Category: Coffee cup science

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Coffee cup science General Home experiments Observations slow

Coffee Damping

vortices in coffee
Vortices behind a tea spoon

How often do you allow yourself to get bored? Or to sit in a cafe and take your time to enjoy your coffee properly, noticing its appearance, the smell ‘landscape’ of the cafe, pausing while you absorb the sounds of the cafe and playing with the feel of the coffee while you create vortices with your tea spoon?

If you regularly drink black coffee, you may have noticed how these vortices form more easily in the coffee once the crema has dispersed. Intuitively this may seem obvious to you, perhaps you wouldn’t even bother trying to form these vortices in a cappuccino, you’d know that they wouldn’t appear. The bubbles of the crema (or the milk in the cappuccino) quickly kill any vortex that forms behind the tea spoon (we’d technically call it ‘damping’ them). But even when we are aware of this, it is still surprising just how quickly the crema stops those vortices. Try forming a couple of vortices in a region of black coffee close to a region of crema. Indeed I thoroughly recommend ordering a good black coffee in a great cafe somewhere and just sitting playing with these vortices all the while noticing how their behaviour changes as the crema disperses.

latte art, flat white art
Latte art at The Corner One. Lovely to look at but not good for the vortices.

The damping caused by bubbles on the surface of a coffee is responsible for another phenomenon that you may have encountered in a cafe but, to be fair, are more likely to have noticed in a pub. Have you ever noticed that you are less likely to spill your cappuccino between the bar and your seat than you are your lovingly prepared filter coffee? Or perhaps, in the pub, you can get your pint of Guinness back to the table more easily than your cup of tea? (At least for the first pint of Guinness)

Back in 2014, a team investigated the damping properties of foam by controlling the size and number of bubbles on top of a liquid as it was vibrated (sloshed) about. They found that just five layers of bubbles on top of the liquid was enough to significantly damp the liquid movement as it vibrated from side to side. That is, five layers of bubbles suppressed the sloshing (try saying that after a couple of pints of Guinness). Much as I dislike emphasising the utility of a piece of science, this work has obvious implications for any application that requires the transportation of liquids such as the transport of oil containers. There is an obvious need to suppress the effect of liquid oil sloshing from side to side as it is transported by boat for example.

The foam on our latte or crema on our long black should indeed give us pause for thought as we sit in a cafe enjoying our coffee.

 

 

Categories
Coffee cup science General Home experiments Observations slow

Coffee ring bacteria

coffee ring, ink jet printing, organic electronics
Why does it form a ring?

We have all seen them: Dried patches of coffee where you have spilled some of your precious brew. The edge of the dried drop is characteristically darker than the middle. It is as if the coffee in the drop has migrated to the edge and deposited into a ‘ring’. It turns out though that these coffee rings are not just an indication that you really ought to be cleaning up a bit more often. Coffee rings have huge consequences for the world we live in, particularly for consumer electronics. Various medical and diagnostic tests too need to account for coffee ring effects in order to be accurate. Indeed, coffee rings turn up everywhere and not just in coffee. Moreover, the physics behind coffee rings provides a surprising connection between coffee and the mathematics of bacteria growth. To find out why, we need to quickly recap how coffee rings form the way they do.

When you spill some coffee on a table it forms into droplets. Small bits of dust or dirt or even microscopic cracks on the table surface then hold the drop in the position. We’d say that the drop is pinned in position.

artemisdraws, evaporating droplet
As the water molecules leave the droplet, they are more likely to escape if they are at the edge than if they are at the top. Illustration by artemisdraws.com

As the drop dries, the water evaporates from the droplet. The shape of the drop means that the water evaporates faster from the edges of the drop than from the top (for the reasons for this click here). But the drop is stuck (pinned) in position and so cannot shrink but instead has to get flatter as it dries. As the drop gets squashed, water flows from the centre of the drop to the edges. The water flow takes the coffee particles with it and so carries them to the edge of the drop where they deposit and form into a ring; the coffee ring. You can see more of how coffee rings form in the sequence of cartoons below and also here.

However in this quick explanation, we implicitly assumed that the coffee particles are more or less spherical, which turns out to be a good assumption for coffee. The link with the bacteria comes with a slightly different type of ‘coffee’ ring. What would happen if we replaced the spherical drops of coffee particles with elliptical or egg shaped particles? Would this make any difference to the shape of the coffee rings?

Artemisdraws
As water evaporates from A, the drop gets flatter. Consequently, the coffee flows from A to B forming a ring. Illustration by artemisdraws.com

In fact the difference is crucial. If the “coffee” particles were not spherical but were more elliptical, the coffee ring does not form. Instead, the elliptical particles produce a fairly uniform stain (you can see a video of drying drops here, yes someone really did video it). The reason this happens is in part due to a pretty cool trick of surface tension. Have you ever noticed how something floating on your coffee deforms the water surface around it? The elliptical particles do the same thing to the droplet as they flow towards the edge. (Indeed, the effect is related to what is known as the Cheerios effect). This deformation means that, rather than form a ring, the elliptical particles get stuck before reaching the edge and so produce a far more uniform ‘coffee’ stain when the water dries.

E Coli on a petri dish
A growing E. Coli culture. Image courtesy of @laurencebu

By videoing many drying droplets (containing either spherical or elliptical particles), a team in the US found that they could describe drying drops containing elliptical particles with a mathematical equation called the Kardar-Parisi-Zhang equation (or KPZ for short). The KPZ equation is used to describe growth process such as how a cigarette paper burns or a liquid crystal grows. It also describes the growth of bacterial colonies. Varying the shape of the elliptical particles in the drying drop allows scientists to test the KPZ equation in a controllable way. Until the team in the US started to ask questions about how the coffee ring formed, it was very difficult to test the KPZ equation by varying parameters in it controllably. Changing the shape of the particles in a drying drop gives us a guide to understanding the mathematics that helps to describe how bacterial colonies grow. And that is a connection between coffee and bacteria that I do not mind.

As ever, please leave any comments in the comments section below. If you have an idea for a connection between coffee and an area of science that you think should be included on the Daily Grind, or if you have a cafe that you think deserves a cafe-physics review, please let me know here.

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Coffee cup science General Observations slow Tea

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.

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Coffee cup science General Observations slow

Coffee & Contrails (II)

vortices in coffee
Vortices forming behind a tea spoon being dragged through coffee.

Drag a tea spoon through your cup of coffee (or tea). Start by dragging the spoon slowly, then faster. Initially, the coffee flows around the spoon smoothly then, as you speed up, small vortices appear at either side of the spoon. Pull the spoon out of the coffee, and the vortices continue to move together through the cup before bouncing off the sides. Such vortices form whenever there is a speed difference between two layers of fluid (gas or liquid), as there is around the spoon being dragged through coffee. It is this effect that is the second connection between the physics of coffee and contrails.

Of course it is not giant tea spoons in the air but aeroplanes. Behind each aeroplane is a series of vortices trailing behind the wings. These vortices do not (normally) cause the contrails, the reason that they form was discussed in Coffee & Contrails (I). However, the vortices do cause some interesting effects in the contrail that we can, occasionally, see.

wake vortex, contrail, coffee in the sky
In this contrail there is a set of protuberances at regular intervals along the lower edge.

As the plane moves through the air, the speed of the air going over the wing is greater than the speed of air under the wing. As well as leading to vortices forming behind the wing, this speed difference results in an air pressure difference (the air pressure under the wing is greater than the air pressure above the wing). The pressure difference (below and above the wing) pushes the plane upwards, or, perhaps more technically, ‘creates lift’ and enables the plane to fly. If you want a good demonstration of the fact that a higher air speed leads to a lower air pressure, get two pieces of flat A4 paper and hold them in front of you such that you are looking through the small gap between them. Now blow into the gap separating the two sheets; they will join together. The reason that they do this is that the air pressure for fast moving air (as you blow) is less than the air pressure for static air (around the paper) and so the difference in air pressure pushes the two sheets together.

Shadowy contrail
Look carefully for another interesting contrail optical effect. There are two contrails here, an obvious one cutting straight down the photo and a second contrail moving more horizontally across the photograph. The second contrail can be seen more clearly by its shadow.

On a clear day, if the air in the higher atmosphere is relatively humid, you will see lots of persistent contrails. These contrails last for a long time in the skies and can drift with the wind. Occasionally at the edge of such a contrail you will see regular protrusions from the contrail, almost as if waves are forming on the contrail and producing white horses in the sky (see picture above). Initially I had hoped that this was a manifestation of the Kelvin-Helmholtz instability however the actual explanation is still quite fascinating. It seems that these protrusions are the result of the “wake vortices“, the vortices that form behind a plane just as the coffee forms vortices behind your spoon. I find it quite impressive to realise that high in the sky, these contrails are showing us that the atmosphere behaves just as if it were a cup of coffee. A definite case for which a coffee is a telescope for viewing the world.

Please leave any comments in the comments box below. If you think of any other connections between the physics of coffee and contrails please share them either here or on my Facebook page.

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Coffee cup science Observations slow

Coffee & Contrails (I)

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

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

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

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

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

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

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

contrails
A hot air balloon in a sky full of contrails

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

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

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Coffee cup science General

Setting the standard for coffee brewing

Chemex, 30g, coffee
A Chemex, how much coffee do you need to make a good cup (or two?)

Serious coffee drinking requires a serious attention to preparation. Various online guides (such as this one from Ineedcoffee.com) specify the ratio of water to coffee that you need and some will dictate the exact quantity of coffee that you should grind ready for your brew (30g for a standard, 500ml Chemex). Different brew methods require different amounts of coffee. Some will insist that the correct ratio of coffee to water is essential for a good coffee. So how can we ensure that 30g of coffee is really 30g? How do I know that what you measure as 30g is what I measure as 30g? It is a question that reveals a fascinating answer. The measurement of mass, the definition of the kilogram, is the only unit of measurement left for which we still use a physical standard as the reference.

This means that there is a physical lump of metal (it is actually a platinum-iridium alloy) that is sitting in a lab somewhere (Paris) against which all our definitions of mass are referenced. If you were to weigh out 1 Kg of coffee, your scales would, ultimately, be referencing this 1Kg lump of platinum-iridium in Paris. My scales reference the same standard and so we can be sure that, assuming our scales are accurate, your 30g is equivalent to my 30g. Many years ago (in 1884), forty replicas of this standard of measurement were made and distributed throughout the world. The idea was that rather than have to always refer to the Parisian standard, there would be a more local ‘standard’ that people could refer to. The problem of course is that the standards diverge, they have to be regularly re-calibrated so that the Kg in Paris weighs the same as the Kg in London (well, just outside London in Teddington, at the National Physical Laboratory).

gold weights, standard weights, not Kg
A set of gold weights from China in the British Museum collection. © Trustees of the British Museum

The reason appears to be because the standards get dirty. The surface of the metal adsorbs contaminants from the air which make the standard seem heavier. Admittedly, this may not be by much, only perhaps tens of μg, but over many tonnes, this small difference is going to add up. And if you trade in commodities (such as coffee beans) and are paying by weight of coffee then such differences, in large amounts, may be costly. So what is the solution? One method involves finding new ways to clean the standards so that they are contamination free. A more long term solution is to move away from measuring relative to a physical standard at all. After all, length is no longer measured with reference to a stick in a lab but with reference to the distance that light travels in a certain amount of time. Research is now being done into exactly this in metrology labs around the world. At some point in the not to distant future, it is very likely that the Kg will be defined with reference to an electrical measurement, for example, rather than with reference to a physical block of metal. For the meanwhile, we have to hope that the standards labs around the world keep their blocks of metal very clean otherwise, how would we ever expect to get the correct amount of coffee in our Chemex?

Comments always welcome, please click the link below to add a comment.

Categories
Coffee cup science Observations Science history

Perpetual motion in a coffee cup

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

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

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

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

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

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

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

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

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

Categories
Coffee cup science Coffee review General Observations slow

Rain drops at Notes, Covent Garden

Notes Covent Garden, rain, puddles
No one wanted to sit outside when we visited Notes at Covent Garden

It was a cold and wet afternoon in early January when I finally had the opportunity to try Notes (Covent Garden branch). Inside, there were plenty of places to sit while warming up and drying off enjoying a coffee. Although it seems small from the outside, inside, the branch feels quite open, with the bar immediately in front of you as you come through the door. One of the attractions of Notes to me, was the fact that I knew that they served different single estate brewed coffees. I think I tried a “La Benedicion” coffee, or at least that is what I seem to have scribbled in my notepad. We took a stool-seat at the window to look out at the rain as my coffee arrived in a 0.25L glass jar. It is always nice to try different single estate coffees and generally, if I know that a café serves single estate coffees I will seek them out to try them for the Daily Grind.

The reflection of the Notes sign board in a cup of tea
The reflections in a cup of tea

Watching the rain form puddles outside, my thoughts were turned to the reflections bouncing off the water in the puddle. It struck me that the appearance of puddles depends on the water molecules behaving both as individual molecules and as molecules within a group. The rain creates ripples in the puddle which can only occur because each molecule is (weakly) attracted to the other water molecules in the puddle, forming a surface tension effect. A ripple is a necessarily collective ‘action’. On the other hand, the reflection of the lights from the street is the response of each individual water molecule to the incoming light. The reflected image is made from the response of many individual molecules. Reflection is more of an individual molecule thing.

Warning sign, train, turbulence
Such turbulence should be familiar to anyone who has stirred a cup of coffee.

I continued thinking about this when I got home where it occurred to me that there was another connection between rain and coffee. It is often said that “rain helps clear the air”, or something similar. Yet this is not quite true. If you have a coffee in front of you at this instant, take a moment to drag a spoon through it. Note the vortices that form behind the spoon. Such vortices form around any object moving through a fluid. In the case of the coffee it is the spoon through the water. For the rain, as the rain drop falls through the air it creates tiny vortices of air behind it. Just as with the coffee spoon, the size of these vortices depend on the speed and size of the falling drop. These vortices pull and trap the atmospheric dust bringing it down to earth more quickly than rain alone could do. The air is cleaned more by this ‘vacuum cleaner’ action than by the ‘wet mop’ of the rain itself.

I’m sure that there are many other coffee-rain connections that you can make if you sit in a café as I did on a rainy day. Let me know your thoughts on this or indeed, on anything that you notice and think interesting while sitting in a café. There is so much to notice if we just put down the phone or close the laptop while enjoying our brew.

Edited to add: Sadly, this article was posted just as Notes Covent Garden was closing down. Notes still has branches at Trafalgar Square and in Moorgate and is opening new branches in Kings Cross and Canary Wharf in February I believe. Hopefully they will all serve single estate brewed coffee and have good window seats from which to observe the rain when it falls.

Categories
Coffee cup science General

Getting my teeth into some latte art

LatteArt_CoffeeworksEach year, in the UK, there are approximately 160 000 hip or knee replacements. Additionally, many of us will have a dental implant during our lifetime. How is this linked with coffee? The answer lies in the differences between a latte and a cappuccino.

To support the artwork that can be seen on many a latte, the milk foam used for the drink is a fine “micro-foam”. It is quite a soft structure. On the other hand a cappuccino is more rigid, being made out of a larger foam structure. The different way that a barista froths the milk for a cappuccino compared to a latte means that the peak structures that can be formed in the cappuccino, are far more difficult to create in a latte, the cappuccino has more of a “meringue like” froth.

Joint replacements and dental implants were traditionally made from solid metal. This meant that the majority of the load that was put on the joint (by walking or chewing for example) was carried by the implant. It is thought that this was one of the reasons that joint replacements and dental implants eventually failed; the bond between the bone and the implant became progressively weaker in a process called “aseptic loosening”. In recent years there have been many improvements to joint replacements/implants so as to avoid these problems. One such improvement is to manufacture the implant out of a metallic foam instead of solid metal.

Cappuccino showing peaks in the foamJust like a latte or a cappuccino, the way that the metal foam responds to stress (and its rigidity) is dependent on many factors including the size of the bubbles in the foam and exactly what the foam is made from. (Imagine comparing a cappuccino with a soya milk cappuccino). By manipulating the structure of the metal foam, an implant can be made that behaves almost exactly as bone does when stress is placed on it. Together with the inherently stronger bone-implant bond created by the bone growing into the ‘bubbles’ of the implant foam, this is thought to reduce the risk of implant failure owing to ‘aseptic loosening’.

I am indebted to Michaela and Juan of Poppy’s Place for patiently showing me the art (and science) of making coffee. With good coffee (from Climpson & Sons) and knowledgeable barista-teachers, it is a place that I am sure that I will return to very soon. Michaela and Juan assured me that if I would like to see a properly rigid milk foam I should order a “babyccino”. There are however limits to the amount that I am prepared to ‘suffer for my science’ and the babyccino is it. If you would like to properly investigate the effect of bubble structure on the ability of an implant (dental or otherwise) to take stress, I suggest you compare a latte with a babyccino. If, like me, you like your coffee, a cappuccino will definitely suffice.

 

Categories
Coffee cup science Coffee review Home experiments

Red Door, Greenwich

Red Door Greenwich, Red Door
Interior of Red Door cafe, Greenwich

Red Door in Greenwich is a great escape from the bustle of the busy streets surrounding it. Although it was crowded when we visited, it was still possible to find a table and have a conversation without too much background noise. I had heard good things about Red Door and wasn’t disappointed. Good coffee (from Monmouth), nice cake and warm surroundings. Definitely a place to go to when in Greenwich. The music that was playing was coming from a record player in the corner. A proper turn-table playing vinyl records. Suddenly, there were so many possibilities for stories for a Daily Grind article. There was the fact that records are analogue based (as opposed to the digital CDs), or perhaps I could write about the physics of a valve amplifier and how it relates to the evaporation of water from coffee (some of the physics is very similar). However what I started to get obsessed with is: what would happen if you put a coffee on a record player?

Now, I am an experimentalist and I do have a record player at home but before I could say “what would happen if…” my plans for experimentation with the record player were blasted out of the water. So I had to make a model record player out of a rotating spice rack. This probably worked better as I could control the speed of rotation, though it did make taking photographs tricky.

record player, turntable
The record player at Red Door

So, what would happen if we put a coffee at the centre of a turntable? The movement of fluids in cups and on record players is extraordinarily complex and is indeed very far from my ‘area of expertise’. However, we can start to understand what might be happening in the cup by making some approximations. Our first approximation is that the coffee in the mug rotates as a ‘rigid body’, meaning that it rotates as a whole. As the coffee cup rotates about its central axis on the “record player” the coffee inside the cup will (eventually) also rotate at the same angular velocity (speed of rotation). The fact that there is a rotation means that there is a force acting on the particles in the coffee liquid. This force produces an acceleration that increases with increasing distance from the axis of rotation. Each coffee particle is of course also subject to the vertical action of gravity. The combined acceleration means that each particle is simultaneously being pulled downwards and inwards. As the acceleration due to rotation increases with increasing radius, the horizontal acceleration becomes increasingly dominant away from the centre of the cup. This leads to the familiar curved surface (a dip at the centre of the mug) that we see with rotating fluids.

vortices, turbulence, coffee cup physics, coffee cup science
This polystyrene cup was rotated about its axis before being stopped. The water inside continues to rotate causing turbulent layers at the edges. These have been visualised with a small amount of blue ink.

Yet we know that this cannot be the full story. If we suddenly stop rotating the mug, the coffee in the mug continues to rotate for a while but does not do so indefinitely; it slows down. We can understand this by refining our approximation that the coffee inside the mug rotates as a rigid body. In fact, the coffee is a viscous liquid and the viscosity means that the layer of coffee immediately adjacent to the mug walls will move at the same speed as those walls: Stationary wall, stationary coffee. The coffee towards the centre of the cup meanwhile continues to rotate for a while. Imagine suddenly stopping the record player so that the mug is now still but the coffee inside continues to spin around the central axis. Stress is being produced between the stationary ‘layers’ of coffee next to the mug wall and neighbouring ‘layers’ of rotating coffee. This stress leads to turbulence. We can make this turbulence visible if, instead of coffee we use a mug of water. Rotate the mug of water as before and then suddenly stop the mug rotating. As with the coffee, the water continues to rotate. Now drop a tiny amount of water soluble ink or food colouring into the very edge of the water (I used a cocktail stick dipped in ink and held against the mug wall so that a small amount dripped into the water). As the water continues to spin, the ink is caught up in the turbulence and the vortices it produces can be seen. These concepts of boundary layers and turbulence are important for many applications including weather systems and car design. We need to understand how liquids (or gases) flow past each other in order to predict the weather and we need to know how they flow past solid objects in order to make cars more aerodynamic. In the coffee however I think that this turbulence is one of those things that is worth just creating and appreciating. A great demonstration of beauty, art and science in a mug of coffee.

Please do share your pictures of these coffee cup vortices if you manage to create them, particularly if you are able to see the effect with cream in coffee. You could either write about your results in the comments section below or email me photographs of your coffee and I will include them on this page. As always, enjoy your coffee.

My thanks to Kate & Edward of Red Door for sending me the photos of Red Door.

Extra photos of vortices in a rotating coffee:

Rotating coffee

An attempt at visualising the vortices using cream in coffee. Not so successful though you can see at least 2 well defined vortices in the top left of the image. Introducing the second liquid right at the edge of the mug seems critical, not so easy with cream as it is with ink!