Tag: tea

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Coffee cup science Coffee review General Observations Sustainability/environmental Tea

A cup of tea for a light bulb moment at Ginger and White, Hampstead

Coffee, Ginger and White, Hampstead
Coffee at Ginger and White, Hampstead

It was late afternoon by the time we stopped by Ginger and White in Hampstead. The warm weather meant that we could enjoy time spent sitting outdoors in the little alleyway in front of the café. We had been taking a friend around the various sights (and foodie places) of London and so stopped here before heading back home. The long black, cortado and soya latte were all very well done and, while the others had enjoyed a crepe at La Creperie de Hampstead just around the corner, I took the opportunity to try the excellent banana bread on offer at Ginger and White. There was a fairly good selection of cakes on offer, but sadly those that the staff could confidently affirm were nut free were far fewer. However, the moist and tasty banana bread was a good option anyway. Coffee was roasted by Square Mile and there were also Square Mile beans available for purchase should you wish to take some home with you. While the café was fairly busy, it was nevertheless a relaxing place to sit and watch the people of Hampstead go by.

The interior of Ginger and Whites
Everything is connected. From the lights to your cup of tea.

As I was looking around, wondering what the physics part of this cafe-physics review would be, I had what you could call a “light-bulb moment”. The walls of the building opposite were reflected in the windows of the café but looking inside, I noticed the lights which appeared to be LED lightbulbs set-back into the ceiling. Along with requiring less energy to power than conventional or halogen lightbulbs, LED lightbulbs in a café offer another, more poetic advantage for the café: they have a connection to the drinks being served and particularly tea. It’s all about diffusion.

At the heart of an LED light, there are two materials that form a junction. On one side of the junction is a semiconductor material that conducts electricity by means of electrons. Electrons conduct electricity in metals and are the ‘normal’ way that we consider electrical current to  be carried. On the other side of the junction is a different semiconductor, one that still conducts electricity but this time does so with carriers called ‘holes’. You can view the electrons as having a negative charge and the holes as having a positive charge.

tea bag, tea cup, diffusion, turbulence
What happens when you put a tea bag into a cup of cold water. How long until the water becomes ‘tea’?

But what happens at the junction? Is there really a sharp barrier between these two types of material? Think about putting a tea bag in a cup of cold water, does the tea bag just sit there or does it slowly, very slowly, start to diffuse tea into the cold water? It is a similar thing for the two materials. Slowly the electrons diffuse into the hole material and the holes into the electron material. In fact, mathematically, the same equations describe the process in the junction as in the tea cup. But unlike tea, in the LED, the holes and electrons have an electric charge associated with them and so, as they diffuse away from the junction, they set up an electric field across the junction. It is this electric field that eventually stops any further diffusion of electrons or holes across the junction and sets up the conditions necessary for LEDs to emit light. It would be like having a tea bag that diffuses tea into the cup until it is perfectly brewed and no further.

Of course, there is much more than this to understanding LEDs. If you’re interested, there is further information here. I find it fascinating however that what happens in your tea cup, is also happening on many different scales in many places in the universe. And of course, in the lighting of cafés and coffee houses around the world.

Ginger and Whites is at 4a-5a Perrins Court, NW3 1QS

 

Categories
General Home experiments Observations Science history Tea

Coffee and Pluto

Three billion miles away, on an object formerly known as the planet Pluto (now sadly demoted to the dwarf planet Pluto), there exists a plain of polygonal cells 10-40 km across, extending over a region of about 1200 km diameter. Last year, the New Horizons mission photographed this region and these strange shapes (see photo) as the probe flew past Pluto and its moon Charon. But what could have caused them, and perhaps more importantly for this website, can we see the same thing closer to home and specifically in a cup of coffee? Well, the answer to those questions are yes and probably, so what on Earth is happening on Pluto?

Plutonian polygons
What is causing these strange polygons on the surface of Pluto. Image © NASA

Pluto moves in an highly elliptical orbit with an average distance to the Sun of 5.9 billion km (3.7 billion miles). Each Pluto year is 248 Earth years but one day on Pluto is only 6½ Earth days. As it is so far from the Sun, it is very cold on Pluto’s surface, somewhere between -238 to -218 ºC. The polygons that were photographed by New Horizons are in the ‘Sputnik Planum’ basin where the temperatures are at the lower end of that scale, somewhere around -238 ºC. At this temperature, nitrogen gas (which makes up 78% of the Earth’s own atmosphere) has not just liquified, it has solidified; turned into nitrogen ice. These polygons are made of solid nitrogen.

But solid nitrogen is a very odd type of solid and in fact, at the temperatures on Pluto’s surface, solid nitrogen is expected to flow with a very high viscosity (like an extremely gloopy liquid). And it is this fact that is the clue to the origin of the odd polygons (and the link to fluids like coffee). Pluto is not just a cold dead rock circling the Sun, but instead it has a warm interior, heated by the radioactive decay of elements in the rocks making up Pluto. This means that the base of the nitrogen ice in the Sputnik Planum basin is being heated and, as two groups writing earlier this summer in Nature showed, this leads to the nitrogen ice in the basin forming convection currents. The warmer nitrogen ‘ice’ at the bottom of the basin flows towards the surface forming convection patterns. It is these nitrogen convection cells that appear as the polygons on the surface of Pluto.

Rayleigh Benard cells in clouds
Rayleigh-Benard cells in cloud structures above the Pacific showing both closed and open cell structures. Image © NASA image by Jeff Schmaltz, LANCE/EOSDIS Rapid Response

Of course, convection occurs in coffee too, we can see it when we add milk to the coffee and watch the patterns form or by observing the dancing caustics in a cup of tea. So why is it that we see stable polygons of nitrogen on the surface of Pluto but not coffee polygons on the surface of our coffee? The first point to note is the time-scale. Although the polygons on Pluto are moving, they are doing so much more slowly than the liquid movement in a cup of tea or coffee, at a rate of only a few cm per year. But secondly, the type of convection may be different. Although both of the papers in Nature attributed the polygons on Pluto to convection, they differed in the type of convection that they considered was happening. McKinnon et al., suggest that the viscosity of the nitrogen on Pluto is much greater on the surface of the basin than in the warmer interior and so the surface flows far more slowly. This leads to cells that are much wider than they are deep. We would not expect such a drastic change in the viscosity of the coffee between the (cool) top and (warm) bottom of the cup! In contrast, Trowbridge et al., think that the cells are Rayleigh-Bénard convection cells,  circular convection cells that form such that the cells are as wide as they are deep. This sort of convection is seen in a coffee cup as well as in the sky on cloudy days: On the Earth, clouds often form at the top (or bottom) of Rayleigh-Benard cells, where hot humid air meets cold dry air (more info here). But to form cells that you can see in your coffee (such as are on the surface of Pluto) you would need the coffee to be in a fairly thin layer and heated from below. You would also need some way of visualising the cells, either with an infra-red camera or with powder suspended in the liquid, it would be hard I think to see it in coffee alone. However, you can see these cells in cooking oil as this video shows:

As well as providing the link to the coffee, the different types of convection on the surface of Pluto hypothesised by Trowbridge and McKinnon have consequences for our understanding of the geology of Pluto. If the cells are formed through Rayleigh-Bénard convection (Trowbridge), the basin has to be as deep as the cells are wide (meaning the basin has to be 10-40km deep with nitrogen ice). If McKinnon is correct on the other hand, the basin only needs to be 3-6 km deep. It is easy to imagine that an impact crater could cause a shallow crater such as that needed for McKinnon’s mechanism. A deeper crater would create another puzzle.

If you do manage to heat coffee (or tea) from below and form some lovely Rayleigh-Bénard cells while doing so I’d love to see the photos or video. Please do contact me either by email, Facebook or Twitter. Otherwise, if you just enjoy watching the patterns form on your coffee, it’s worth remembering that there could be an entire cosmos in that cup.

Categories
General Observations slow Tea

The process or the cup?

effect of motivation on experience of pleasure while drinking coffee
Are you a hedonist or a utilitarian when it comes to drinking coffee?

Which part of the process of making and drinking coffee do you enjoy most? How do you rate the importance of smell, taste, touch (even hearing and sight) to the enjoyment of the process of brewing your cup?

It appears that your answer to this question may well be affected by your motivation for drinking your coffee in the first place. Last year, a group of researchers from Switzerland published a study that investigated whether the reason that you drank coffee (i.e. either for sensory enjoyment or just for the caffeine kick) influenced your enjoyment of the experience of making and drinking the coffee.

The researchers looked at how the participants in the study rated their own levels of enjoyment and satisfaction as they progressed through four stages of making coffee.

  1. Water heating
  2. Jar handling*
  3. Cup preparation
  4. Cup drinking

The 60 participants were divided into two groups of 30, those who drank coffee for enjoyment (the hedonists), and those who drank for stimulation (the utilitarians). After checking that both groups of participants rated their levels of pleasure and satisfaction similarly before the experiment started (they did), the participants were repeatedly interrupted while they made their coffee and asked to rate their levels of enjoyment and the importance that they attached to different sensory experiences (smell, sight, touch etc).

Kettle drum at Amoret
A very enjoyable coffee, but which part of the process of making and drinking coffee do you enjoy most?

After stage 4, when both groups had finally managed to drink their coffees, both groups reported similar levels of enjoyment, satisfaction etc. The difference came in the process. Overall, the group that were drinking the coffee purely for stimulation found the experience of making coffee less pleasant than the group who drank coffee because they enjoyed it. Meaning, those that drank coffee because they liked the taste seemed to enjoy the entire process of making and then drinking the coffee more than those who were just looking for a pick-me-up. Moreover, the ‘hedonists’ also attached more importance to the satisfaction of the smell and the taste of the coffee than did the ‘utilitarians’. Interestingly though, vision played an important role throughout the whole process for both groups of participants.

So how much we enjoy the process of making coffee depends on why we are drinking coffee in the first place. What about you? How do you rate the time that you spend brewing your coffee (I think that we can extrapolate this to tea too)? Are you a hedonist, a utilitarian or somewhere in between and does it matter? Please share your thoughts either here, on FB, or on Twitter.

 

*The study was performed at the Nestlé Research Centre so presumably used instant, hence the ‘jar’. Does this affect the conclusions of the study for ‘speciality’ coffee drinkers? Are you a utilitarian speciality coffee drinker who nonetheless enjoys the entire process? Please share your thoughts in the comments section below.

Categories
cafe with good nut knowledge Coffee review Home experiments Observations Tea

Electrifying coffee at the Black Penny

Black Penny coffee London
The Black Penny on Great Queen St

Back in the seventeenth and eighteenth centuries, coffee houses were places to go for debate, discussion or even to learn something new. The Grecian was known for science. Maths instruction (particularly for gambling) could be found with Abraham de Moivre (1667-1754) at Old Slaughter’s on St Martin’s Lane. Other coffee houses were meeting centres for literature, politics, philosophy or even espionage*. Coffee houses became known as “Penny Universities”. The Black Penny on Great Queen St is a café that wants to continue this tradition, with a downstairs “seminar pit” ready to host such discussions. Although the events page still says “coming soon”, if the events do indeed come, this is very much something that’s worth keeping an eye on.

Even without the seminars though, The Black Penny is definitely worth a visit. Entering from the street, the bar is on the left and is stocked with a good looking selection of cakes. We were shown through to the relatively large, bright and airy seating area at the back where a jar of water (infused with cucumber and mint) had been put on the table for us. I had a very good long black and a lovely apple and blackberry muffin with which to take in my surroundings. The muffin was confidently asserted to be nut-free, and so the Black Penny gets a tick in the ‘good nut knowledge’ section on the Daily Grind. The coffee beans were roasted by the Black Penny themselves and while it still says that they serve ‘Alchemy’ coffee on their website, this no longer appears to be the case.

Duracell batteries as coat hooks, battery, batteries
A strange form of coat hook? The things that catch your eye in cafes

Inside, there are some very interesting architectural features to notice, the remains of a ceiling for example (now removed to reveal the roof) and the acoustics introduced by the speaker positioning. Downstairs in the seminar pit there is apparently a very old stove, though I didn’t get to see that on my visit. However, what immediately struck my eye was what appeared to be a series of coat hooks that looked very similar to a well known brand of battery. Quite what these hooks were for or why they looked like batteries I didn’t manage to ascertain, however, it did get me thinking, can you use coffee-power to light an LED?

You may have heard of a potato battery, or a lemon battery. These are often used in science outreach experiments in schools to demonstrate electricity, or the concepts of current/voltage. Made from an ordinary potato (or a lemon), a copper wire is stuck into one end of the potato and a different metal (usually zinc) is stuck into the other end of the potato. At the Black Penny, there were three things left on the table. My coffee, the mint and cucumber infused water and the tea of my accomplice in many of these reviews (I’d eaten the muffin). Which of these would perform better as a battery?

coffee power
Can 6 coffee ‘cells’ with aluminium and copper electrodes light up an LED? (The answer may be in the photo)

Although people suggest using galvanised screws as the source of the zinc electrodes, I didn’t have many of those to hand and so had to manage with aluminium foil for one electrode, copper wire for the other. By putting the aluminium on one side of a shot glass, the copper wire on the other and then filling the glass with coffee, I was able to get 0.5-0.8V across the electrodes when I measured it with my digital multimeter (DMM). Fantastic you may think, almost an AA battery, but then if you were to measure the voltage across the water rather than coffee, you will find that you get a voltage of 0.6-0.7V. The result for tea was, perhaps unsurprisingly, about 0.6V.

But voltage is not the whole story. A battery does not just supply a voltage, it gives a current. The current depends on the electrical conductance of the liquid that the electrodes are in. In the case of the potato or the lemon battery, the acid (phosphoric or citric respectively) means that there are free hydrogen ions in the ‘battery’ between the electrodes which mean the electric current can flow through the circuit. Coffee consists of many acids (chlorogenic, quinic, citric etc etc.) and so it seems sensible to ask if coffee could be used to produce a battery with a current that could power an LED? LEDs require both voltage and current, (1.6V and 10mA for the LEDs used here). Hooking up a series of coffee battery-cells meant that, by 6 ‘cells’, I had 3V across the contacts. However the electric current through the coffee battery was very low (the maximum current I recorded using the low acidity Roasting House Sierra de Agalta Honduran coffee prepared in a cafetière was 155 μA). Although this was higher than the current through water (max 81 μA), it is much lower than the current through white vinegar (770 μA under the same conditions). Consequently, in order to light the LED connected to my coffee battery, I had to add salt to each coffee cell which serves as a way of massively boosting the current through the coffee (salt forms a solution of Na+ and Cl- ions that conduct electricity through the coffee). Though even then, my LED only lit dimly and intermittently.

battery, Volta, Como museum, Como
How it should be done. The “Alessandro Volta Temple” in Como, Italy, is a fantastic place to learn about the history of electricity

Sadly then, I do not see coffee power as a future for lighting in our cafés, (unless you want to use bulletproof coffee with salted butter). However, it has started to make me wonder, could we use a single coffee-cell to monitor the acidity of our coffee? If you find a method of brewing or a particular coffee especially acidic, it should produce a higher current for the same voltage through the cell, or equivalently, the resistance of the coffee-cell should decrease as the acidity of your coffee increases. Although obviously, it would be a bad idea to drink the coffee after putting it into a cell with copper and zinc (or aluminium) electrodes, you could pour a small amount of your coffee into a shot glass to test it while you were drinking the rest of the coffee. I intend on testing this hypothesis over the next couple of weeks but in the meanwhile, if you have thoughts on this to share (or the results of your experiments), please let me know either via the comments section, email, Facebook or Twitter.

The Black Penny is at 34 Great Queen St, WC2B 5AA

* A history of coffee houses can be found in “London Coffee Houses”, Bryant Lillywhite, (1963)

 

Categories
General Home experiments Observations Sustainability/environmental

Clouds in my coffee

clouds over Lindisfarne
How do clouds form?

Does your coffee appear to steam more next to a polluted road than in the countryside?

This is a question that has been bothering me for some time. Perhaps it seems an odd question and maybe it is, but it is all about how clouds form. Maybe as you read this you can glance out the window where you will see blue skies and fluffy white clouds. Each cloud consists of millions, billions, of water droplets. Indeed, according to the Met Office, just one cubic metre of a cloud contains 1 hundred million water droplets. We know something about the size of these droplets because the clouds appear white which is due to the way that particles, including water droplets, scatter sunlight. Clouds appear white because the water droplets scatter the sunlight in all directions. In contrast, the particles in a cloudless sky scatter blue light (from the Sun) more than they scatter red. Consequently, from our viewpoint, the scattered light from the clouds appears white while the sky appears blue. The sort of directionless light scattering that comes from the clouds happens when the scattering sites (ie. the water droplets) are of a size that is comparable to, or larger than, the wavelength of light. This means that the water droplets in a cloud have to be larger than about 700 nm in diameter (or approximately just less than a tenth of the size of the smallest particle in an espresso grind). The particles in the atmosphere on the other hand scatter blue light more than they scatter red light because they are smaller than the wavelength of the blue light. You can find out more about light scattering, blue skies and cloudy days, with a simple experiment involving a glass of milk, more details can be found here.

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

So each of the one hundred million water droplets in a cubic metre of cloud is at least about a micron in diameter. We can then estimate how many water molecules make up one droplet by dividing the mass of a droplet of this size by the mass of one water molecule. This turns out to be more than 1000 million water molecules that are needed to make up one droplet of cloud. So, 1000 million water molecules are needed for each of the 100 million drops that make up one, just one, cubic metre of cloud. These numbers are truly huge.

But can so many molecules just spontaneously form into so many water droplets? Unlike a snowball, the water droplet in a cloud cannot start very small and accumulate more water, getting larger and larger until it forms a droplet of about a micron in size. Water droplets that are much smaller than about a micron are unstable because the water molecules in the drop evaporate out of it before they get a chance to form into a cloud (precise details depend on the exact atmospheric conditions). Water droplets need to come ‘ready formed’ to make the clouds which seems unlikely. So how is it that clouds can form?

Condensation on mug in CGaF
Look carefully at the rim of the mug. Do you see the condensation?

It turns out that the water droplets form by the water condensing onto something in the atmosphere. That something could be dust, or salt or one of the many other sorts of aerosol that are floating around in our skies. Just as with a cold mug filled with hot coffee, the dust in the air gives the water molecules a cold surface onto which they can condense. This sort of water droplet can ‘snowball’ into the bigger droplets that form clouds because the water is now condensing onto something and so does not evaporate off again so easily. At the heart of each water droplet in a cloud is a bit of dust or a tiny crystal of salt. Which brings me back to my question. It is much more dusty along a polluted road  than it is in the clean air of the countryside. Is this going to be enough of an effect to affect the probability of cloud formation? Does your coffee steam more as you cross the road than when you walk through the park?

It is a question that demands an experiment (and associated video). Last year, the Met Office suggested this simple experiment for observing clouds in a bottle. Unfortunately however, I have yet to make this experiment work in a way that would allow me to test whether polluted air produces thicker clouds than cleaner air. If you have any suggestions as to a good experiment (that will work on camera!) please let me know either in the comments section, by emailing me, or on Facebook. In the meanwhile, I’d be interested to know what you think, so if you think this post is about you, please let me know.

 

 

Categories
General Home experiments Observations Science history Tea

Caustic Coffee

A post that applies equally to tea, just swap the word “tea” for “coffee” throughout!

A cusp caustic in an empty mug of coffee
Have you seen this line?

Look deep into your coffee. Do you see the secrets of the cosmos being revealed? Well, neither do I usually but there is something in your coffee that could be said to have ‘cosmic implications’ and I’m sure it’s something that you’ve seen hundreds of times.

Now, admittedly it is easier to see this effect if you put milk in your coffee. Imagine drinking your (milky) coffee with a strong light source (the Sun, a lightbulb) behind you. You see that curved line of light that meets in a cusp near the centre of the cup? You can see various photos of it on this page. Yes, it is indeed the reflection of the light from the curved mug surface but it is far from just that. It is what prompted a professor at Duke University to say “It’s amazing how what we can see in a coffee cup extends into a mathematical theorem with effects in the cosmos.” To understand why, perhaps it is worth reflecting a bit more on our coffee.

The shape of the curve is called a ‘cusp’  and the bright edge is known as a ‘caustic’. It is fairly easy to play with the angle of the cup and the light so that you can see the first cusp curve but you can go further and create caustics that are the result of multiple reflections. Such multiple reflections can give heart shaped curves or “cardioids” so, in a certain sense adding milk to your coffee is good for (seeing) the heart.

caustic in a cup of tea or coffee
A cusp reflection is just visible in a cup of (soya) milk tea

Caustics were first investigated by Huygens and Tschirnhaus in the late 17th century. Mathematically, the cusp curve is termed an epicycloid, you can draw one by tracing the shape made by a point on the circumference of a circle rotating around a second circle, as this graphic from Wolfram mathematics demonstrates. There is a lot of maths in milky coffee. But just how is it that these curves reveal the “Cosmos in a cup of coffee“? It turns out that once you start to see caustics you start to see them everywhere. Caustics are not just going to be formed on the inside of your coffee mug, they can be formed by light waves getting bent by ripples on the surface of a stream or even by gravity, in a phenomenon known as “gravitational lensing”.  Gravitational lensing is when a massive object, such as a black hole or a galaxy, bends the light travelling past it so that it acts analogously to a lens in optics (but a very big one). It is this last type of caustic that prompted the headline quoted above. In a series of papers published in the Journal of Mathematical Physics, Arlie Petters of Duke University and coworkers calculated how light from distant objects was focussed through gravitational lensing and the effects of caustics. Their predictions (and in particular any exceptions to their predictions) could lead to a new way to search for the elusive dark matter, which is thought to contribute to much of the Universe’s mass. They are now waiting for the Large Synoptic Survey Telescope (LSST) to start mapping the sky in order to test their theories.

multiple caustics from multiple LEDs
Multiple light sources are being reflected in this cup.

Before concluding this discussion of cosmic coffee, it is worth taking another look at the mathematician Tschirnhaus. As well as maths, he was known for his philosophy and his chemistry. In fact, it seems that he was responsible for the invention of European porcelain. As noted elsewhere, it has been argued that it was the ability of Europeans to start making their own porcelain that explained the rapid rise in consumption of tea and coffee during the eighteenth century in Europe. Interestingly, one of the tools that allowed Tschirnhaus to succeed in manufacturing porcelain in Dresden where others elsewhere failed was his use of “burning mirrors” to focus the heat and to achieve higher furnace temperatures than were otherwise available. He was using those caustics that he and others had so thoroughly studied mathematically in order to produce the type of cup in which we most often encounter the easiest caustics. A lovely little ‘elliptical’ story on which to end this Daily Grind.

In order to see the caustics in your coffee, it is necessary that the coffee reflects the light incident on it. Meaning, you need to add milk to your coffee. I knew there had to be a good reason to add milk to coffee at some point. Please do share your photos of caustics in your coffee either here or on Facebook or Twitter.

 

 

Categories
General Home experiments Observations Science history Tea Uncategorized

Predicting the weather with a cup of coffee?

What do the bubbles on the surface of your coffee tell you about the weather?

weather, bubbles, coffee, coffee physics, weather prediction, meteorology
There is a lot of physics going on with the bubbles on this coffee, but can they be used to predict the weather?

You have just poured a cup of freshly brewed coffee into your favourite mug and watched as bubbles on the surface collect in the middle of the cup. It occurs to you that it is going to be a good day, but is that because you are enjoying your coffee or because of the position of the bubbles?

There are a large number of sayings about the weather in the English language. Some of the sayings have a basis in fact, for example the famous “red sky at night, shepherd’s delight, red sky in the morning, shepherd’s warning“. Others though seem to verge on the superstitious (“If in autumn cows lie on their right sides the winter will be severe; if on their left sides, it will be mild”), or unlikely (“As August, so the next February”).  In 1869, Richard Inwards published a collection of sayings about the weather. “Weather Lore” has since undergone several new editions and remains in print although Inwards himself died in 1937. Amongst the sayings contained in the book is one about coffee:

“When the bubbles of coffee collect in the centre of the cup, expect fair weather. When they adhere to the cup forming a ring, expect rain. If they separate without assuming any fixed position, expect changeable weather.“

A quick search on the internet shows that this example of weather lore is still circulated, there is even a ‘theory‘ as to why it should be true. But is it true or is it just an old wives’ tale? Although I have consumed a lot of coffee I have never undertaken enough of a statistical study to find out if there could be an element of truth in this particular saying. The number of bubbles on the surface of the coffee is going to depend, amongst other things, on the type of coffee, the freshness of the roast and the speed at which you poured it. While the position of the bubbles will depend on how you poured the coffee into the mug, the surface tension in the coffee and the temperature. It would appear that there are too many variables to easily do a study and furthermore that the mechanism by which coffee could work as a weather indicator is unclear. It is tempting to write off this particular ‘lore’ as just another superstition but before we do that, it is worth revisiting another old wives tale which involves Kepler, Galileo, the Moon and the tides.

tides, old wives legends, Kepler, Galileo, Lindisfarne, bubbles in coffee
The pilgrim path between Lindisfarne and the mainland that emerges at low tide is marked by sticks. But what causes the tides?

Back in the mid-17th century, Newton’s theory of universal gravitation had not yet been published. It was increasingly clear that the Earth orbited around the Sun and that the Moon orbited around the Earth, but why exactly did they do that? Gilbert’s 1600 work De Magnete (about electricity and magnetism) had revealed what seemed to be an “action at a distance”. Yet the scientific thought of the day, still considerably influenced by Aristotelianism, believed that an object could only exert a force on another object if it was somehow in contact with it. There was no room for the heavenly bodies to exert a force on things that were found on the Earth. Indeed, when Kepler suggested that the Moon somehow influenced the tides on the Earth (as we now know that it does), Galileo reproached him for believing “old wives’ tales”: We should not have to rely on some ‘magical attraction’ between the moon and the water to explain the tides!

The point of this anecdote is not to suggest that a cup of coffee can indeed predict the weather. The point is that sometimes we should be a little bit more circumspect before stating categorically that something is true or false when that statement is based, in reality, purely on what we believe we know about the world. We should always be open to asking questions about what we see in our daily life and how it relates to the world around us. It will of course be hard to do a proper statistical study of whether the bubbles go to the edge or stay in the centre depending on the weather (whilst keeping everything constant). Still, there are a lot of people who drink a lot of coffee and this seems to me to offer a good excuse to drink more, so perhaps you have some comments to make on this? Can a cup of coffee predict the weather? Let me know what you think in the comments section below.

 

Weather legends taken from “Weather Lore”, Richard Inwards, Revised & Edited by EL Hawke, Rider and Company publishers, 1950

Galileo/Kepler anecdote from “History and Philosophy of Science”, LWH Hull, Longmans, Green and Co. 1959

Categories
General Observations Tea

Dynamical similarity

vortices in coffee
A vortex … (Dragging a spoon through a cup of coffee)

Science involves designing experiments to test theories. I do not want to get distracted here by how a theory is defined or the precise ways in which a theory is tested by experiment. The point of this week’s Daily Grind is to look at the role of experiments in physics, where they can be used, where it is more difficult to use experiments to test hypotheses and, how this can be connected with coffee. Some physics can be relatively easily tested by observation or experiment: we can for example take photographs of distant no-longer-planets to test theories about the evolution of the solar system or measure the viscosity of a liquid as we add something to it. Yet there are some areas of physics where it is not immediately obvious how you would test any theory that you develop. One such area is atmospheric physics where the limitations of living on one planet with one atmosphere where many different things all happen at once, could potentially be a bit of a problem for doing experiments on the theories of atmospheric physics.

vortices, turbulence, coffee cup physics, coffee cup science
… is a vortex… (What happens if you put a coffee on a record player?)

Fortunately, there is a way in which atmospheric physicists can test their theories with experiment and, perhaps unsurprisingly for the Daily Grind, that way involves a cup of coffee (or tea). The route out is called “dynamical similarity” and it is a consequence of the fact that the same mathematics describes much of that which happens in a cup of tea as it does the atmosphere. It is true that a tea cup is a lot smaller than the atmosphere but a vortex in a tea cup is the same as a vortex in the atmosphere even if one is only a centimetre across while the other has a core size of many kilometres. The mathematics will be the same. This allows people to test hypotheses formed about the atmosphere in an environment that they can control and repeat.

A vortex in the atmosphere
… is a vortex.
(Typhoon Nangka, Image Credit: NASA image courtesy Jeff Schmaltz, LANCE/EOSDIS MODIS Rapid Response Team at NASA GSFC. Caption by Kathryn Hansen)

A couple of months ago, I wrote an article in Physics World about the connections between coffee and physics. Shortly after it came out, I got an email from Paul Williams alerting me to an article that he had written in the journal Weather called “Storm in a tea cup“. It turns out that the subject of his research had been to study the impact on the weather of the interaction of two types of atmospheric waves: Rossby Waves and Inertia-gravity waves. The method that he had used to test this was, if not quite a tea cup, a bucket which he could rotate. Rossby waves and inertia-gravity waves are both present in the atmosphere and can be induced, albeit on a smaller scale, in a bucket. He was using the concept of dynamical similarity to explore what happens in our atmosphere. And the experiment was important. Before his experiments, it had been thought that the effect of the interaction of these two sorts of waves was minimal. His experiments revealed that this may not be the case, the inertia-gravity waves can significantly affect the Rossby waves. Given that Rossby waves are responsible for cold/warm fronts and weather phenomena in mid-latitude regions of the world (such as the UK) his results, and his cup of tea, were potentially very important.

I’m always very happy to hear about what others are doing with science in a tea cup or a coffee mug. Please share any thoughts in the comments section below.

Paul Williams “Storm in a tea cup” can be found in Weather, 59, (4), p.96 (2004) 

With apologies to Gertrude Stein.

Categories
General Tea

On nuclear fusion and making tea

tea bag, tea cup, diffusion, turbulence
How not to prepare tea

Although largely a coffee drinker, occasionally I will order tea in a café. When I do so, one of my pet hates is being served a cup of hot water with an individually wrapped tea bag sitting on the saucer beside it. Quite apart from the unnecessary environmental cost of individually wrapping tea bags, there is the problem with the resultant cup of tea. Hot water poured onto tea (preferably in a pot) allows the tea to infuse by a mixture of turbulence, convection and diffusion as the hot water swirls around carrying the tea with it. A tea bag placed into hot water on the other hand relies on infusion by convection and diffusion only and so takes a lot longer to brew. Oddly enough, there is at this moment, a major scientific project being built in the south-west of France that has the opposite problem. The aim of the project is to generate electricity by nuclear fusion in extremely hot clouds of gas that are confined into the shape of a doughnut. To achieve this, they must reduce the turbulence within their doughnuts. Unlike the tea, nuclear fusion seems to require diffusion and convection to prevail over turbulence.

Supplying the growing energy demands of the planet is a major problem for us all. How can we simultaneously generate the electricity that we want while limiting our carbon dioxide emissions to levels that will cause minimal damage to our planet? Renewable energy is part of the solution, some have argued that nuclear fission could be another part of the solution (all of our current “nuclear” power plants run by nuclear fission). The “ITER” project in the Provence-Alpes-Côtes d’Azur region of France aims to demonstrate the feasibility of nuclear fusion to supply our energy needs instead.

Sun, heat, nuclear fusion
The Sun is powered by nuclear fusion. Could we generate electricity on Earth with a fusion generator? Image © NSO/AURA/NSF

Unlike nuclear fission which works by exploiting the decay of radioactive elements, nuclear fusion ‘fuses’ elements together to produce energy. Gazing up at the sky you can see thousands of nuclear fusion generators: Each star (including our Sun) produces light and heat, by nuclear fusion. First the stars fuse hydrogen into helium (as our Sun does now), then, as the star ages, the heavier elements combine until finally iron is formed in the core of the dying star. All the elements found on our planet and elsewhere in space have, ultimately been formed in the core of a star (or in reactions as the star dies in a final explosion). Every atom in us has been formed by such reactions in stars and so it is very true to say “from dust you came and to dust you will return”, the dust in question being star dust. If we can exploit it on Earth, nuclear fusion offers a method of providing energy with no long term radioactive by-products and limited carbon dioxide emissions. It is a possible, but very long term, route out of our quandary about energy generation.

doughnut tokamak
A photo to demonstrate “doughnut shaped” was probably unnecessary, but it did provide a good excuse for an unhealthy breakfast.

So why can’t we start using it immediately? A clue comes from the fact that the nuclear fusion reactors that we know of (stars) are very hot and relatively dense. It is not easy to smash two hydrogen atoms together such that they fuse, it requires them to have a lot of energy (ie. be very hot) and be quite close together. To build a nuclear fusion reactor requires us to heat a gas until it becomes a ‘plasma’ which means heating the gas to temperatures of around 150 million ºC. At this temperature we need to confine the plasma with very high magnetic fields so that it does not hit the walls of its container and it turns out that the best way to do this is to manipulate the plasma into a ring doughnut shape. This doughnut confinement, known as a ‘Tokamak’ has become the standard way of confining the plasma. At the moment, we cannot keep the plasmas hot enough for long enough (the current record is 6min30 sec confinement) for fusion to generate more energy than is required to form the plasma in the first place. One of the things limiting the lifetime of the plasma is the fact that the plasma cools down and one of the things that cools the plasma down is the turbulence in the plasma carrying the heat energy from the centre to the edge of the doughnut. Increasing the time it takes for the heat to escape from the centre of the doughnut to the outer edge is one of the challenges facing the ITER team. Just as with the pot of tea, were the cooling by diffusion and convection only, the plasma would take a lot longer to cool down. Understanding the turbulence inside the plasma is one of the challenges facing the team at ITER.

Our method of making tea can tell us a lot, not just about the problems for nuclear fusion generators, but also about diffusion and turbulence generally. It is worth pondering that brew a little more deeply next time you make your pot.

 

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.