Category: Observations

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Coffee cup science Observations slow

Coffee & Contrails (I)

contrail, sunset
A set of criss-crossing contrails taken in the evening.

If you gaze up at the sky on a clear day, you will often see a few contrails tracing their way across the blue. Formed as a result of water in the atmosphere condensing onto exhaust particles from aeroplanes, contrails are a regular feature of the skies in our modern life. There are at least two ways that I can think of, in which the physics of the contrail is connected to the physics of the coffee cup, so, there will be two Daily Grind articles about them. This first one, about the physics of how we see them, and a second post (scheduled for 10th June) about interesting effects that we can see in them.

Perhaps now would be a good point to go and make a cup of coffee before coming back to this post. Make sure that you notice how the steam clouds form above the kettle spout as the water boils. Do you see the steam at the spout itself, or just a few centimetres above it? With the cup next to you, notice the steam rising above it. Does the steam seem more obvious on some days than others? For example, the coffee always seems to me to steam more on cold damp days in winter than on warm-ish days in late spring. Both of these observations (about where and when we see the steam clouds) are mirrored in the contrails, it’s time to take a closer look at the coffee.

V60 from Leyas
The clouds above a coffee cup are a rough indicator of the relative humidity.

The difference in the day to day visibility of the steam above the coffee cup is an indicator of the relative humidity of the atmosphere. If we prepare our cup of coffee on a day when the relative humidity is already high, adding that extra bit of water vapour from the cup leads to clouds of steam above the mug, as the water condenses into droplets of liquid water and forms clouds. If our coffee was instead prepared on a day with low relative humidity, the water vapour above the coffee cup is less likely to condense into clouds. Contrails are formed high in the atmosphere when the relative humidity is quite high. Exhaust particles from the engines of the plane offer a surface onto which the water in the surrounding (humid) atmosphere can condense to form clouds. We know that it is mostly the atmospheric moisture that is forming the contrails (rather than water from the exhaust itself) because of research done by NASA. In research flights, the amount of water vapour leaving the aeroplane engine was 1.7 grammes per metre of travel while the mass of water in the contrail was estimated to be between 20.7 and 41.2 kilograms per metre. This means that contrails can give a clue as to the weather: on dry days, contrails will not form because the water in the atmosphere is likely to remain a gas and therefore invisible to us, it is only when the air is already quite humid that contrails are likely to form and persist.

glass of milk, sky, Mie scattering
A glass of (diluted) milk can provide clues as to the colours of the clouds in the sky as well as the sky itself

Then there is the question of why we see them at all. Contrails appear as white clouds trailing behind the plane. We see them as white because of an optical effect caused by the size of the condensed droplets of water (actually ice) in the contrail. Objects appear as having different colours either as a result of light absorption by chemicals in the object (leaves are green because of chlorophyll) or as a result of light scattering from the object. A water droplet is colourless and so the colour we see coming from the droplet must be purely a consequence of light scattering rather than a light absorption effect. Clouds appear white because the water droplets within the cloud are as large, or larger than, the wavelength of visible light (0.7 μm). Droplets this size will scatter all wavelengths of visible light and so appear white. If the droplets were much smaller than the wavelength of light they would scatter different wavelengths by different amounts. It is because the atmosphere is full of such tiny particles (and molecules) that blue light is scattered more than red light in the atmosphere and so the sky appears blue to us from our vantage point on the Earth’s surface. Milk is composed of large fat droplets (which will scatter a white light) and smaller molecules which will preferentially scatter blue light, just as the sky. This is why you can mimic the colours of the sky in a glass of milk. It is because the water droplets have formed a few cm above the kettle spout that you can see them scattering the light. For exactly the same reason, the contrails in the sky appear as white clouds.

contrails
A hot air balloon in a sky full of contrails

Contrails can persist in the sky for anything from a few minutes to a few days. Just like clouds, contrails affect the way that light (and heat) is reflected from the Sun or back towards the Earth. However, unlike normal clouds they are entirely man-made, another factor that could have an unknown effect on our climate. A few years ago, a volcano eruption in Iceland caused the closure of UK airspace (as well as the airspace of much of Europe). I remember being in the queue to buy a cup of coffee in the physics department and hearing the excited conversation of two atmospheric physicists behind me. For the first time they were able to study some particular atmospheric effects without the influence of any contrails. In effect they could start to understand the influence of contrails by this unique opportunity of taking measurements during their absence. What was a major pain in the neck for so many travellers in 2010 meant a lot of extra (but presumably very interesting) work for them.

Coffee & Contrails (II) is about the structures you can sometimes see within the contrail. If you can think of any other connections between coffee and contrails (or coffee and clouds) why not let us know in the comments section below.

Categories
cafe with good nut knowledge Coffee review Observations Science history

Crystal Perfection at Workshop, Holborn

Workshop coffee Holborn
Diamonds are forever, Workshop coffee Holborn

The brand identifier of Workshop coffee is a diamond, a representation of which hangs on the wall as soon as you enter the Holborn branch. I had arrived at Workshop in order to try their coffee after I’d had a great espresso made with beans roasted by Workshop at Knockbox in Lamb’s Conduit Street. The coffee brewed in their own café certainly did not disappoint. I enjoyed a very good La Soledad filter coffee and a cake (which was confidently nut free, this brings me to another plus point for Workshop, they know the ingredients of their cakes!). The interior of the cafe, just beside Holborn Viaduct, is quite spacious and, if you sit at the back, you get a great view of the workings of the espresso machine as different people come in to get their ‘take out’ coffee. It is very possible to spend quite some time here in order to relax and enjoy your coffee while taking in your surroundings. To a physicist who studies materials (like me), the diamond logo of Workshop represents a fantastic material. A material in which the structure of the crystal determines so much about its properties. Were the carbon atoms in diamond bonded slightly differently, they would form the soft, pencil lead material ‘graphite’, rather than the hard, transparent material of diamond.

unit cell, repeating structure
The floor at Workshop reminds me of my crystallography text books.

Whether it was intentional or not, the crystal theme of the logo was replicated in the floor tiling of the Holborn branch. Crystallography is a branch of science that probes the building blocks of solids. It reveals how the atoms that make up different solids are arranged to form the solid. The atoms could be arranged in a simple cubic arrangement (as with salt) or hexagonally (as is the case for graphite). To establish the crystal structure you need to find the smallest repeating unit in the whole. Many introductory solid state physics or crystallography text books use 2D examples of repeating structures to help the student to understand how to build up these “unit cells” into full blown crystals. Many of the examples of such lattices look stunningly similar to the floor at Workshop.  Fundamentally, the idea of the crystal is that it is a simple repeating structure, just like the floor of Workshop. Indeed, the word “crystal” as used by Pythagoras implied perfection, harmony and beauty, a sense that is really conveyed by the crystal structure of the diamond logo of Workshop.

Crystal cake, LaFeSi cake, grape atoms
When a colleague left our lab, we made her a  cake that was a representation of part of the crystal structure of the material that she had worked on. Chocolate grapes and profiteroles represent different atoms in the structure.

The ancient Greek term for “crystal” actually implied the type of hard ice that is wonderfully clear and transparent. And it is ice that connects the area surrounding Workshop with a famous chemist who won a Nobel prize for his work in crystallography in 1962.  Max Perutz (1914-2002) described crystallography as a technique that “explains why diamond is hard and wax is soft, why graphite writes on paper and silk is strong”. Once you have enjoyed your coffee at Workshop, if you head down the stairs on the viaduct and descend to Farringdon Road you quickly get to Smithfield Market. It was here that, during the Second World War, Perutz helped to develop the material Pykrete. A “secret weapon” of World War II, Pykrete was developed five floors below Smithfield Market in a room cunningly disguised with animal carcasses. The planners in the war effort had wanted to design a boat made of ice but the problem was that when it was shot at, ice shattered. Could scientists develop a type of ice that would not shatter if it got hit by enemy fire? Pykrete was the answer. Pykrete uses the fact that materials such as plastics can be strengthened by adding fibres to them. In the case of Pykrete the “fibres” were sawdust and the material to be strengthened was ice. Not only does it not shatter when shot at (instead, the bullet creates a crater in the ‘boat’), it takes a lot longer to melt than ordinary ice. The sawdust encased in the ice acts to insulate the ice and increase its longevity.

Perutz’s Nobel prize was for his work to determine the crystal structure of haemoglobin, it took ‘just’ 25 years to do so. The field of crystallography continues to enrich our understanding of the behaviour of solids, though now we’re expected to get results more quickly than the 25 year time frame Perutz enjoyed. If you know of a good café where lots of physics goes on, or of a good café near a site of special (or unexpected) scientific interest, (or even just a good café) please do share your story either in the comments section below or by contacting me on email, Twitter or Facebook.

Workshop Holborn is at 60 Holborn Viaduct, EC1A 2FD

Quotes and other useful facts taken from:

“In our time, 29th November 2012: Crystallography“, (BBC Radio4)

Max Perutz “I wish I’d made you angry earlier” (2002),

Ichiro Sunagawa “Crystals, Growth Morphology and Perfection”, Cambridge University Press (2005)

Categories
Home experiments Observations slow

Patterns in a tea cup

light patterns on the bottom of a tea cup
Looking into my peppermint tea. Dancing filaments of light are just visible

Have I been unfair to tea drinkers? It has been pointed out to me on more than one occasion recently that tea is also a good source of science in a cup. So, last week, I drank a large amount of tea and started gazing into my (peppermint) tea cup. I watched as dancing lines of light played on the bottom of the cup. Never staying in one position for long, the filaments moved around, snaking across the tea cup. You can possibly see them in the picture on the right, although you would get a better view of them if you watched them dance yourself in a cup of freshly made tea. Similar lines can often be seen at the bottom of the swimming pool. Such lines of light must be caused by something in the water (or tea) bending the light from the surface into concentrated patches on the bottom. But are the two effects, though visually similar, caused by the same thing? And, what can this possibly have to do with forensic science and drug dealers?

straw, water, glass
When light travels from one medium to another (e.g. air to water) it gets bent by refraction

When light passes from air into a transparent medium (eg. into tea) it gets ‘bent’, in a process called refraction. This is why a spoon (or straw) put into a glass of water looks bent when viewed from the side (see picture). The amount that the light bends is dependent on the angle at which it hits the tea surface and by the density of the tea. The fact that you have to be able to see the bottom of the cup to see this effect, makes tea ideal for viewing it. (If your coffee is transparent enough to view these dancing lines of light, you may well want to check that you are brewing it correctly).

I’m not an optics person but it strikes me that there are at least two easy ways for these light patterns to form. Firstly, small waves on the surface of the water/tea will cause the light hitting the waves to be refracted by different angles as they go through the water. The patterns that form on the bottom of the pool/cup will therefore move with the waves. It is easy to see how such waves could form in a swimming pool, it is not so easy to imagine them in a tea cup. A second way to form these patterns is if the light is refracted through regions of different density, such as slightly hotter and slightly cooler tea. Such regions will occur in a tea cup because the tea is being cooled at the surface by contact with cool air and so there will be a continuous convection process in the cup. Warm water is less dense than cold water* and so will refract light slightly less than cold water will. Consequently, as the slightly cooler and slightly warmer regions of tea bend the light by slightly different amounts you should see patterns forming on the bottom of the cup as different amounts of light get to the bottom at each point.

So we have two possible causes for the light patterns on the bottom of a tea cup. How could we distinguish between them? Perhaps it would be an idea to get two identical cups, one filled with cold water, one with hot water (or a clear tea such as peppermint). Which shows the dancing filaments? Both of them, neither of them? Another experiment could be to observe the filaments in a cup of hot tea and then wait for the tea to cool. Do the light patterns fade as the tea cools?

tea pot science
Not always coffee. Tea can be interesting too.

The link to forensic science comes from the fact that light passing through transparent substances of different density will be ‘bent’ by different amounts. Imagine a drug dealer has been caught with some illegal substance wrapped in cling film. Although it looks to us like any other piece of cling film, that piece of film has been made in a specific factory at a specific time. This means that the roll of cling film that this piece was taken from will share variations in thickness and density with the cling film wrap. A type of cling film ‘finger print’. The density variation in the cling film can be photographed with a technique called the Schlieren photograph which exploits the fact that the light is refracted by varying amounts as it passes through these varying densities. If the police can get hold of the cling film in the suspected drug dealers home, this too can be imaged. If the ‘finger prints’ (changes in density etc.) of the two samples of cling film match, the suspect may be in significant trouble. The motto of this: Ensure that you have a decoy roll of cling film to hand before wrapping anything or, what is probably much better, spend time contemplating your tea in a café instead.

What do you think causes these patterns? What do your experiments reveal? Comments always welcome, please leave them in the box below.

 

* Between 0-4ºC the density of water decreases with decreasing temperature. For the purposes of this blog article it is assumed that you are drinking normal tea at around 60ºC rather than ice tea.

Categories
Coffee review General Observations Science history

Molecular reality at the Turkish Deli, Borough

“Just as the air is more dense at sea-level than on a mountain top, so the granules of an emulsion, whatever may be their initial distribution, will attain a permanent state where the concentration will go on diminishing as a function of the height from the lower layers, and the law of rarefaction will be the same as for the air” (Jean Perrin)

Turkish Deli, Turkish Coffee
The Turkish Deli, Borough Market

I have long had a fascination for the history of coffee and the different styles of brew. So it should be no surprise that I went to try The Turkish Deli in Borough Market for the Daily Grind. Very close to Monmouth, the Turkish Deli serves Turkish-style coffee and a delicious looking array of Turkish delights. Although quite far from the brew bars and single estate coffee types of some cafés now in London, Turkish coffee nonetheless offers the opportunity to slow down and appreciate the moment. Perhaps even more so than an espresso, since you are forced to wait for the coffee to be ready. The coffee is presented to you, straight from the Ibrik, in a small cup with a fantastic looking crema on top of it. At this point you are told that you will have to let it settle for at least four minutes before even thinking about starting to drink it. Indeed, the person in front of me in the queue was advised that he could “sit down, watch the world go by” while waiting for the crema on the coffee to turn a very dark (black) colour, indicating that the coffee was finally ready.

before settling, Turkish coffee
Waiting for the coffee to be ready

If you take sugar in your coffee you have to add it right at the start, before the coffee is warmed to the point of boiling (though it is not boiled). The reason is fairly obvious if you think about it. Turkish coffee has a large amount of sediment, this is the reason that you need to leave it for four minutes for the sediment to ‘settle’. Adding sugar during this settling time would mean that you would need to stir the coffee which would disturb the sediment and prevent it from quickly settling. Instead, you either take your coffee sugar-less or you add your sugar before starting this settling process.

Jean Perrin, (author of the quote at the start of this week’s Daily Grind) used the gradient of sediment in a different liquid (gamboge – a bright yellow paint pigment) to confirm the existence of molecules, just over one hundred years ago. He was exploring Brownian motion, the seemingly random motion of bits of dust, sediment etc, on the top of the coffee cup which had been explained in terms of “molecules” in the coffee (or water, or paint), hitting the bits of dust on the surface. Jean Perrin (1870-1942), realised that if Brownian motion was being caused by molecules, they would not just be causing the movement of the dust (and sediment) on the surface, it would be a three dimensional effect. Measuring the gradient of sedimentation would be a way to prove the molecular theory of Brownian motion and, simultaneously, to prove the existence of molecules.

Turkish coffee
The surface of the coffee reminded me of a coastline, itself connected (mathematically) to Brownian motion

Imagine a bit of sediment in the middle of the liquid (it could be a Turkish coffee, for Perrin it was the paint). That piece of sediment is going to be pulled down by gravity but in addition, it is going to be pushed up by molecules from below and down by molecules in layers above it. This is the bit that is related to Brownian motion. We know that even after leaving it for a long time, much of the sediment is still suspended mid-way up in the cup. It follows that the total forces acting downwards on the sediment (from gravity and the molecules above it) must be the same as the total force acting upwards (from the molecules below).

This means that the mass of sediment held at any particular level in the coffee must decrease with height. If the size of each piece of sediment is identical (which was ensured by Jean Perrin in his paint but is not the case for the Turkish coffee), then the number of pieces of sediment held aloft in the coffee/paint would decrease with height from the bottom to the top. All Perrin had to do therefore was to count (with a microscope) the number of bits of sediment as a function of height in order to test whether the molecular theory for Brownian motion was correct.

Turkish coffee, Borough market, sedimentary, sedimentation
The sediment at the bottom of the cup, don’t drink this bit!

To obtain statistics, Perrin and his assistants would count 11000 particles in one emulsion and repeat this experiment 1000s of times, but his patience paid off. By 1910, (only a few years after starting his observations), Perrin could claim that “the molecular theory of the Brownian movement can be regarded as experimentally established, and, at the same time, it becomes very difficult to deny the objective reality of molecules”. In 1926 he received the Nobel prize in recognition of this work.

Returning to the coffee, it is a very good drink with which to slow down and watch the world go by, perhaps while pondering molecular reality. When you get towards the bottom, do not drink the sediment but do take time to appreciate the mouthfeel and flavour as you drink this beverage that, in many ways represents an early chapter in the coffee story and one that continues to be made very well at the Turkish Deli.

The Turkish Deli is in Borough Market, Stoney Street, London, SE1 9AA

Quotes taken from “Brownian Movement and Molecular Reality”, Jean Perrin, 1910

Categories
Coffee Roasters Home experiments Observations

Coffee bean degassing

coffee, Roast House
Coffee from the Roasting House, one light roasted one dark roasted. They were roasted within an hour of each other.

How long do freshly roasted coffee beans take to  degas? Should you let the beans lose the carbon dioxide inside them for 24h, 72h, one week, more? Do dark roasted beans degas for fewer days than light roasted beans? As readers of Bean Thinking will hopefully know, one of the aims of Bean Thinking is to bring science, and particularly experimental science, onto everybody’s coffee tables. Is there an experiment (or experiments) that you can do to measure the amount, and duration, of degassing with equipment that you will have in your kitchen?

To help me in my coffee bean degassing experiments, I got in contact with the very helpful people at Roasting House. Based in Nottingham (UK) they will deliver freshly roasted beans to you by bicycle if you live in the Nottingham area or, for the rest of us, by Royal Mail. Together with the cycling aspect of their business, they also have a commitment to supporting those people who produce the coffee. It is important I think, not just that coffee tastes good, but that everybody involved in the coffee process (from grower to consumer inclusive) gets a good deal. Lastly, and very importantly for the degassing experiment, Roasting House offer their beans roasted to the degree that you specify. While they helpfully recommend a particular style of roasting for each bean (dark roast for one bean type, a lighter roast for another), they do give you the option of choosing which you would prefer.

They are also very knowledgeable about their coffee. As I was discussing the degassing issue with them, they suggested a coffee (Daterra, Bourbon Yellow) that they thought would degas quite a lot. Not just that, but the coffee concerned would taste great as both a dark and a light roast (I do drink the coffee after all). All in all, this experiment could not have been done without the help and input from Roasting House and I am very grateful to them for their support in my little project. So, onto the experiment.

The Experiment:

water acidification via coffee beans
Red cabbage liquid approx 96h after roasting and then being sealed in a jar with coffee beans. Note the colours.

To discover the time period over which the beans degas, I decided to utilise an effect that (for reasons unconnected to coffee beans) is currently having an alarming environmental effect: the acidification of water by carbon dioxide. Carbon dioxide dissolves in water to form carbonic acid. With the rising atmospheric levels of CO2, this is leading to ocean acidification, which is another factor in the “global weirding” phenomenon. For the degassing experiment however, if the roasted beans are sealed in a jar with some water, appreciable CO2 degassing will lead to the water becoming acidic, something that is easily measurable.

Experiment 1 – Red Cabbage, Do the coffee beans really degas CO2?

red cabbage, acidity, indicator, natural indicator, coffee bean degassing
The colours of the red cabbage liquid on tissue. Control sample is on the left, light roast in the middle, dark roast on the right

An acidity indicator that you may well have in your kitchen is red cabbage. Liquid extracted from red cabbage is initially purple but will turn blue in the presence of an alkali or red if it is exposed to an acid. For the experiment, three (identical) jars were prepared each containing 60 ml of red cabbage indicator and three (identical) shot glasses. Each shot glass contained either 10g of dark roast, 10g of light roast or nothing (as a control). The coffee beans were kept dry and out of the water by placing them in the shot glasses. The jars were closed, sealed with sellotape and then left. On opening the jars, (approximately 96h after roasting) the two that had contained the coffee beans had turned red (indicating acidity) while the control jar remained purple – see pictures. It is a pretty way of showing the acidification of the water by carbon dioxide and confirms that the beans are degassing. To establish the duration of degassing, it would be necessary to refresh the red cabbage liquid and measure for a further period of time.

Experiment 2 – testing the pH more systematically.

I headed off to a pet shop to get a pH indicator used by people who keep fish (Nutrafin Test). As with experiment 1, the coffee (10g) was sealed in jars (with 30 ml of water) together with a control. When the jars were first opened (at the same time as the red cabbage jars), the jars containing the coffee showed really low (acidic) pH values (approx 6.0 – 6.5). The control water was neutral or slightly alkali (approx 7.5). The water in each jar was then emptied, the jar rinsed and the water replaced with 30 ml of fresh water which was then sealed in the jar, again for 48h. The picture below shows the evolution of the pH with time (measured as hours after roasting) for the jars containing both roasts. The jar containing the dark roast showed a reduced acidity by 192h (8 days) after roasting (the test tube in the picture is greener), compared with about 288h (12 days) for the light roast. Even after this amount of time however, the water was still becoming slightly more acidic than the control, indicating that the beans were still degassing a little.

pH testing, coffee bean degassing
Testing the pH of the water exposed to the coffee bean degassing. The light roasted beans are on the top row, the dark roast on the bottom row. The ‘hours’ is the number of hours after roasting. The pH is measured by comparing the colour of the liquid in the tube to the colour chart.

Experiment 3 – using a bubble system to ‘catch’ the CO2

A third experiment to try to ‘catch’ the CO2 degassing from the beans (in an adaptation of this experiment) sadly did not work on either occasion that I tried it. If you try it and get it to work with with equipment that you can find around the house, please let me know via the comments section below.

Conclusions:

The coffee tried here, Daterra, Bourbon Yellow, degassed significantly for 6 days after the roasting date. The time over which the beans degassed, was dependent on the roast type, with the dark roast degassing for less time, consistent with the thoughts expressed here. Degassing certainly continued for many days after the critical ’72’ hours. Even 10 days after roasting, some degassing was still occurring. To be pedantic about things, the gas was not identified in these experiments. However, the acidification of the water in proximity with the coffee beans is consistent with the gas being CO2.

Please do try this at home and send me your results and pictures. Let me know what you find out, whether you use red cabbage or a bubble system that works. One thing that these experiments did not do at all of course was monitor how the beans tasted over a similar time frame to the degassing experiment. Perhaps you have thoughts on this. Please send your comments via the form below, comments are moderated but will (hopefully) be approved pretty quickly after you submit them.

Thanks again to Roasting House for being very efficient about sending me freshly roasted coffee and also to Tyla for helping to independently test the red cabbage experiment.

Categories
Coffee review Observations Science history

A ‘brief’ encounter at Coffee Affair

Coffee Affair, Queens Road Station
The exterior of Coffee Affair, yes it really is inside the station

It was a few weeks ago now that I dropped into Coffee Affair on a Saturday afternoon and met Michael (who runs Coffee Affair along with ‘Mags’). What can I say? This place is worth visiting for so many reasons. Firstly of course there is the coffee, so much care and attention to detail was taken when I ordered a pour over Burundi coffee from Round Hill Roastery. I was warned that my coffee would take some time to prepare before the filter was carefully rinsed and the coffee weighed and ground. The final coffee having been made with such attention that I started to understand why they had chosen the name ‘coffee affair’. It is clear that coffee is a passion.

Parquet floor at Coffee Affair
The floor at Coffee Affair.

Then there is the knowledge that Michael brings to the coffee and is happy to share. Thoughts about the best temperature to drink the coffee for example, or details about different brew methods (there is a lovely array of coffee brewing equipment on the wall of the cafe). One thing that really appealed to me though was the place. There are only a couple of tables and a bar but this emphasises the space that Coffee Affair inhabits: A preserved old ticket office. There are windows looking into the station with bars on them through which the tickets used to be sold. There is the oak table that has had years of ticket sellers leaning on it, presumably with a lamp next to their counter as it would have been a lot darker when it was used as a ticket office. Then there is the flooring, original parquet flooring that dates from the time that the station was built.

If you take a seat towards the back of Coffee Affair and look at the floor you can see where the floor has worn down just that little bit as ticket sellers from years ago shuffled at their counters while selling tickets. Like the toe of St Peter, the floor has been worn away by the number of people in contact with it over the years. Between the counters you can see where someone has tried to polish the parquet to minimise this ‘dip’ but has instead managed to produce lines in a slightly more polished floor. Thinking about the wear of the floor reminded me of Charles Darwin’s musings about the erosion of the Weald in the South East of England.

Goudhurst area
How long does it take for such landscapes to form?

In the first edition of Darwin’s book “On the Origin of Species” (1859), Darwin included an estimate for the age of the Weald of Kent, the area between the chalk hills of the North and South Downs. Based on his observations of coastal erosion, Darwin calculated that the Weald must have been at least 300 million years old. This was perfectly long enough for the gradual evolutionary steps of natural selection to have occurred. As Darwin said “What an infinite number of generations, which the mind cannot grasp, must have succeeded each other in the long roll of years!”* Looking at the floor at the Coffee Affair, you can get a similar idea as to the number of generations that have stood at the ticket windows.

Darwin’s estimate of the age of the Weald led him into an argument with William Thomson (Lord Kelvin) about the age of the Earth (which you can read more about here). It was Kelvin’s argument about the age of the Earth that Darwin considered “the single most intractable point levelled against his theory during his lifetime”†. The argument was eventually settled in Darwin’s favour, once new physics had been discovered, but only after both Kelvin and Darwin had died. So I’ll leave Darwin the last words for today’s Daily Grind, relevant too for those who have the opportunity to study the floor at Coffee Affair: “He who most closely studies the action of the sea on our shores, will, I believe, be most deeply impressed with the slowness with which rocky coasts are worn away”.*

 

Coffee Affair is in the old ticket office at Queenstown Road Station, Battersea,

* Quotes from “On the origin of species”, Charles Darwin (Oxford World Classic’s edition, 2008)

†Quote taken from “Charles Darwin, The Power of Place”, Janet Browne, Princeton University Press, 2002

Categories
Coffee cup science Observations Science history

Perpetual motion in a coffee cup

V60 from Leyas
Could your coffee be used to power a perpetual motion machine?

There can be no such thing as a perpetual motion machine right? Yet less than two hundred years ago it seemed possible that there could be. Not just that, the source of this perpetual motion machine was in your coffee cup. How would you explain Brownian motion?

Brownian motion is the random movement of small bits of dust or coffee/tea particles on the surface of your brew. To see it, you may have to use a microscope though you should take care not to confuse Brownian motion with motion caused by convection currents. There will be Brownian movement even a long time after the coffee has got cold. What causes this continuous movement? When he observed it for the first time in 1827, Robert Brown (1773-1858) had thought it was to do with a ‘life force’. He had been observing pollen suspended in water and noticed that the pollen kept moving under his microscope lens. In 1827, this was a very reasonable explanation, after all, weren’t several people looking for a motion, a force, that gave life?

Sphinx, Brownian motion
Brown used some dust from the Sphinx (shown here with the Great Pyramid) to show that ‘Brownian’ motion could occur in inorganic materials. Postcard image © Trustees of the British Museum

So, he checked if he saw the effect in pollen that was one hundred years old (he did) and then in truly inorganic matter, he looked at the dust from a fragment of the Sphinx. Again he saw the dust fragment move in the water. He had therefore shown that it was not associated with a life force but was something that happened for every small particle suspended in a liquid. What was driving it?

Without knowing what caused it, some people in the nineteenth century had already suggested a device to exploit it, using tiny levers to carry the energy from this continuous motion into devices. Others insisted on finding out what was causing the motion but it was here that the physics of the day hit a philosophical problem. It was proposed that molecules in the water could be hitting the dust on the surface and moving the dust in seemingly random directions. And yet there is a problem with this explanation. At that time there was no way of seeing or measuring molecules. How could physics postulate a theory – or suggest a reality – that could not be tested?

Nasa, Norway, coastline, fratal
How long is a section of coastline? Coastlines can be described as fractal like. Mathematics that grew out of studying random walks and Brownian motion. Image credit NASA Visible Earth/Jeff Schmaltz

An answer came one hundred years ago in a paper published by Albert Einstein (1879-1955) in 1905. In it he made some mathematical predictions that, for the first time, allowed the theory (that it was molecules causing Brownian motion) to be tested by experiment. Jean Perrin (1870-1942) of the Sorbonne, Paris, was the experimentalist who, by careful observation of droplets of water containing a pigment used by water colour artists, provided evidence for Einstein’s theory of Brownian motion. The experiment was so important that Perrin later wrote “.. the molecular kinetic theory of Brownian movement has been verified to such a point in all its consequences that, whatever prepossession may exist against Atomism, it becomes difficult to reject the theory.”

The consequences for our world have been profound. The mathematics that describes Brownian motion is that which we use as the basis to predict the movements of the stock exchange. Extensions of the mathematics have been used to develop new areas of mathematics such as fractals. Even art has grasped the theory of Brownian motion, the Anthony Gormley sculpture “Quantum Cloud” is based on mathematics describing Brownian motion. Everywhere you look there are phenomena described by the movements in your coffee cup. What we have yet to do is find that perpetual motion machine.

Categories
Coffee review Home experiments Observations

Sugar castles at Iris and June, Victoria

Iris and June, Victoria, coffee in Victoria
The exterior of Iris and June

This post has been a long time coming. Over the past few months I’ve been popping into Iris&June to get take away coffee now and then and have got quite fond of the friendly service and good coffee. What I have not really had the opportunity to do (until recently) was sit and enjoy a coffee inside. Fortunately that’s now changed and I can add Iris&June to the Daily Grind.

So, how is I+J? Well, it is a 5 minute walk from Victoria train station and a welcome break for good coffee. They serve Ozone based espresso, with a brew bar which features guest roasts (also from Ozone) made with the V60 or Aeropress. There are a good looking selection of cakes on offer, though sadly, on the day that I could sit inside with my drink, they all had nuts in them. Hopefully another time.

Sugar jar, I&J, I+J
A jar of sugar at Iris and June

I took a seat on the cushioned bench near the wall and started to look at what was going on. It is the sort of place that is very good for people watching. My eye though was drawn to what was on my table: a jar of sugar. It is not that I take sugar in my coffee, it is that I was reminded of a tutorial I once had as a student. I cannot remember the exact conversation but it concerned piles of sand. My tutor (a theoretical physicist) had said something along the lines “Ah yes, well, of course, everyone knew the maximum angle that a pile of sand could make before it became unstable and then how it started to collapse…. Until of course someone measured it.” [laughed] “We’d got it entirely wrong.”

This ability to laugh at what we do not know, (or what we assume we do know and then measure it and find out that in fact we do not)  is one of the pleasures of physics. We are trying to understand the world we live in, we have not yet got there. Sometimes it is the smallest things that are not yet understood, such as how and why (dry) sand forms avalanches as it is piled up. Yet these small things can turn out to have big consequences (as was also the case for the understanding of coffee stains). In this case, the experiment had tested the way that a pile of sand collapsed in response to different shaped grains of ‘sand’. It had relevance then (and continues to have relevance now) not only in terms of granular dynamics: how do we predict landslides/avalanches? But also in terms of crucial theoretical models about how these processes behave. Theoretical models that are applied to systems as diverse as knowing how electrical devices (resistors) work to understanding the noise on the luminosity of stars. Realising that we were wrong enabled us to probe the question more deeply and thereby to understand it more.

There are similarities between sugar and sand, but also key differences. Although it was tempting to start building sugar castles in the sugar jars on the tables at Iris and June, I was aware of the impression that I may have made to those who go to I+J to people watch (see above). I will therefore leave it as a home experiment. How steep a sugar castle do you think you can make? And how steep can you in fact make it, what is the role of water in building sand castles?

Please leave any reports of experimental results for how steep you can make a pile of sugar in the comments section below and feel free to send me your sugar-castle pictures.

Iris and June is at No 1 Howick Place, SW1P 1WG

Categories
Coffee review General Observations

Reflections at Knockbox, Lamb’s Conduit Street

Knockbox, Knock box, coffeeKnockbox coffee is on the corner of Lamb’s Conduit Street and Dombey Street. It is a small place and we had to go twice in order to get a seat, though the compensation is that there are views all around the cafe (it being on a corner). I enjoyed a very good americano, made using Workshop coffee. Complementary jugs of mint infused water were dotted around the cafe which is always a nice touch. Sadly, I tried Knockbox just after lunch and so didn’t try any of the edibles on offer. This does mean however that I will just have to go back to try them at some point (and of course, to enjoy another coffee).

There were a lot of things to notice around Knockbox that day. There were the air bubbles in the water that had become stuck around the mint leaves. There were the light bulbs (that you can see through the windows in the picture). And there was the espresso machine: A gleaming piece of machinery that sat majestically on the counter. Looking at the espresso machine it was impossible not to be struck by the reflections from the surface. The reflections are not only testament to how much the staff at Knockbox must polish the machine; how reflections work is the subject of today’s Daily Grind.

espresso machine, metal, reflection
The gleaming espresso machine at Knock box

The interaction of materials with light is one of those fascinating areas that reveal physics at its most fundamental. I’ve often taught undergraduate physics students who are looking forward to learning about quantum mechanics because it is “weird”. This is true, quantum mechanics can be quirky, but electromagnetism (which is about light) can be just as odd. To get such elegant and surprising physics out of what is essentially all classical, nineteenth century theory, is one of those joys about learning about (and teaching, using and experiencing) this subject.

However, to return to the espresso machine and light.  How light interacts with objects reveals how the electrons are distributed in the material which in turn tells you something about the atoms that make up the espresso machine. (For how to experience electrons in your coffee, see Bending Coffee, Daily Grind, 26 Nov. 2014). As the electrons are electrically charged, they respond to light which is, ultimately, an oscillation of electric (and magnetic) field. Electrons in a metal are shared in an “electron sea” between all the atoms in the metal. Consequently, when light falls on a metal surface, the electrons can respond to the electric field oscillation of the light and they re-emit the light backwards as a reflection.

ImpFringe, #ImpFringe, Fox's Glacier Mints, linearly polarised light
Sugar rotates linearly polarised light. The ‘device’ above is made from layers of Fox’s Glacier Mints and 2 linear polarisers (eg. a pair of polarised sunglasses). Photographed at ‘Lit Up’, an Imperial Fringe event held at Imperial College London, that was free to the public.

On the other hand the electrons in the atoms of the plastic of the grinder (or the glasses on the top of the espresso machine) are held firmly to each atom. Therefore most of the light that we see will go straight through these substances with each atom acting to propagate the light forward but not able to completely block it for a reflection. Coffee beans too contain electrons that are held in place by the atoms in the molecules that make up the bean. Unlike the glass though, the electrons in coffee beans are held in atomic bonds that happen to have an “excitation energy” that is at a visible light frequency. Rather than let the light through, they absorb certain colours of light (more info in the Daily Grind here). The result is the opaque, deep brown of the coffee bean.

This year is the international year of light, a year which is intended to celebrate our understanding of light. There are so many light based processes occurring all around us at every moment. Why not stop in a cafe and see how many you can spot in your coffee cup?

Categories
Home experiments Observations

Levitating water

V60 from Leyas
Time to look more closely at the surface of your black coffee.

Have you ever sat watching the steam that forms above a hot Americano? Beneath the swirling steam clouds you can occasionally see patterns of a white mist that seem to hover just above the dark brew. Bean Thinking is about taking time to notice what occurs in a coffee cup and yet I admit, I had seen these mists and thought that it was something that was just associated with the evaporation of the water and that “someone”, “somewhere” had probably explained it. So it was entirely right that I was recently taken to task (collectively with others who have observed this phenomenon and taken the same attitude) for this assumption by the authors of this paper who wrote “The phenomenon that we studied here can be observed everyday and should have been noticed by many scientists, yet very few people appear to have imagined such fascinating phenomena happening in a teacup.”

ineedcoffee.com, espresso grind
The water particles in the white mist are a similar size to the smallest particles in an espresso grind. Photo courtesy of ineedcoffee.com, (CC Attribution, No Derivs). The coin shown is a US nickel of diameter 21.21 mm

The authors of the study show that the white mists (these “fascinating phenomena”) are, in fact, layers of water drops that have a typical diameter of around 10 μm (which is roughly the size of the smallest particles in an espresso grind). Although the white mists exist above tea and even hot water as well as coffee, they are probably easiest to see against the black surface of the Americano.

More surprising than the fairly uniform distribution of water droplet size though is the fact that the authors of this study showed that the droplets were levitating above the coffee. Each water droplet was somehow literally hovering above the surface of the coffee at a height of between 10 – 100 μm (which is, coincidentally, roughly the particle size distribution in an espresso grind).

white mists, slow science
You can (just about) see the white mists over the surface of this cup of tea (which is a still from the video below)

One of the questions that the authors of the paper have not yet managed to answer is what is causing this levitation? Could it be the pressure of the hot coffee evaporating that keeps these particles held aloft? This would explain the observation that the mists form patterns similar to those caused by (heat) convection currents. Alternatively perhaps the droplets are charged and are kept away from the coffee by electrostatic repulsion, an explanation that is suggested by the behaviour of the droplets when near a statically charged object (eg. hair comb, balloon, try it). Perhaps the levitation is caused by the droplets spinning and inducing an air cushion under them? Why not design some experiments and try to find out. It would be great if we can drink hot black coffee in the name of science. Let me know the results of your observations in the comments section below. In the meanwhile, here is a video of the white mists in tea, enjoy your coffee:

You can read the study at: Umeki et al., Scientific Reports, 5, 8046, (2015)