home experiments

Clouds, condensation and coffee

Clouds in my coffee. There is, perhaps unsurprisingly, plenty of atmospheric physics you can encounter in your cup.

As we approach the end of the year, it is a good time to notice the changes in the weather. If you are in the northern hemisphere, the nights grow longer as the days grow colder. If you are in the southern hemisphere it is the opposite. And yet around the world, we have things in common. There may be days when it is more cloudy and days when there is a heavy dew (or even in some places a frost) on the grass. But what has this to do with coffee?

It’s to do with some experiments that you can do at home or on your way to work. And, in particular, with two effects you can see in your coffee cup.

To start with the dew, perhaps you’ve noticed the condensation around the rim of the cup or the coffee pot when you brew the coffee and the hot steam condenses onto the cold mug around it. Condensation happens because the temperature of the mug is lower than the ‘dew point’ of water at that humidity and pressure. Below the temperature of the dew point, the water vapour will condense into the liquid droplets that we then see dotted around the mug.

coffee bowl pour over
You can see the condensation on the V60 brewer here. Looking at the dew formed in the mornings, what does it tell you about the temperature of space?

It is a similar effect on the grass: the temperature there is lower than the point at which the water vapour in the air starts to condense out of the air and so you get dew. William Charles Wells published his “Essay on Dew” in 1814. The result of more than two years of careful observation, Wells found that dew formed only under certain weather conditions and only on certain space (sky) facing surfaces. Wells’ results can be used to show that the space around the earth is much colder than the surface of our planet. His results (together with some back of the envelope calculations) can therefore also be used to show that the Earth is in a delicate balance and has a natural greenhouse effect. As the weather changes this year and you notice the dew, can you see how Well’s could come to this conclusion?

The second coffee experiment we could do at this time of year is to see whether pollution affects our steaming take-away coffee. While generally it’s always a better idea to sit in a cafe and take the time to enjoy your coffee, there are occasions when a take-away is necessary. Just as with the dew, clouds start to form when the air temperature drops below the dew point. However, water droplets in the air are unstable to evaporation and so as soon as a pure water droplet is formed, it will evaporate unless it has a diameter larger than about 0.1 µmª. This may seem small and yet to spontaneously form a droplet with this diameter would take the accumulation of several million water molecules (I will leave it to you to do the estimate!). This represents a very improbable occurrence and yet we can see that clouds are everywhere, how can this be?

contrail, sunset
Contrails are caused by condensing water droplets behind aeroplanes. But why are they white and what does that tell you about the water droplets within them?

The answer comes from the dust. Fortunately we are a dusty planet and these bits of dust in the atmosphere act as ‘nucleation’ points for water to condense onto. This makes the condensation of water into droplets much more likely and so clouds – which are an accumulation of droplets – can form.

Which brings us back to the coffee. If clouds require dust in order to form droplets, and the steam above your coffee is a grouping of water droplets, does it not make sense that your coffee should be steamier next to a polluted road than in the middle of a park (for the same temperature coffee)?

It’s an idea that I’ve never been able to test but the shift to colder weather here offers a(nother) perfect opportunity.

Does your coffee steam more when you take it away from a city cafe?

I look forward to hearing about the results of your experiments, in the comments here, on Twitter or on Facebook.

ª Introduction to Atmospheric Physics, Andrews, Cambridge University Press, 2008

Coffee (beans) in the blood?

Brazil nut effect

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

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

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

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

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

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

Brew&Bread, latte art Sun, KL latte art

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

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

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

On rings, knots, myths and coffee

vortices in coffee

Vortices behind a spoon dragged through coffee.

Dragging a spoon through coffee (or tea) has got to remain one of the easiest ways to see, and play with, vortices. Changing the way that you pull the spoon through the coffee, you can make the vortices travel at different speeds and watch as they bounce off the sides of the cup. This type of vortex can be seen whenever one object (such as the spoon) pulls through a fluid (such as the coffee). Examples could be the whirlwinds behind buses (and trains), the whirlpools around the pillars of bridges in rivers and the high winds around chimneys that has led some chimneys to collapse.

Yet there is another type of vortex that you can make, and play with, in coffee. A type of vortex that has been associated with the legends of sailors, supernovae and atomic theory. If you add milk to your coffee, you may have been making these vortices each time you prepare your brew and yet, perhaps you’ve never noticed them. They are the vortex rings. Unlike the vortices behind a spoon, to see these vortex rings we do not pull one object through another one. Instead we push one fluid (such as milk) through another fluid (the coffee).

It is said that there used to be a sailor’s legend: If it was slightly choppy out at sea, the waves could be calmed by a rain shower. One person who heard this legend and decided to investigate whether there was any substance to it was Osborne Reynolds (1842-1912). Loading a tank with water and then floating a layer of dyed water on top of that, he dripped water into the tank and watched as the coloured fluid curled up in on itself forming doughnut shapes that then sank through the tank. The dripping water was creating vortex rings as it entered the tank. You can replicate his experiment in your cup of coffee, though it is easier to see it in a glass of water, (see the video below for a how-to).

Reynolds reasoned that the vortices took energy out of the waves on the surface of the water and so in that way calmed the choppy waves. As with Benjamin Franklin’s oil on water experiment, it’s another instance where a sailor’s myth led to an experimental discovery.

chimney, coffeecupscience, everydayphysics, coffee cup science, vortex

In high winds, vortices around chimneys can cause them to collapse. The spiral around the chimney helps to reduce these problem vortices.

Another physicist was interested in these vortex rings for an entirely different reason. William Thomson, better known as Lord Kelvin, proposed an early model of atoms that explained certain aspects of the developing field of atomic spectroscopy. Different elements were known to absorb (or emit) light at different frequencies (or equivalently energies). These energies acted as a ‘fingerprint’ that could be used to identify the elements. Indeed, helium, which was until that point unknown on Earth, was discovered by measuring the light emission from the Sun (Helios) and noting an unusual set of emission frequencies. Kelvin proposed that the elements behaved this way as each element was formed of atoms which were actually vortex rings in the ether. Different elements were made by different arrangements of vortex ring, perhaps two tied together or even three interlocking rings. The simplest atom may be merely a ring, a different element may have atoms made of figure of eights or of linked vortex rings. For more about Kelvin’s vortex atom theory click here.

Kelvin’s atomic theory fell by the way side but not before it contributed to ideas on the mathematics (and physics) of knots. And lest it be thought that this is just an interesting bit of physics history, the idea has had a bit of a resurgence recently. It has been proposed that peculiar magnetic structures that can be found in some materials (and which show potential as data storage devices), may work through being knotted in the same sort of vortex rings that Kelvin proposed and that Reynolds saw.

And that you can find in a cup of coffee, if you just add milk.

 

Theme on a V60

bloom on a v60

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

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

science in a V60

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

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

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

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

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

evaporation V60 in contact with scales

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

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

control

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

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

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

V60 Halley

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

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

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

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

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

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.

 

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.

 

 

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 bean degassing

coffee, Roast House

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

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

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

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

The Experiment:

water acidification via coffee beans

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

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

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

red cabbage, acidity, indicator, natural indicator, coffee bean degassing

The colours of the red cabbage liquid on tissue. Control sample is on the left, light roast in the middle, dark roast on the right

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

Experiment 2 – testing the pH more systematically.

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

pH testing, coffee bean degassing

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

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

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

Conclusions:

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

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

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

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.