected loss of water through evaporation out of the V60 (though to get an accurate value, we would need to account for, and quantify the reproducibility of, the drift on the scales).

V60 Halley

The full set: How much water was really lost through evaporation?

Halley went on to estimate the flow of water into the Mediterranean Sea (which he did by estimating the flow of the Thames and making a few ‘back of the envelope’ assumptions) and so calculate whether the amount of water that he observed evaporating from his pan of water at “heat of summer” was balanced by the water entering the sea from the rivers. He went on to make valuable contributions to our knowledge of the water cycle. Could you do the same thing while waiting for your coffee to brew?

Let me know your results, guesses and thoughts in the comments section below (or on Twitter or Facebook).

¹As this was written during Plastic Free July 2017, I’d just like to take the opportunity to point out that Roasting House use no plastic in their coffee packaging and are offering a 10% discount on coffees ordered during July as part of a Plastic Free July promotion, more details are here.

²E Halley, “An estimate of the quantity of vapour….” Phil. Trans. 16, p366 (1686-1692) (link opens as pdf)

Now you see it now you don’t at Bond St Coffee, Brighton

Outside Bond St Coffee Brighton

Bond St on Bond St, Brighton

A couple of weeks back, I tried the lovely Bond St. Coffee in Brighton on the recommendation of @paullovestea from Twitter. It was a Saturday with good weather and it turns out that this particular café is (understandably) very popular and so, sadly, to begin with we could only sit outside. That said, it was a lovely spring day (sunny but a bit chilly) and so it was quite pleasant to watch the world go by (or at least Bond St) while savouring a well made pour-over coffee. All around the café, the street decoration hinted at times past. Across the road what was obviously a pub in times gone by has turned into an oddities store. Air vents to a space underneath the window seating area in Bond Street café itself suggested an old storage space. A seat in the window appeared to have been re-cycled from an old bus seat.

But it was the pour-overs at Bond St. Coffee that had been particularly recommended and they certainly lived up to expectations. I had a Kenyan coffee roasted by the Horsham Roasters. The V60 arrived at our bench seat/table in a metal jug together with a drinking glass. The angle of the Sun caught the oils on the surface of the coffee, reminding me of Agnes Pockels and her pioneering experiments on surface tension. Pouring the coffee into the glass I thought about the different thermal conductivities of glass as compared to metal and how I had put both down on the wooden bench. How was heat being transferred through these three materials? And then, as I placed the metal jug back on the bench I noticed the reflections from the side of the jug and thought, just why is it that you can see through the colourless glass but the metal is grey and opaque?

Metal jug and transparent glass

Metal jug, glass cup. V60 presentation at Bond St Coffee

On one level, this question has a simple answer. Light is an electromagnetic wave and a material is opaque if something in the material absorbs or scatters the incoming light. In a metal, the electrons (that carry the electric currents associated with the metal’s high electrical conductivity) can absorb the light and re-emit it leading to highly reflective surfaces. In glass there are no “free” electrons and few absorbing centres ready to absorb the light and so it is transmitted through the glass.

Only this is not a complete answer. For a start we haven’t said what we mean by ‘glass’. The glass in the photo is indeed transparent but some glasses can be more opaque. More fundamentally though, there is a problem with the word ‘opaque’. For us humans, ‘visible’ light is limited to light having wavelengths from about 400nm (blue) to about 780nm (red). ‘Light’ though can have wavelengths well below 400 nm (deep into the UV and through the X-ray) and well above 780 nm (through infra-red and to microwaves and beyond). We can see the spread of wavelengths of light visible to us each time we see a rainbow or sun dog. Other animals see different ranges of ‘visible’ light, for example, bumble-bees can see into the ultra-violet. So, our statement that glass is transparent while metal is opaque is partly a consequence of the fact that we ‘see’ in the part of the spectrum of light for which this is true.

Sun-dog, Sun dog

Sun dogs reveal the spectrum of visible light through refraction of the light through ice crystals.

For example if, like the bumble-bee, we could see in the UV, some glass may appear quite different from the way it does to us now. Even though the glass in the photo lacks the free electrons that are in the metallic jug, there are electrons in the atoms that make up the glass that can absorb the incident light if that light has the right energy. There are also different types of bonds between the atoms in the glass that can also absorb light at particular energies. The energy of light is related to its frequency (effectively its colour*). Consequently, if the energy (frequency/ wavelength) of the light happens to be at the absorption energy of an atom or an electron in the glass, the glass will absorb the light and it will start to appear more opaque to light of that colour. Many silicate glasses absorb light in the UV and infra-red regions of the electromagnetic spectrum while remaining highly transparent in the visible region. High purity silica glass starts to absorb light in the UV at wavelengths less than approx 160nm†. Ordinary window glass starts to absorb light in the nearer UV†. In fact, window glass can start to absorb light below wavelengths of up to ~ 300 nm, the edge of what is visible to a bumble bee: The world must appear very different to the bumble bee. At the other end of the scale, chalcogenide based glasses absorb light in (our) visible range but are transparent in the infra-red.

Looking at how materials absorb light, that is, the ‘absorption spectrum’, enables us to investigate what is in a material. It is in many ways similar to a ‘fingerprint’ for the material. From drugs discovery to archaeology, environmental analysis to quality control, measuring how a material absorbs light (over a wider range of frequencies than we can see) can tell us a great deal about what is in that material.

Perhaps you could conclude that whether something is opaque or crystal clear depends partly on how you look at it.


Bond St Cafe is on Bond St, Brighton, BN1 1RD

*I could add a pedantic note here about how the colour that we see is not necessarily directly related to the frequency of the light. However, it would be fair to say that a given frequency of light has a given ‘colour’ so blue light has a certain frequency, red light a different frequency. Whether something that appears red does so because it is reflecting light at the frequency of red light is a different question.

†”Optical properties of Glass”, I Fanderlik, was published by Elsevier in 1983.

Something brewing in my V60

kettle, V60, spout, pourover, v60 preparation

The new V60 “power kettle”

It was my birthday a short while ago and someone who knows me well got me a perfect present: a kettle specially adapted for making pour-over V60 style coffees. Until this point I had been struggling with a normal kettle with it’s large spout but now, I can dream that I pour like a barista. Of course, it is important to try out your birthday present as soon as you receive it. And then try it again, and again, just to make sure that it does really make a difference to your coffee. So it is fair to say, that recently I have been enjoying some very good coffees prepared with a variety of lovely beans from Roasting House and my new V60/V60 kettle combination.

Spending the time to prepare a good coffee seems to make it even more enjoyable (though it turns out that whether you agree with this partly depends on why you are drinking coffee). Grinding the beans, rinsing the filter, warming the pot, the whole process taken slowly adds to the experience. But then, while watching the coffee drip through the filter one day, I saw a coffee drop dance over the surface of the coffee. Then another one, and another, a whole load of dancing droplets (video below). Perhaps some readers of Bean Thinking may remember a few months back a similar story of bouncing droplets on soapy water. In that case, fairly large drops of water (up to about 1cm wide) were made to ‘float’ on the surface of a dish of water that had been placed on a loudspeaker.

Sadly, for that initial experiment the coffee had been made undrinkable by adding soap to it. The soap had the effect of increasing the surface viscosity of the droplets which meant that the drop could bounce back from the vibrating water surface before it recombined with the liquid. Adding soap to the coffee meant that these liquid drops could ‘float’ (they actually bounce) on the water for many minutes or even longer (for more of the physics behind this click here).

science in a V60

A still from the video above showing three drops of coffee on the surface.

On the face of it, there are some similarities between the drops dancing on the coffee in my V60 and these bouncing droplets. As each drop falls from the filter, it creates a vibration on the surface of the coffee. The vibration wave is then reflected back at the edges of the V60 and when the next drop falls from the filter it is ‘bounced’ back up by the vibration of the coffee.

But there are also significant differences. Firstly, as mentioned, there was no soap added to this coffee (I was brewing it to drink it!). This means that the viscosity of the drops should be similar to that of ordinary water. Although water drops can be made to bounce, the reduced viscosity means that this is less likely. Secondly, the water is hot. This acts to reduce the viscosity still further (think of honey on hot toast). Perhaps other effects (such as an evaporation flux or similar) could be having an effect, but it is noticeable that although the drops “live” long enough to be caught on camera, they are not very stable. Could it be that the vibrations caused by the droplets hitting the coffee are indeed enough to bounce the incoming droplets back up but that, unlike the soapy-water, these “anti-bubbles” do not survive for very long? Or is something deeper at play? I admit that I do not know. So, over to you out there. If you are taking time to make coffee in a V60, why not keep an eye out for these bouncing droplets and then do some experiments with them. Do you think that the bounce vibration is enough to sustain the bouncing droplet – does the speed of pour make a difference? Is it associated with the heat of the coffee? I’d be interested to hear what you think.

(The original soapy-coffee bouncing droplet video).

If you see anything interesting or odd in your coffee, why not let me know, either here in the comments section below, e-mail, or over on Twitter or on Facebook.