Categories
Coffee review General slow Sustainability/environmental Tea

Coffee and a horse box at Blue Tin, Oxfordshire

coffee Nuffield Oxfordshire Blue Tin
The cafe is in here. The new farm shop at Blue Tin.

Blue Tin is not an easy café to find, one that you can wander into just off the street. In fact, although it serves coffee and cake, Blue Tin is not really a café at all but a friendly place for a drink attached to a farm shop. Open to walkers and passing cyclists, horse riders and drivers, Blue Tin seems to be almost in the middle of nowhere. Approximately 4km by road from Nuffield, Blue Tin can be found just off a single lane country road. You will know when you actually arrive at the farm because of the sign and the box advertising fresh eggs (complete with honesty box for when the shop is closed).

We first came across Blue Tin (when it was a shop in a shed rather than a shop/café in a building) while walking in the area. There are some good walks in the area which can be very pretty when the bluebells are out and so it is well worth combining a visit to the café with a walk in the Oxfordshire countryside. Despite only opening in December 2017, the café associated with Blue Tin has put a lot of effort into ensuring that their coffee is ethically sourced, great tasting and locally roasted. The coffee is roasted by Horsebox Coffee and is of course also available to purchase in the shop. The espresso based drinks use the Dark Horse espresso blend (though I wasn’t sure whether this was occasionally rotated when the seasonal espresso is available). We had an Americano and a latte. The Americano indeed had the chocolate notes described on the coffee bag, while the milk of the latte really complemented the espresso base. Cakes are also available though we didn’t try on this occasion.

Provenance Information Blue Tin, old board, no coffee info
On the wall of the cafe is a schematic showing where each product sold in the shop originates. Seen here is the earlier version that does not include the coffee (roasted within 5 miles). Everything is local and the meat is particularly local having been brought up in the farm next to the shop.

There is plenty of space to sit down inside and contemplate the shop while enjoying your coffee in this welcoming environment. The arrangement also gives you time to consider the farm and space for your mind to wander. One place my mind was wandering that day was to the importance of our beliefs in our decision making, even while they are informed by science.

For example, it is often said that we could significantly help climate change by becoming vegetarian or including one meat free day per week in our diet. In a 2013 paper, the authors calculated the emissions associated with farming, producing, packaging and storing 66 categories of food item that were sold in a (modelled) medium sized supermarket. In order to calculate this various assumptions about the produce had to be made¹.

To summarise the paper (though it is well worth taking the time to read it), the study suggested that avoiding meat altogether could reduce our individual carbon footprint due to food by 35%. However even introducing a meat free day (combined with a switch to poultry rather than beef and significant reduction in the amount of food we waste/packaging used) could introduce a reduction of 26%.

Horsebox coffee Oxfordshire
Americano, Latte and shop. The coffee-space at Blue Tin farm shop, Oxfordshire

Does the science therefore “say” that we should all go vegetarian? It is worth looking more closely at the paper and considering our own belief systems. In the report, beef had a total CO2 equivalent emissions of 25.13 Kg per kg of food¹. One suggestion of the authors was to swap beef for poultry. Poultry has a total emissions of 4.05 Kg CO2 equivalent per Kg. But looking through the table of calculated emissions for each food type, “spirits and liqueurs” had a total emissions of 3.16 Kg CO2e per Kg. Perhaps (hopefully) you could say you would take a long time to drink a kg of spirits, but even wine has 2.41 Kg CO2e per Kg. As a rough estimate, 1 bottle of wine (750 ml) is 3/4 Kg, so the bottle of wine with dinner is contributing roughly equivalently to the shared roast chicken. There are nutritional arguments for eating meat. Can we say the same of drinking wine? What of coffee? (not included in the table I’m relieved to say!)

The study shows the quantity of emissions associated with each type of food stuff. It does not show us how to act. Each decision we make (eliminate meat/go teetotal/all things in moderation) is based on what we believe about the world. Even the idea that it would be a good thing to reduce our greenhouse gas emissions in order to try to limit climate change is a moral one, not a scientific one. These decisions depend on what we think is ‘good’, what is ‘bad’, what life is about. In short it depends on our beliefs about the world and our place in it rather than purely the facts. These are philosophical, or dare I say it, even religious questions. Science can inform us of the damage that we are doing but it cannot help us to decide whether that matters, nor even if it does matter, what we should do about it.

Interior of Blue Tin with flower
You won’t need the sugar. Another view of the cafe at Blue Tin.

Whether we decide to buy better quality² meat less frequently, go vegan or even do nothing are not decisions that we are making entirely based on the science. Although informed by the science they are nonetheless based also on our existing beliefs about the way the world is and the way the world should be. We may decide for example that, while we should reduce our consumption of “cheap” beef/imported lamb, we should take care to buy more expensive meat from somewhere that takes care of their animals throughout the farming (and slaughtering) process but eat it less often. Depending on the rest of our diet, this could similarly reduce our food-related greenhouse gas emissions (if for example we mostly ate vegetarian with occasional meat consumption). We may similarly decide that eating meat is intrinsically wrong and so go entirely vegetarian.

These are choices we make, informed by the science but based on our ideas of morality. They are not easy choices nor are they choices we will necessarily agree with each other about. To make these choices requires time set aside to think about what matters to us, about what we believe in. And this means that there is no intrinsic conflict between science and religious belief any more than there is a conflict between science and the decisions we make in general. We need science to inform our beliefs but we need also to recognise the role of our beliefs (conscious or subconscious) in our decision making.

In short, we need to slow down, pause and really think about things. And where better to do so than in quiet and comfortable places with good coffee (and cake) such as Blue Tin?

 

¹Interestingly, the authors assumed that for beef, the effective emissions were increased owing to deforestation and land clearance that is associated with beef production in some places. If this is not accounted for, the effective emissions from beef are still calculated to dwarf much of the other food stuffs but not by quite so much. The footprint would therefore be less for beef raised on an existing local form, slaughtered and purchased locally. Similarly, the ‘vegetarian’ diet that was modelled by the authors contained dairy (with similarly high emissions as beef).

²Again ‘quality’ is a subjective term that cannot be easily ‘scientifically’ quantified, instead it is argued based on what we believe ‘good’ means. Do I mean that the animal had healthy conditions during life? Was well cared for? That it tastes good? That I could speak to the people who farmed the animals? I’ve left it deliberately slightly ambiguous here.

Blue Tin Produce is at Garsons Farm, Ipsden, OX10 6QU

Categories
General Observations

Coffee (beans) in the blood?

Brazil nut effect
A green bean ‘floating’ in coffee grounds. When you pour your beans into your grinder, do they behave like a liquid flow or do they have their own type of ‘granular’ flow?

When you first learn about liquids, solids and gases, you may learn about the fact that a solid keeps its shape whereas a liquid flows. A solid is rigid and can be moved as one block whereas a liquid will spread and change shape. Solids can be stacked up like bricks though this is not true of liquids.

A slightly unfair question is then put to you. What about sand? (Or, in the context of this website, what about coffee beans?). A pile of beans will initially stack but as the pile builds, avalanches will occur to prevent the tower being too vertical. When you pour your beans into your grinder hopper, the beans will level out, in much the same way as the eventual coffee will in the cup. Do the collection of coffee beans move more as if they are a liquid or a solid?

Clearly to some extent the question is wrong, the beans represent their own class of structure but perhaps a better way of asking the question would be, how do a collection of coffee beans flow? It is a question with consequences beyond the coffee hopper. From pharmaceuticals to civil engineering projects and beyond, understanding how granular materials flow is an important topic.

Beans on a plate. The aspect ratio of the coffee bean is similar to that of the particles used in a new study to analyse granular flow.

And yet it has apparently been difficult to analyse this problem owing to the difficulty in tracking individual coffee beans (tablets or particles of cement) as they are pushed in one direction or another. A start was made nearly 20 years ago when a team at the University of Chicago used Magnetic Resonance Imaging (MRI, yes, the same MRI as you get in hospitals) to image individual mustard and poppy seeds as they flowed between two cylinders. The imaging allowed researchers to track the position and velocity and packing density of the seeds as they moved around the cylinders. Then, last year a new study used X-ray tomography to watch individual particles in a rectangular box as they were subjected to being pushed at various pressures in different directions. This, more recent study used plastic ellipses with a minor axis of 6.35mm and an aspect ratio of 1.5. Sadly, not real coffee beans but a fairly large plastic equivalent. While the aspect ratio will of course vary from varietal to varietal and even bean to bean, the coffee beans in my hopper at the moment have an aspect ratio of 1.3 (and a minor axis of 4.5mm) which makes them pretty close to the plastic used in the study.

Brew&Bread, latte art Sun, KL latte art
The structures in milk allow the milk to be ‘frothed’ and so enable latte art. They also make milk an example of a complex fluid.

By tracking each bean, the study discovered that such granular collections moved as if they were “complex fluids”. Which is all very well but does makes you wonder, what is a complex fluid? Is coffee a complex fluid?

Does the definition help? The definition on the Physics (APS) website says that: complex fluids “can be considered homogeneous at the macroscopic (or bulk) scale, but are disordered at the “microscopic” scale, and possess structure at an intermediate scale.”. What does that mean? Well, it seems to mean that complex fluids contain things that are larger than the molecules that make up the liquid and so affect how the fluid flows. Milk has long chains of proteins and fats (which give it the foam like qualities when it is frothed in a cappuccino) and so is a complex fluid. Chocolate and blood are other complex fluids as are emulsions and gels. Pure water would not be a complex fluid and my guess is that coffee (which contains water molecules and various molecules associated with the coffee itself) is also not a complex fluid. Were you to have a latte or a cortado though, the milk would transform your coffee into a complex fluid. Although I much prefer to keep my coffee simple, it would seem that there is more to the saying “you have coffee in your blood” than it would at first appear, particularly as regards the coffee beans. It may be time for some experimental tests of coffee bean (and coffee or latte liquid) flow….

Categories
Coffee review General Observations Science history Tea

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.

 

Categories
General Home experiments Observations Science history slow Tea

A tense moment for a coffee…

capillary bridge
A bridge formed by water between a cup and a cafetière.

Each and every coffee represents an opportunity to uncover an unusual bit of science. Sometimes the connections between what happens in your cup and the wider world are fairly obvious (e.g. the steam above your coffee and cloud formation), but sometimes the connections seem a little more obscure. On occasion, your observations may lead to philosophical speculations or stories from history. Every coffee is an opportunity to discover something, if you just slow down and ponder enough.

It was with this in mind that I looked at my freshly made French Press coffee a few weeks ago. I had positioned my cup very close to the cafetière such that a small water bridge had formed between the cup and the cafetière (see photo). Such “capillary bridges” have been studied for a couple of centuries and yet there is still more work to do. Caused by the surface tension of the water, understanding the way these bridges form and the shape of the surfaces produced is important for fields such as printing and powder processing. Yet it is only in the last 150 years or so that we have started to understand what surface tension is. Moreover, much of the pioneering work on this subject was done by an amateur scientist who just noticed things (and then designed some very clever experiments to discover more).

Agnes Pockels (1862-1935) is now regarded as a surface science pioneer but in 1891 she was a complete unknown. Although she had wanted to study physics, she was prevented from going to university because she was female. Consequently, all her study of the subject had to be through her brother Friedrich’s books and letters. It is not known what prompted her investigations but from 1880 she had been experimenting with a device to measure the surface tension of water. The device used a sliding weight to measure the force required to pull a 6mm diameter wooden disk off of the surface of a trough of water.¹ The design of this device was so successful that, a few years later, Irvine Langmuir adapted it slightly in order to study the surface of oils. He went on to receive the Nobel Prize for his work in 1932. Yet it is a device that could also be built in your kitchen, exactly as Agnes Pockels did².

reflections, surface tension
The effects of surface tension can be seen in the light reflected from a coffee

Pockels measured the surface tension of water contaminated by oil, alcohol, sugar, wax, soda crystals and salt (amongst other things)¹. She discovered how the surface tension of the water could be affected by pulling the surface or introducing metal objects onto it. She discovered the “compensating flows” that occurred between regions of different surface tension (you can see a similar effect with this soap boat). Yet all of this remained hidden from the wider world because Pockels was unable to publish. Not having access to the contemporary literature about surface tension and moreover unknown, unqualified and female, no journal would look at her work let alone publish it. Nonetheless, she was clearly a brilliant experimentalist and capable physicist.

Things changed when Pockels read a paper by John William Strutt (Lord Rayleigh) in about 1890. Rayleigh was quite the opposite of the unknown Pockels. As well as his work on sound, electricity and magnetism and the (co-) discovery of Argon, Rayleigh is known for his work on understanding why the sky is blue. (Which is another phenomenon that you can see while preparing your coffee if you drink your coffee with milk.) In his paper on surface tension, Rayleigh had come to similar conclusions as Pockels’ work but Pockels had gone further. Unable to publish herself, she instead wrote to Rayleigh, in German, detailing her experimental technique and results. Rayleigh responded by forwarding her letter to the scientific journal Nature together with an introductory paragraph:

“I shall be obliged if you can find space for the accompanying translation of an interesting letter which I have received from a German lady, who with very homely appliances has arrived at valuable results respecting the behaviour of contaminated water surfaces. The earlier part of Miss Pockels’ letter covers nearly the same ground as some of my own recent work, and in the main harmonizes with it. The later sections seem to me very suggestive, raising, if they do not fully answer, many important questions. I hope soon to find opportunity for repeating some of Miss Pockels’ experiments.”¹

Coffee Corona
You may have seen white mists form over the surface of your coffee (seen here by the rainbow effect around the light reflection). But what are they and how do they form? This is still not really known.

Rayleigh’s introduction and Agnes Pockels’ letter were published in Nature on 12 March 1891. The paper enabled Pockels to publish further results in both Science and Nature as well as in other journals. In 1932 she received an honorary doctorate in recognition of her work.

It seems that this coffee-science story has two main messages. The first is to emphasise how much we gain by ensuring everyone has access (and encouragement) to study physics (or indeed whatever subject they are motivated by). What would we have lost if Agnes Pockels had not had the books of her brother and made the decision to write to Rayleigh? But the second message is that Agnes Pockels managed all this, at least initially, by merely noticing what was going on in the liquids around her. Being curious she designed and built a piece of equipment that enabled her to measure what she was intrigued by and by taking a systematic series of data she discovered physics that was unknown to the wider community at the time. So the question is, what do you notice when you look at your coffee? How does it work, what can you discover?

Please do share any interesting physics that you see in (or around) your coffee either here in the comments section below, on Facebook or on Twitter. Tea comments would also be welcome, but whatever you do, slow down and notice it.

 

¹Rayleigh, Nature 1891, 43, 437-439, 12 March 1891 (full text here)

²Reference to the kitchen is here.

Categories
General Home experiments Observations Science history

A lawyer, an accountant and and emperor walk into a cafe…

Strata, geology
This is not a resonance in a coffee cup but the concentric circle pattern is similar to a resonance that you could frequently see.

Have you ever noticed concentric rings on the surface of your coffee, forming as the table under the coffee cup vibrates slightly? Perhaps you have seen more complicated patterns. You may have observed, as you have played with your coffee, that some patterns are more stable than others. The one that is formed from concentric circles is fairly easy to form and to see. A more complex one looks like a chequer board, you may perhaps of seen others. These patterns are what are known as ‘resonances’ on the surface of the coffee and they are the consequence of standing waves being set up on the coffee surface. Many people who have gone through an undergraduate physics degree will immediately be reminded of Chladni figures and there is a good reason for this. Ernst Chladni (1756 – 1827) was a pioneer in investigating such resonances, one of the reasons that he has been described as “the father of experimental acoustics”.

And yet Chladni was not a physicist in the way that we now think of the term. In fact, by training he was a lawyer, a consequence of following his father’s rather insistent ‘advice’. Obediently, Chladni had trained in law and had started working as a lawyer in 1782 when his father died. Chladni appears to have taken this event as an opportunity to start to investigate the scientific problems that he was actually interested in and so re-invented himself as an acoustician testing the theories of music developed by people like Bernoulli and Euler¹.

transmission lines, electrical noise
Like strings on a guitar. Resonances on a string can be used to make musical notes.

Did Chladni drink coffee in eighteenth century coffee houses while admiring the resonances in the cup? Sadly what comes down to us in history is not his coffee habit but his experiments with sand covered metal plates secured onto wooden rods. Chladni caused resonances on these plates by rubbing them with a violin bow. By exciting resonances similar to those you can see on the surface of your coffee, Chladni was able to test theories about the sounds made by curved metal surfaces (e.g. bells). Indeed, these experiments became so important to understanding acoustic theory that Chladni started a European tour demonstrating his plates and their relevance to designing musical instruments. It was presumably through one of these tours that he met an Emperor of the time, Napoleon Bonaparte.

But despite this great experimental progress, the mathematics used to understand these resonance patterns, was developed by another physicist with a non-typical career path, Friedrich Bessel (1784-1846). Bessel had trained as an accountant but with the good fortune of timing, he had apprenticed into an exports company. At this time, such companies would have been interested in the problem of longitude and so Bessel gained an opportunity to indulge his interest in astronomy. As a consequence of this work, particularly his work on the orbit of Halley’s comet, Bessel secured a job in an astronomical observatory and it was there that he started the work that would eventually lead us to be able to describe, mathematically, the resonances on the surface of your coffee.

Did Bessel drink coffee? Had he seen Chladni demonstrate his plates? We don’t know the answer to those questions and in many ways it is not relevant because Bessel’s mathematics did not concern such resonances at all. Instead, almost to underline the idea that everything is connected, particularly with physics and coffee, Bessel was working on the problem of how to calculate the gravitational attraction between multiple objects.

Kettle drum at Amoret
The note made by a drum is a function of the size and shape (therefore resonance pattern) of the drum and also the gas filling the drum. Would this drum-table sound the same if banged on Venus as on Earth?

Perhaps you remember from school Newton’s famous description of the gravitational attraction between two bodies as being F = GMm/r² (where F is the force, G the gravitational constant, M and m the masses of the two bodies and r the distance between them). That’s all very good but what if there were three bodies, or four, or…

It was this problem that Bessel was working on and by so doing he solved the problem of Chladni’s patterns. The maths that describes the many body problem also describes the way that these resonances form. Those patterns in your coffee are described by the same maths as allows us to calculate complex gravitational problems.

And so perhaps it is not quite correct to title this post as a lawyer, an accountant and an emperor walk into a café, but it would be fair to say that each time you catch those resonances in your coffee cup, the  influence and interests of these investigators of nature are infused within the brew.

You can find a sketch of Chladni entertaining Bonaparte with his metal plates here.

¹Harmonius Triads, Physicists, musicians and instrument makers in nineteenth century Germany, MIT Press, 2006

 

Categories
General Home experiments Observations Tea

Freezing point

coffee and ice in New Cross on a wooden table
Isn’t it a fact that water boils at 100C and freezes at 0C?

Water boils at 100ºC and the ice in your iced latte is at 0ºC. These are facts that we think we know about water: it boils at 100ºC and it melts at 0ºC. A sharp observer may point out that these are pressure dependent and that if we were at the top of a mountain, the water would boil at a slightly lower temperature (I once had a student argue that this was a good reason to only ever drink green tea at high altitude). But if we are at ground level and it is a normal day, we will be fairly certain that the water for our coffees would boil at 100ºC and ice would form at 0ºC.

Yet these ‘facts’ hide some complicated physics and some oddities about our planet. Pure water, that is, water without any impurities in a clean vessel (such as a clean, scratch free glass) does not boil at 100ºC but at temperatures significantly higher than that. Nor does pure water freeze at 0ºC but at temperatures significantly below that. These are phenomena known as superheating and supercooling respectively and, if you are observant, you could see them occasionally in your coffee cup. To see why, and how, we need to think a bit more about how water freezes.

blue tits, mint water, mint infusion, mint leaves in water
If you put pure water into the freezer, you may find that it freezes at a temperature considerably lower than 0C

If you fill an ice cube tray with water and put it in your freezer, you would expect ice cubes to start forming at about 0ºC. We expect the freezing temperature to be the same as the melting temperature, that is the temperature at which the ice cubes would melt. And yet, if you make the water very pure (even distilled water would be a start) and put that in a clean, defect free container (such as a clean glass jar) in the freezer, the freezing process will not begin until much lower temperatures. It’s because the water has to crystallise and change state from a liquid to a solid and to start this process, there needs to be a seed, a surface on which the ice can form. Called a “nucleation site”, this seed could be a piece of dust, a small impurity in the water, a scratch on the surface of the container holding the water, or in fact anything that allows the bonds of ice to start to form. The same is true at the other end of the temperature scale. When the liquid water turns into steam, nucleation sites are needed so that the gas bubbles can start to form at those sites. In the absence of impurities in the water, the water will not boil until temperatures high above 100ºC.

Fortunately in tap water, or in your super-filtered water that you make your coffee with, there are plenty of such nucleation sites so the water boils and freezes at roughly the temperatures you’d expect them to. The same is not true however for clouds in the sky where some (high altitude) clouds have been shown to contain water droplets that are at -35ºC, well below the “freezing temperature”. Exactly why this occurs is still puzzling and a topic of research, but when you stop and think about it, how would you actually measure this temperature? If you supercooled a cup of water and then put a thermometer into it, the thermometer would provide a nucleation site and the water would immediately freeze. How can you measure the water’s temperature without a thermometer?

kettle, V60, spout, pourover, v60 preparation
You are unlikely to see superheating when you boil the water for your coffee in a kettle like this.

Recently a study reported in Physical Review Letters used a laser to measure the diameter of a series of supercooled liquid droplets by determining the energy of a resonance that depended on the droplet’s size. To calculate the temperature of the droplet, the authors then used the principle that as water evaporates, the droplet from which it is evaporating will become colder at the same time that it shrinks in size. Measuring the size of the droplet allowed them to calculate the evaporative loss and therefore the temperature of the drop. They double checked this new technique by measuring (with the same laser) the energy of a particular atomic bond in water that has a known temperature dependence (at higher temperatures). The temperature determined from the drop’s size corresponded with the extrapolation of the energy of this atomic bond and so the team were fairly confident that they had measured liquid water to very cold temperatures indeed. In fact, the authors suggested that it was still possible to have liquid water at 230.6±0.6 K which, in more every-day units corresponds to -42.55ºC, well below the nominal ‘freezing point’.

So pure, liquid, water can get very cold indeed. But could you ever see this in your coffee cup? Although you may like to try some experiments with freezing ultra-pure water, it is easier to see the phenomenon of superheating in your coffee. However, given the possibility of an accident, it may be safer to watch the effect on the video below. The idea is that if you put very pure water in a clean cup into a microwave, it is possible to superheat it well above 100ºC without it boiling, because there are no nucleation sites in the cup or the water on which the steam bubbles could start to form. When you take the cup out and put a nucleation site in (perhaps a spoon or maybe even instant coffee granules), the water will boil suddenly as a result of those new nucleation sites and can even explode. Obviously if you were anywhere near the water when this happened you could get seriously burnt and so it is probably safer to watch the Mythbusters do it with their robotic arm. Enjoy the video, enjoy your coffee, preferably far from superheated:

 

 

 

Categories
Coffee review Coffee Roasters General Observations slow Sustainability/environmental

Seeing the light at Redemption Roasters

Coffee Bloomsbury
Redemption Roasters Cafe on Lamb’s Conduit St.

At the top end of Lamb’s Conduit Street there is an unassuming café in a fairly modern building at the corner of Long Yard. In recent weeks I had been hearing a lot about Redemption Roasters and their café. First came the review by Double Skinny Macchiato, then various comments on Twitter, in Caffeine magazine and finally, an article in the FT. In an ideal world, it seems to me that cafés can act as seeds towards forming a better society. Local and independent, a friendly place where you can chat with the baristas (or café owners), and so where communities can form and develop. All that I had heard about Redemption Roasters café fitted, in some way, into this ideal which meant that it was not going to be long before I headed towards Bloomsbury and tried this new café.

Plenty of seating could be found inside the café, with tables of two or four and benches around the space. The counter was immediately in front of us as we went through the door and the friendly barista took our order (long black and soya hot chocolate, what else?) while we took our seats. There were a fair few staff in the café when we visited, so many in fact that we weren’t initially sure who were staff and who were customers. Nonetheless, their joviality transformed the café’s fairly austere decor more into the feel of the welcoming space of a living room.

blue shadow, hot chocolate
A layered hot chocolate? No, just the reflection of the saucer in the glass.

Having taken our seats and started to look around, we found that much could be said about the science in this café. From the SMEG refrigerator and individual radiators to the light reflection off individual sugar crystals in a glass on the table. Moreover, when our drinks arrived, the reflection of the (blue) saucer in the hot chocolate glass made it appear as if the hot chocolate were layered. In fact it was an optical illusion caused by the way our minds process the colour blue in shadows, more on that in this great article about colour, Goethe and Turner. But it was to a different lighting effect that my thought train eventually turned. Above the counter are a series of hanging lights with angular shades over them. Above our table were LED bulbs inset into the ceiling.

The way that the LEDs above us had been placed produced two shadows from the spoon on the saucer of my cup. A dark shadow and a light shadow at a slightly different angle. One reason that LEDs have caught on as a light source is that they are more efficient and so better environmentally and cheaper financially. So you may think that LEDs are one way of reducing our (collective) environmental footprint. But does this work? According to a study that measured the outdoor light levels around the world from 2012 to 2016, the answer is no. It would appear that while on a local level, people are enjoying cheaper lighting, on larger scales (nationally, globally), this decreased cost is leading to us installing more lights. Consequently, on the global scale, the area of land that is lit has increased by 2.2% per year with very few countries showing a reduction or even a stabilisation of the amount of outdoor areas that are lit.

shadows from a coffee Redemption Roasters
Determining a presence by noticing an absence. The two shadows of the spoon came from the light bulbs inset into the ceiling.

Does this matter? Well, it is something that is affecting us, the way we view our world and the wildlife that we share our planet with and so it is something that we ought to be thinking about. In brightly lit areas of the UK, trees have been shown to produce buds up to 7.5 days earlier than in darker areas. Artificial light is causing problems for nocturnal insects and animals, with knock on effects for crop pollination. And when was the last time you looked up at the sky on a clear night and saw seven of the Pleiades let alone the Milky Way? How does it change our psychology and philosophical outlook when we can no longer gaze at the night sky with wonder and without the glow of streetlights?

Some astronomers have called for increased shielding of street lighting as a way for us to both enjoy well lit streets and be able to enjoy looking up at the night sky. Shielding such as that over the lights over the counter at Redemption Roasters café, where the light is efficiently directed downwards rather than be allowed to escape into the sky. Small steps that can make a big difference. It is interesting to notice that around central London at least, many newer lampposts are more efficiently shielded than older ones. Pausing for a coffee in Redemption Roasters café is a great moment to consider this problem and your reaction to it. Have you stopped to gaze at the night sky recently?

After leaving the café, I realised I had lost an opportunity to notice something else. Frequently, after visiting a good café, I will look up the area in my London Encyclopaedia¹ to see whether there is anything of interest historically in the area of the café. As expected, Lamb’s Conduit St was named after a conduit made from a tributary of the river Fleet restored by one William Lamb in 1577. But Lamb also donated 120 buckets for poor women of the area to use for collecting their water, which explains the statue of a woman with an urn at the top of the street. However what was also mentioned was that at the entrance to Long Yard (ie. very close to Redemption Roasters) there is an ancient stone inset into a wall with a description about the Lamb’s Conduit. Somehow I missed this though Double Skinny Macchiato evidently found it. So if you do visit Redemption Roasters café, and I would very much recommend that you do, as well as taking some time to savour the coffee and to notice the surroundings, please do look out for this elusive stone and if you find it, do let me know.

¹The London Encyclopaedia, Weinreb, Hibbert, Keay and Keay, MacMillan, 2008

Redemption Roasters Cafe is at 84 Lamb’s Conduit St, WC1N 3LR

Categories
General Home experiments Observations Science history slow Sustainability/environmental Tea

Coffee and the world

Welcome to the first post of 2018, Happy New Year! But before embracing 2018, perhaps let’s take a moment to remember those things that we discovered in 2017 that connect your coffee cup (or brewing device) with the physics of what occurs in the wider universe. Here are some of the highlights for me this year, if you want to share your highlight, please comment in the section below.

latte art, flat white art
A properly made latte. But what if you add hot espresso to the milk instead of the other way around?

1) Latte layering

In mid-December a study was published in Nature Communications that explored the complex, but elegant, physics involved in making lattes (ok, not quite by the technique that you would hopefully find in your neighbourhood café but keep with this…). When a hot, low density, liquid (espresso) was poured into a hot higher density liquid (milk) contained within a cold mug, the competition between the density gradients of the liquid (vertical) and the temperature gradient from the cup wall to the liquids (horizontal) produced multiple layers of varying coffee/milk concentration in the cup. Too late for a 2017 Daily Grind article, this looks to be too good an experiment to pass by, hopefully it will appear on the Daily Grind in early 2018.

 

science in a V60
Could this V60 mystery now be solved?

2) Bouncing drops

November 2017 saw research published about what happens when a cold droplet falls onto a hot liquid (think milk and coffee). The temperature difference causes currents to be established within the droplet (and in the main liquid) that in turn create air flows between the droplet and the liquid bath that prevent the droplet from merging with the bath. The research can explain why it is that you can sometimes see raindrops staying as spheres of water on the top of puddles. It may also explain a puzzling phenomenon that I have seen while brewing coffee in a V60.

 

Vortex rings get everywhere.

3) Vortex rings in coffee

June 2017 and it is again about adding milk to coffee (why do I drink coffee black?). When one liquid (such as milk) is dripped into another (such as coffee), it is very likely that you will observe the milk to form “vortex rings”. These rings are related to smoke rings and have, in the past, been proposed as an atomic model. This year however it was suggested that these vortex rings could form as a type of magnetic nanostructure. Mathematically impressive, beautiful, perhaps quite useful and mathematically similar to something you can find in your coffee.

 

bloom on a v60
How do craters form?

4) Crater shapes

April 2017. What happens while brewing a pour over? As you drip water onto a granular bed (or, in coffee terms, ground coffee in a V60 filter), each drop will create a crater. The size and shape of the crater will depend on the density of the granular bed (espresso puck or loose grounds in a filter) and the velocity of the falling drop. Fast frame photography revealed how the shape of the crater changed with time for different scenarios.

 

Coffee bag genuinely home compostable
How it started.
The Roasting House bag before it went into the worm composter.

5) A home experiment

Perhaps not quite in the theme of the other four stories but this is an experiment that you can do at home. Some have proposed compostable coffee cups as a more environmentally conscious alternative to ordinary, disposable, coffee cups. But how “compostable” are compostable cups and compostable packaging? Between May and September 2017, #howlongtocompost looked at how long it took the Natureflex packaging (used by the coffee roasting company Roasting House for their ground coffee) to compost in a worm composting bin. This one worked quite well. Within 17 weeks, it had been eaten by the worms. In comparison, the “completely compostable” take away coffee cup is still in the worm bin (although considerably degraded) 37 weeks after the start of the experiment. If you are interested, you can follow #willitcompost on twitter. Will it finally compost? I’ll leave you to place your bets but you may decide that a link to Brian’s coffee spot guide to re-usable cups will be helpful.

 

What will 2018 bring? Certainly there will be more composting experiments as I have a coffee bean bag from Amoret coffee, 3 different compostable cups and a compostable “glass” to try with the worms. But in terms of the science? We’ll have to wait. Meanwhile, if you have a coffee-science highlight from 2017, please do share it either here in the comments section, on Twitter or on Facebook. Happy New Year to you all.

 

 

 

 

 

Categories
General Home experiments Observations slow Tea

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.

 

 

 

Categories
Allergy friendly Coffee review Coffee Roasters General Observations

Focussing the sound at Spike and Earl

soya latte ginger beer
Soya latte and a ginger beer at Spike and Earl.

A few months ago, news came that the coffee roasting company Old Spike had opened a new café, Spike and Earl, down in Camberwell. Operating on similar principles to Old Spike, Spike and Earl aims to serve excellent coffee (and food and cocktails) with a social conscience. By employing those who have previously been homeless, Spike and Earl offers an employment (and training) route for people who may not easily otherwise have the opportunity. So although Camberwell is a bit of a trek, I was looking forward to trying this new place. As it was a late afternoon in November and the menu suggested that the dairy alternatives were only soya or oat, I decided to try a soya latte. (For any reader with a nut allergy, the current fashion of using almond milk means that you should always ask first if your cappuccino contains nuts). The baristas were friendly and confident in assuring me that they do not use almond milk (no danger of nut-cross contamination) but that their brownies did contain nuts (so I sadly had to pass on the brownie opportunity). My partner in these café reviews opted for a ginger beer.

There were a series of high tables with stools on the left hand side of the café. Presumably many people can therefore be accommodated when it gets crowded. However, at the time of our visit, it was fairly empty and we made our way to the rear of the café. Behind us, and behind closed glass doors, was a coffee roaster that we later discovered was part of the Old Spike roasting expansion. It’s always a nice touch to see coffee roasting happening as you drink but perhaps we needed to arrive earlier for that.

Bricks with holes Spike and Earl
Holes in bricks at Spike and Earl. Just a foot-hold or a suggestion for a great piece of engineering?

Drinks arrived together with complementary water and the soya latte was very smooth. Almost caramel like in the sweetness and very drinkable. It makes a pleasant change to have a latte once in a while. Light was playing tricks around the room as the sun was setting and the inside lights were becoming more prominent. But the striking thing about Spike and Earl was that the bricks used to support the tables and line the walls all had holes in them. On the wall running along the side of the café, (windows were on the other side), pot plants were placed in the holes giving the impression of the beginnings of a green wall. The holes in the bricks supporting the table meanwhile made an excellent footstool and were complemented by holes in the stools. A latte of course is largely made up of holes, or at least bubbles. The foam structure consisting mostly of air. How is it that some structures can be made better owing to what they don’t contain rather than what they do?

For example, if you imagine the difference between a latte and a cappuccino (but made out of metal rather than milk) that can be the difference between a successful tooth implant and a failure. We know from our coffees that bubble size can have a significant structural effect. But how about more fundamental properties, can the holes in bricks change things such as the way sound propagates?

Interior wall at Spike and Earl
More bricks with holes at Spike and Earl, this time with some plants escaping from them. The start of a green wall?

You may have heard about how different structures can be engineered to make materials “invisible” to certain frequencies of light. Imaginatively named “invisibility cloaks” are made by designing materials with patterns on them that change the path of an incident light beam. Because the effect on the light beam is due to the structure in the material rather than purely from the material itself, these materials have become known as ‘meta-materials’. When you remember that microwaves are a form of light, it is perhaps easy to see some of the applications of this research and one reason that it has attracted a lot of funding.

However there is an acoustic type of metamaterial that is far more similar to the bricks in Spike and Earl and that may find applications in medical imaging (ultrasound). Earlier in 2017, a team from the universities of Sussex and Bristol published a study about acoustic metamaterial ‘bricks’. Each brick had a differently shaped hole through the centre of it which delayed the incident sound wave by a specific phase interval (you can say it ‘slowed’ the wave). In order to work efficiently, the brick had to be of a height equal to the wavelength that the researchers were interested in and a width equal to half that wavelength. As they were investigating ultrasound, the bricks were therefore about 4.3mm square and 8.66 mm high.

By assembling the bricks together, the researchers found that they could steer a focussed beam of sound or even change the shape of the sound beam. This would have applications as diverse as targeting cancer cells with ultrasound to levitating a polystyrene bead. You can read more about their research here (or, if you have access to Nature Communications, their paper can be downloaded here).

soya latte Spike and Earl
Layering at the end of my soya latte. What would you think about?

Just for fun, assuming that the bricks supporting the table at Spike and Earl could be similarly turned into acoustic metamaterials, we could calculate the musical note that they would best work with. Estimating the brick at about 15cm square and remembering that is approximately  half the wavelength (λ/2) and using the speed of sound in air to be 330 m/s, we can calculate the frequency to be:

f = c/λ

f = 330/0.3 = 1100 Hz

Which is the musical note C#6 (with an explanation of nomenclature here).

As I finished my soya latte, strata of milk lined the cup. Reminiscent of the Earth’s layers or perhaps, metaphorically, our strata of understanding, there is certainly plenty more to ponder at this interesting new(ish) addition to the London café scene. So next time you are in Spike and Earl, do let me know what you end up thinking about, you never know where these thought trains may take you.

Spike and Earl is at 31 Peckham Road, SE1 8UB