kitchen top science

Latte Art

Latte art scutoid tulip
The physics of bubbles. What links latte art to the shape of cells as an embryo develops?

An odd one out competition: which of the following is not a type of latte art? Tulip, heart, swan or scutoid? You may well ask, “what on earth is a scutoid?” and so identify this as the odd one out and, to some extent you would be right. Scutoids are not a type of latte art. But I would wager that you can still occasionally see them in your coffee.

Twitter can be a great thing and I was recently alerted there to a New York Times article about Karen Uhlenbeck by @Bob_Mat_Phys. Uhlenbeck is a mathematician at the University of Texas who has just won the Abel Prize in mathematics for her work on the maths of bubbles. The article was fascinating in itself but also mentioned in the article was the fact that there may be, on occasion, a connection between a cup of coffee and the cell structures seen in foetal development. And while I’m very well aware of the extraordinary number of connections that can be made between coffee and the science of the everyday world, I’ll admit, that one surprised me.

Metal jug and transparent glass
More bubbles in your coffee. But what determines their shape? And what shape are they?

By this point you may be unsurprised to hear that the connection is made via the scutoids, but what are they? A new type of shape, they were first described in a Nature Communications article about the development of cells as organisms such as fruit flies grew. Scutoids formed as the embryonic cells grew to form tubes or egg shapes. On one surface of the tube the cell was contacting a different number of cells to that which it contacted on the other surface (so perhaps the cell looked like a pentagon on the top and a hexagon on the bottom). In order for the cell to do this, it formed a further triangular face along one side of the cell and it is this cellular shape that is the scutoid.

Where is the connection with a coffee? Well, the amazing thing is that this shape can be the result of the physics that determines the shape of bubbles, in this case when they are confined between two curved surfaces, such as two cylinders. The shape of a bubble is the result of the minimisation of the surface energy of the bubble. So, in free space, the bubble will be spherical but somehow squash bubbles into a box and you can form a cube shaped bubble in the middle of the box. The shapes that form are the result of the minimum surface energy of the bubble surface. Now, if we return to the curved surfaces and the scutoids. The idea is that if there is a single layer of bubbles between two curved surfaces and that these surfaces are then moved away from each other, the bubbles will first resemble prisms and then, as the surfaces are stretched further, some bubbles will form a prism shape but with a triangular surface at one of the bounding walls: a scutoid.

latte art by Mace, Eiffel Tower and hot air balloon
It is astonishing what you can see in a coffee when you look closely enough.

The paper that showed this (published in Philosophical Transactions but you can read the full version here) combined mathematical modelling of the minimisation of surface energy with experiments involving two cylinders and some soap suds. They then photographed the resultant bubble structures. The results suggest that the minimisation of energy (ie. the physics of the bubble shape) could be a first approximation for explaining the cell structures that form in foetal development. But can you see them in your coffee?

You would need a coffee mug or French press and a smaller cylinder that fits neatly inside it. You would then need to form a foam somehow. Soap suds are obvious, some form of milk texturing would be more interesting. You can then look closely and see, can you in fact see scutoids in your latte art?

Coffee Rings: Cultivating a healthy respect for bacteria

coffee ring, ink jet printing, organic electronics

Why does it form a ring?

It is twenty years since Sidney Nagel and colleagues at the University of Chicago started to work on the “Coffee Ring” problem. When spilled coffee dries, it forms rings rather than blobs of dried coffee. Why does it do that? Why doesn’t it just form into a homogeneous mass of brown dried coffee? Surely someone knew the answer to these questions?

Well, it turns out that until 1997 no one had asked these questions. Did we all assume that someone somewhere knew? A bit like those ubiquitous white mists that form on hot drinks, surely someone knew what they were? (They didn’t, the paper looking at those only came out two years ago and is here). Unlike the white mists though, coffee rings are of enormous technological importance. Many of our electronic devices are now printed with electrically conducting ink. As anyone who still writes with a fountain pen may be aware, it is not just coffee that forms ‘coffee rings’. Ink too can form rings as it dries. This is true whether the ink is from a pen or a specially made electrically conducting ink. We need to know how coffee rings form so that we can know how to stop them forming when we print our latest gadgets. This probably helps to explain why Nagel’s paper suggesting a mechanism for coffee ring formation has been cited thousands (>2000) of times since it was published.

More information on the formation of coffee rings (and some experiments that you can do with them on your work top) can be found here. Instead, for today’s Daily Grind, I’d like to focus on how to avoid the coffee ring effect and the fact that bacteria beat us to it. By many years.

There is a bacteria called Pseudomonas aeruginosa (P. aeruginosa for short) that has been subverting the coffee ring effect in order to survive. Although P. aeruginosa is fairly harmless for healthy individuals, it can affect people with compromised immune systems (such as some patients in hospitals). Often water borne, if P. aeruginosa had not found a way around the coffee ring effect, as the water hosting it dried, it would, like the coffee, be forced into a ring on the edge of the drop. Instead, drying water droplets that contain P. aeruginosa deposit the bacteria uniformly across the drop’s footprint, maximising the bacteria’s survival and, unfortunately for us, infection potential.

The bacteria can do this because they produce a surfactant that they inject into the water surrounding them. A surfactant is any substance that reduces the surface tension of a liquid. Soap is a surfactant and can be used to illustrate what the bacteria are doing (but with coffee). At the core of the bacteria’s survival mechanism is something called the Marangoni effect. This is the liquid flow that is caused by a gradient in surface tension; there is a flow of water from a region of lower surface tension to a region of higher surface tension. If we float a coffee bean on a dish of water and then drop some soap behind it, the bean accelerates away from the dripped drop (see video). The soap lowers the surface tension in the area around it causing a flow of water (that carries the bean) away from the soap drop.

If now you can imagine thousands of bacteria in a liquid drop ejecting tiny amounts of surfactant into the drop, you can hopefully see in your mind’s eye that the water flow in the drying droplet is going to get quite turbulent. Lots of little eddies will form as the water flows from areas of high surface tension to areas of low surface tension. These eddies will carry the bacteria with them counteracting the more linear flow from the top of the droplet to the edges (caused by the evaporation of the droplet) that drives the normal coffee ring formation. Consequently, rather than get carried to the edge of the drop, the bacteria are constantly moved around it and so when the drop finally dries, they will be more uniformly spread over the circle of the drop’s footprint.

Incidentally, the addition of a surfactant is one way that electronics can now be printed so as to avoid coffee ring staining effects. However, it is amusing and somewhat thought provoking to consider that the experimentalist bacteria had discovered this long before us.

The impact of water on coffee

lilies on water, rain on a pond, droplets

What is the crater shape produced by falling droplets of water on freshly ground coffee?

Recently there has been considerable discussion about the impact of water on the taste of your coffee. Although this is interesting not only from a chemistry perspective, but also an experimental design and an environmental one, Bean Thinking is probably not the best place to explore such effects of chemistry on coffee taste. If you are interested, there is a recent article about it in Caffeine Magazine, click here. Instead, on Bean Thinking, the idea would be to go a little more fundamental and ask instead what is the impact of water on coffee? What effect does dripping water have on the craters produced in freshly roasted coffee grinds?

You may have noticed craters produced by rain drops on sand or paused while preparing your drip brew to think about the different ways that water percolates through a filter compared to an espresso puck. But have you stopped to consider what determines the shape of the crater that is produced as a falling droplet impacts a loose bed of granular material (such as coffee). Perhaps you have looked at images of the Chicxulub crater on the Yucatan peninsula and wondered about asteroid impacts on the Earth or craters on the Moon but what about something closer to home? What if the impacting object were liquid and the impact surface more sand like? It’s a problem that affects how rain is absorbed by soil as well as the manufacture of many drugs in the pharmaceutical industry. But it is also something that we could experiment with in coffee. Is there a difference between craters formed in espresso pucks compared to those in the coffee in the filter paper of a V60?

bloom on a v60

Bubbles in a V60 filter – but what is the impact of individual drops of water on the dry grains of coffee? The ultimate in slow coffee.

Recently, a study appeared in Physical Review E that investigated the crater shapes produced by water droplets on a bed of dry glass beads (imitating sand). The effect of the impact speed of the water droplet as well as the packing density of the granular bed (sand/coffee) was studied. A high speed camera (10 000fps) was used in combination with a laser to reveal how the shape of the craters changed with time, from the initial impact right through until the crater was stable. The authors came up with a mathematical model to consider how the energy of the falling droplet was distributed between the impacting drop and the sand bed. Does the droplet of water deform first or does the energy of the impact go into displacing the sand and so forming the crater?

Perhaps unsurprisingly, when drops of water fell onto dense beds of sand (think espresso pucks but not quite so packed), the craters produced were quite shallow. It would take a lot of energy to displace the densely packed sand but not quite so much to deform the droplet. But when the drops fell onto looser sand beds (think drip brew coffee) the crater produced formed in two stages and depended on the velocity of impact. A deep crater was formed as the drop first impacted the sand. Then as the camera rolled, the sides of the crater started to avalanche producing much wider craters that had different shapes in profile (from doughnut to pancake type structures). For looser beds of sand, the faster the impacting drop, the wider the final crater. You can read a summary of the study here.

So what would happen for craters produced during making an espresso compared to those produced making a drip brew? A first approximation would be that the espresso coffee is more densely packed, so the craters should be shallower and less wide than those produced in the loose packed filter coffee. However then we need to think that the water used in making espresso is forced through the puck with high energy. In contrast, in drip brewing techniques, the water used has a lower impact energy, (it could be said that the clue is in the name). So the energy of the impact would form larger craters in the espresso pucks and smaller craters in the drip brewers, an opposite expectation from that of the packing densities, which effect wins?

coffee ground in a candle holder

Early experiments with coffee grind craters: There are advantages to working with glass beads and high speed cameras.

But is there anything else? Grind size! Espressos are made using finely ground coffee beans, with a typical “grain size” being about 10μm (0.01mm). Drip brewed coffee is somewhat coarser, a typical medium grind being compared to grains of sand (which vary between 0.05-2mm, 50 – 2000μm but we’d expect ‘medium’ ground coffee to be at the lower end of that). This is fairly similar to the ‘sand’ used in the study in Phys Rev E which used grains of size 70-110 μm. A slightly earlier study had shown how the crater shape depended on grain size for ‘sand’ ranging from 98 to 257 μm. That study had revealed that how the water interacted with the different grain sizes depended in turn on whether those grains were hydrophilic (wettable) or hydrophobic (water proof). It is probably safe to assume that the coffee used in an espresso grind has the same hydrophilic properties as the coffee used in drip brew but even so, we still have those three variables to contend with, packing density, impact energy and grind size. So, happy experimenting! Let’s find out how the impact craters left in coffee change with preparation method. And whatever else, it’s a perfect excuse (if one were really needed) to drink more coffee while slowing down and properly appreciating it.

With thanks to Dr Rianne de Jong for pointing me in some interesting directions (not all of which fitted in this piece) towards the interaction of water with coffee, more coming soon I hope.



Making a splash

You spilled your coffee, a terrible accident or an opportunity to start noticing?

Why do some droplets splash  while others stay, well, drop like? It turns out that there is some surprising physics at play here. When a drop of water, or coffee, falls from a height and onto a flat surface (such as glass), we are accustomed to seeing the droplet fracture into a type of crown of smaller droplets that form a mess over the surface. Visually spectacular, these splashing droplets have even been made into an art form (here).

Fast frame-rate photography reveals how each micro-droplet breaks away from the splashing drop:

Video taken from Vimeo – “Drop impact on a solid surface”, a review by Josserand and Thoroddsen.


So it perhaps surprising to discover that there are many things about this process that we do not yet understand. Firstly, if you reduce the gas pressure that surrounds the drop as it falls, it does not make a splash. In the extreme, this means that if you were to spill your coffee in a vacuum, you would not see the crown-like splashing behaviour that we have come to expect of falling liquids. Rather than splash, a droplet falling in low pressure spreads out on impact as a flattening droplet. This counterintuitive result was first described in a 2005 study (here) that compared the effect on splashing of droplets with different viscosities (methanol, ethanol, 2-propanol) falling through different gasses.

cortado, Brunswick House, everyday physics, coffee cup science

Don’t spill it!
But would a latte splash more or less than a long black?

The authors of the study ruled out the effect of air entrapment surrounding the droplet as it falls as high speed photography had not indicated any air bubbles in the droplet just before impact. Instead they considered that whether a drop splashes on impact – or not – depended on the balance between the surface tension of the falling liquid and the stress on the drop created by the restraining pressure of the surrounding gas. Calculating these stresses led to a second surprising result. Whether a drop splashes on impact or not depends on its viscosity (as well as the gas pressure and the speed of impact). But the surprising bit is that the more viscous the liquid, the greater the splash.

From a common-sense perspective (that may or may not have any bearing on the reality of the situation), an extremely viscous liquid like honey should not splash as much as a less viscous liquid like coffee. This suggests that there is an upper-limit in viscosity to the relation predicted in the 2005 study. After all, although the authors did change the viscosity of the liquids, the range of viscosity they studied was not as great as the difference between coffee and honey. This sounds like a perfect experiment for some kitchen-top science and so if any reader can share the results of their experiments on the relative splashes formed by coffee and honey, I would love to hear of them.


Bouncing Coffee

floating, bouncing drops

Water droplets ‘floating’ on a bath of water (actually they bounce rather than float).

Perhaps you remember the video about how to ‘float’ coffee droplets on water posted on the Daily Grind a few weeks ago? The video featured an experiment that you could do at home in which droplets of water (or coffee, or even, if you were feeling adventurous, tea) could be made to stay as spherical droplets on the surface of a shallow dish of water for minutes at a time. Of course there were a few tricks: The water had soap added to it (10ml of soap to 100ml of water) and the shallow dish was on a loudspeaker which was playing music at the time. The whole experiment was very pretty. But hopefully as well as appreciating the aesthetics, you were asking ‘how’ and ‘why’? Why does the addition of soap mean that these globules of liquid appear to float on the liquid surface? And is the rumour you have heard about a connection with quantum physics true?

Well it turns out that people have known about these floating droplets for over a hundred years but why they behave as they do is still being investigated. It is another case of cutting-edge science appearing in your coffee cup*. So it’s worth taking a look at what is going on and why we needed to add soap and vibration for the droplets to remain stable on the water surface.

lilies on water, rain on a pond, droplets

When it rains, the rain drops don’t float on the pond

It seems to appeal to common sense and to everyday experience that if we drop a droplet onto a bath of water, the droplet will merge with the water and become part of the bath. After all, when we bring two drops that we have dripped on a table close to each other, at a certain distance between the two drops, they appear to touch and then rapidly merge into one big droplet (try it). And when it rains onto a pond, we don’t see lots of spherical droplets hovering over the surface of the pond! We know that it is the attractive van der Waals forces that bring the two drops together and then the effects of surface tension that minimise the surface area of the drops so that they become one big drop. So how is it that we can get a droplet to remain, as a droplet, on the surface of a bath of water?

How to bounce water droplets on a water surface

It could be said that the answer can be pulled out of thin air: Before the drops can merge, the air that separates them has to escape from the area between the droplet and the water bath. If the droplet can somehow be made to bounce back upwards before the air separating the droplet from the bath becomes thin enough for the two liquids to combine, the air could be made into a cushion to keep pushing the droplet upwards. This is why the experiment needs to be done with a vibrating dish of water, each time the surface vibrates upwards it is providing the drop with an acceleration upwards that overcomes gravity, like a miniature trampoline: The droplet is not floating, it is bouncing.

So why soap? We all know that the addition of soap decreases the surface tension of the water. But that is not why the addition of soap helps to stabilise the drops in this instance. No, soap has another effect and that is to increase the surface viscosity (and surface elasticity) of the water. Think about the air between the droplet and the dish. As the droplet bounces down (ie. the distance between droplet and water becomes a minimum), the air gets squeezed out of the layer between the droplet and the bath. On the other hand, as the droplet reaches its peak height, air will rush into the gap between the drop and the bath. If the liquid is not very viscous (eg. water), as the air rushes in (or gets squeezed out), it will combine with the liquid and form a turbulent layer on the surface of the droplet. If the viscosity is increased, the air cannot ‘entrain’ the liquid as the droplet bounces and so the drop keeps its shape more easily and is more stable. Soap increases the surface viscosity of the droplet and so helps with this effect. However soap also increases the surface elasticity and makes it harder for the air to flow out of the layer separating the drop from the bath. It is because soap does multiple things to the water (or coffee) that more recent studies have focussed on liquids with controllable viscosity but minimal surfactant effects, i.e. silicone oils. It is just that if you want it to work with coffee, it is easier to add the soap to get the experiment to work.

An “un-cut” video of coffee on water shows how tricky it can be to actually get these drops to be stable on the surface of the water.

Which leaves the quantum link. The experiment shown in the videos show single droplets (or droplet patterns) stabilised by vibrations caused by music. If instead of music you use fixed frequencies to excite resonances through the speakers, it is possible to get the droplet to resonate in a controlled way and, at a certain point, it will move. As the droplet moves, it appears to be guided by the vibrations of the liquid underneath the drop, it is a particle guided by a ‘pilot wave’. It turns out that such walking droplets show behaviour reminiscent of the ‘wave particle duality‘ found in quantum physics where particles (such as electrons and other sub-atomic particles) can be described both as particles and as waves. You can find a video describing the similarities between these bouncing droplets and quantum effects here.


* Ok, so you may not want to add soap to your coffee to see this effect but actually I first observed it in a milky tea. Adding milk to the coffee/tea would increase its viscosity which makes the observation of the bouncing droplets more likely. The ‘milk’ used in the video was actually soya milk which did not appear to increase the viscosity sufficiently to allow the droplets to bounce on the surface without soap.

Coffee baubles

resonating coffee

Not the best image of a resonating coffee but you hopefully get the idea

Most people, at some point in their lives, must have pushed a take-away coffee cup across a table and watched as patterns form on the liquid surface. Sometimes these patterns seem to stand still, we’d say that they form ‘resonances’. On even rarer occasions, on dragging your cup across the surface, you may have seen coffee droplets jump out of the coffee and then dance on the coffee surface for a couple of seconds as the liquid vibrates.

Today’s Daily Grind investigates these ‘floating droplets’ with an experiment in time for Christmas: Decorate your coffee with coffee baubles.

To make these droplets form on your coffee in a controllable way you will need a few bits of equipment:

  1. A couple of loud-speakers with the woofers exposed
  2. Some sort of liquid soap (washing up liquid, hand soap, soap for hand washing clothes etc)
  3. Some water (or coffee but you will do horrible things to it)
  4. A shallow dish (I used the bottom of an old yoghurt pot)
  5. A “dropper”, a pipette or syringe would be ideal, a straw will probably work.

You can do this completely systematically, in which case you’ll also need a signal generator to provide a fixed frequency output to the speakers (I used “ScorpionZZZ’s Lab, Signal Generator Lite for iPhone). Or you can just go straight to the fun bit which is to make these droplets dance to music. It’s Christmas so it’s entirely up to you!

floating drops, resonances, speakers, kitchen top science

Balance a shallow dish on the woofer of a speaker. A roll of sellotape can be used to couple the vibrations of the speaker to the dish if necessary.

Balance your speakers on a flat surface and put the shallow dish so that it sits in good contact with the woofer. Because my dish was ever so slightly larger than the vibrating bit of the speaker, I ‘coupled’ the speaker to the dish with a roll of sellotape. Mix 10ml of soap with 100ml of water (this does not have to be exact but you may want to investigate just how much/little soap you can get away with). If you are using coffee rather than water, you will need to mix 10ml soap with 100ml coffee.

Pour about half the soapy-water into the dish and then turn the speakers on. If you are using a signal generator, watch what happens as you sweep the frequency from 10-200 Hz. Now, either choose a frequency which shows a nice resonance pattern on the water, or start playing the music through the speakers. Music with a good beat will work well (I watched drops dance to Tiesto, Blondie, and Josh Woodward’s “coffee”).

Drip a drop of the remaining soapy-water onto the resonating surface. A video of my playing with these droplets can be seen above. Although not all the drops will float, it is fairly easy to start to form patterns of flowers or rows of droplets and then it’s worth just playing.  How big a droplet can be made to float without collapsing? How many minutes can you get a drop to last before it sinks? What happens if you combine a drop of black (soapy) coffee with a drop of milky (soapy) coffee?

Have fun, and please do share your videos and photos of your experiments with me on Facebook or Twitter.

Disclaimers & Credits:

No coffee was wasted in the making of this video. A very good coffee from Roasting House was thoroughly enjoyed before the remnants were diluted and mixed with soap.

Inspiration & experimental details taken from Jearl Walker’s great article “The Amateur Scientist” in Scientific American, p. 151 (1978).


What haloes and crowns reveal about your coffee

Coffee Corona

Look carefully around the reflected white light. Do you see the rainbow like pattern?

Several weeks ago I had been enjoying some very good black coffee at OJO in Bangsar, KL. As is fairly typical for me, I had been trying to observe the white mists that form just above the coffee. White mists are fascinating, tissue-like clouds that you can often see hovering above the coffee. They form, tear suddenly and then reform into a slightly different pattern. As I was photographing my coffee, I noticed what seemed to be interference patterns on the mists (see picture), just like oil on water, a rainbow-like shimmering over the coffee surface. Yet that explanation did not make sense; interference patterns form because the layer of oil on water has approximately the same thickness as the wavelength of visible light (see more info here). The water droplets that make up the white mists are a good 15 times thicker than the wavelength of light. It is not possible that these mists are producing interference effects, it has to be something else.

Then, last week and back in London, I was walking towards the setting Sun one evening when I saw what looked like a rainbow in a cloud. What caused this and how was it related to what I had seen earlier in my coffee? A short trip to the library later and it was confirmed. What I had seen in the clouds was most likely a Sun-dog. Formed by the refraction of sunlight by ice crystals in the atmosphere, Sun-dogs manifest as bright regions of rainbow. The Sun-dog appeared in cirrus clouds because these are made from the sort of ice crystals that produce brilliant Sun-dogs. These ice crystals are flat and hexagonal so they refract sunlight exactly as does a prism. Just like a prism, red light and blue light will be refracted by differing amounts and so they will appear at different places in the sky. The minimum angle of refraction produces the most intense colouration and, for hexagonal platelets of ice, this occurs at 22º away from the light source.

Sun-dog, Sun dog

A Sun-dog in the clouds to the right of the setting Sun

I do not find degrees a particularly helpful way of thinking about distance but what helped me is that, in terms of the sky, if you hold your outstretched hand out at arms length, the distance from your thumb to the tip of your finger is, approximately, 22º. Hence, if you see a halo around the Sun at about that distance, it is most likely a refraction effect due to ice crystals in the sky and if you see an intense rainbow roughly parallel to the elevation of the Sun, it is very likely to be a Sun-dog.

What does this tell us about the colours in the mists above the coffee? Well, clearly the mists are not made of ice crystals but neither is the ‘rainbow’ colouring as far as 22º from the light source (a light bulb reflected in the coffee). Also, the rainbow is less vivid and, if you look closely, inverted from the rainbow in the clouds. In the cloud, the inner edge of the arc was red and the outer edge blue, in the coffee, the outer edge is more reddish, while the inner is more blue-ish. This is another clue. On the same evening as I had seen the Sun-dog, there was a full moon and around the Moon was a glowing ring, tinged slightly reddish on the outside. The ring was far closer to the Moon than the Sun-dog had been to the Sun. This Moon-ring, and the coffee colouring are the same effect, they are examples of ‘corona’ (literally crown) and they are caused by diffraction of light rather than refraction.

straw, water, glass

It is refraction that makes the straw appear broken in this glass of water.

Refraction we are all quite familiar with, it is the bending of a straw in a glass of water as you look through the glass. Diffraction is a little more tricky, but it is a consequence of how the light moves past an object. It can be understood by thinking about how water waves pass objects in a stream (or by playing with the simulation here). The amount that the wave is diffracted depends on both the size of the object and the wavelength of the wave. As blue light has a much shorter wavelength than red light, the blue will be diffracted by a different amount to the red. If the objects diffracting the light are of a similar size (as water droplets in white mists are going to be) a spectrum, or a rainbow of colour will appear around the light source. The more uniform the droplet size, the more vivid the spectrum in the corona. The thin cloud around the Moon that evening was made up of many different sized droplets and so the rainbow effect was very subtle. In contrast, around the reflection of the light bulb in the coffee, the water droplets in the white mist are a fairly similar size and so the spectrum is more vividly seen.

Seeing rainbow effects in the sky (or in the coffee) therefore gives us many clues as to what is in the sky or indeed, levitating above the coffee. Please do send me any pictures you have of coronae around light source reflections in your coffee, or indeed sun dogs if you are fortunate enough to see them*.

* Sun dogs are in fact apparently fairly common, it is more that we have to be attentive to see them.

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

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.


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.

The hot chocolate effect

hot chocolate effect, Raphas

A ready prepared hot chocolate

This is an effect that reveals how sound travels in liquids. It enables us to understand the milk steaming process involved in making lattes and yet, it can be studied in your kitchen. It has an alternative name, “The instant coffee effect”, but we won’t mention that on this website any further. To study it you will need,

1) a mug (cylindrical is preferable),
2) some hot chocolate powder (no, instant coffee really will not do even if it does work)
3) a teaspoon
4) a wooden chopstick (optional, you can use your knuckle)

Make the hot chocolate as you usually would and stir. Then, remove the spoon and repeatedly tap on the bottom of the mug with the wooden chopstick (you could instead use your knuckle). Over the course of about a minute, you will hear the note made by the chopstick rise (not having a musical ear, I will have to trust that this can be by as much as three octaves).

resonator, mouth organ

The length of the pipes in this mouth organ determine the note heard. Photo © The Trustees of the British Museum

What is happening? Well, just like an organ pipe, the hot chocolate mug acts as a resonator. As the bottom surface of the hot chocolate is fixed in the mug and the top surface is open to the air, the lowest frequency of sound wave that the hot chocolate resonator sustains is a quarter wavelength. The note that you hear depends not just on the wavelength, but also on the speed of sound in the hot chocolate, and it is this last bit that is changing. When you put in the water and stir, you introduce air bubbles into the drink. With time (and with tapping the bottom surface), the air bubbles leave the hot chocolate. The speed of sound in a hot chocolate/air bubble mixture is lower than the speed of sound in hot chocolate without air bubbles. Consequently, the frequency of the note you hear is higher in the hot chocolate without bubbles than in the former case.

Let’s use this to make a prediction about what happens when a barista steams milk ready for a latte. At first, the steam wand introduces air and bubbles into the mixture but it is not yet warming the milk considerably. From above, we expect that the speed of sound will decrease as the bubbles are introduced. This will have the effect of making the ‘note’ that you hear on steaming the milk, lower. At the same time the resonator size is increasing (as the new bubbles push the liquid up the sides of the pitcher). This too will act to decrease the note that is heard as you steam (though the froth will also act to damp the vibration, we’ll neglect this effect for the first approximation). At a certain point, the steam wand will start to heat the milk. The speed of sound increases with the temperature of the milk and so the note will get higher as the milk gets warmer.

So this is my prediction, musically inclined baristas can tell me if there is any truth in this:

1) On initially putting the steam wand into the cold milk, the tone of the note heard as the milk is steamed, will decrease.
2) This decrease will continue for some time until the milk starts to get warm when the note increases again.
3) Towards the end of the process, the note heard on steaming the milk will continue to increase until you stop frothing.
4) It should be possible, by listening to the milk being steamed, to know when the milk is ready for your latte just by listening to it (if you are experienced and always use similar amounts of milk per latte drink).

So, let me know if this is right and, if it is wrong, why not let me know what you think is happening instead. I’d be interested to know your insights into the hot chocolate effect in a milk pitcher.