Home experiments

A short (lived) black

coffee at Story
A black coffee with bubbles on top. The colours on a bubble are the result of light interference. But sometimes the top of the bubble could appear black. What is happening there?

The long black can be distinguished from the Americano by the order in which the espresso and the water are added to the cup. This in turn will affect the type of bubbles on the surface of the coffee. As a guess, the long black (espresso last) will have many more but smaller bubbles than the Americano (water last) which will probably have larger, but fewer bubbles. Perhaps this guess is wrong, this could be an excuse to get out and drink more coffee.

We are used to the coffee being black and the bubbles on the surface reflecting a rainbow of shimmering colours that change with the light and with time before they finally burst. We know the physics of the colours on the bubbles: they are the result of the interference of reflections from the outer and inner surface of the bubble cancelling out certain colours and adding to others dependent on the bubble skin’s thickness. But what about black bubbles? Or, if not entirely black, perhaps the cap of the bubble can, for a short while, appear black just before the bubble bursts?

It is easier to take a short break from coffee and look for this effect in soap films. Like the bubbles on coffee, soap bubbles are caused by the surfactant in the soap solution having a hydrophilic (water loving) and hydrophobic (water hating) end. The hydrophilic end of the surfactant can point into the water (coffee) leaving the hydrophobic end to form a surface. When this is agitated with air, the hydrophilic ends remain contacted with water resulting in bubbles which are thin layers of water surrounded by these surfactant molecules. In coffee the surfactant is not soap but is formed by the lipids and fatty acids. These bubbles are therefore slightly weaker than the soap based bubbles and so while they will form on a coffee, it is not easy to make a film of a coffee bubble in the same way as you can dip a wire loop into a soap solution and come out with a soap film.

However, we can use the stability of the soap film to investigate the colours in the coffee bubbles and watch the colours evolve with time. At this point, I would strongly encourage anyone reading to grab a solution of soap and a wire loop and start playing with soap films.

Soap film in a wire loop held by a crocodile clip.
A soap film in a wire loop showing reflected horizontal coloured bands that are the result of light interference.

Holding the wire loop so that the soap film is vertical with a light source shining at it, we can watch as the film changes from being uniformly transparent to having bands of colour form and move down the film. We watch as there is a red/green band and another red/green band and then on top of the bands there appears a white, or at least pale blue, almost white, band and above that a layer that doesn’t reflect the light at all. If we view the soap film against a dark background looking only at the reflected light, this top portion of the film appears black. Rotating the loop we can see that the bands effectively stay in the same position because it is gravity pulling on this soap film that is causing the film to be thicker at the bottom than at the top. And we recognise that the coloured bands are revealing that thickness change to us by the fact that they are changing throughout the film. If we are careful as we rotate the wire, we could even see vortex like motions as the layers settle into their new position relative to the frame including at the very top where there are swirls and patches of fluid that mix the black layer with the coloured bands. What is going on there?

In fact, this black layer is one of the thinnest things that they human eye can see, and it occurs because of a subtle piece of physics. All waves have a number of properties defined by the position of the peaks and troughs on the wave. The wavelength is the distance between two equivalent points on the wave. The amplitude is the height of the peak (or trough). And the phase is the position of the wave relative to the peak (or trough). When light is reflected at a surface of a material that has a refractive index greater than that which the light is travelling through (eg. air into water, soap, or glass), the reflected wave has a 180 degree phase shift relative to the incident wave. Each peak becomes a trough, each trough becomes a peak. When light is already travelling through water, soap or glass and gets reflected at the surface of the material that is effectively air, there is no phase shift and the light is reflected back with the same phase as the incident wave (a peak remains a peak and a trough a trough).

At the top of the soap film, the layer is so thin that the light reflected from the first surface (180 degree shift) overlays that reflected from the back surface (no phase shift) so that peak and trough cancel each other out and we see no light reflected whatsoever for any visible wavelength; the surface looks black.

As bubbles ‘ripen’ or age, they will become thinner at the top of the bubble. It is at this point that you may be lucky enough to see a region of the bubble from which no light is reflected, this is the black film.

Which leads to some immediate questions. When we look carefully at the soap film, the boundary between the upper white band and the black film is quite sharp, it is not gradual as we may expect if the soap film were completely wedge shaped with gravity. It suggests that the top of the film is very thin and then suddenly gets thicker at the point where we start to see the colour bands. Moreover, the black film does not appear to mix with the thicker film just beneath it. As we watch, just before the soap film bursts, we get turbulence between the black layer and the thicker film, but the turbulent patterns appear like two fluids next to each other, not the same fluid in a continuum. And then, one final question. If we can’t measure the thickness of the black film with light (because it is all reflected as black) how can we know how thick this film is? If we rely on the light interference method, all we can say is how much thinner it is than the wavelength of light.

In fact, careful experiments have revealed two types of black film, which to us experimenting at the kitchen table would be indistinguishable. There is the common black film and the Newton black film. The Newton black film is effectively two layers of surfactant molecules only and is about 5nm thick (which is 5 millionths of a millimetre). The common black film is thicker, but is still much less than 100 nm thick. Investigating how these films behave is still an active area of research.

One last observation may prompt us to play for a bit longer with the soap films. Johann Gottlob Leidenfrost (1715-94) noted that if you put a sharp object such as a needle through the region of the soap film that showed the coloured bands, the film could self-heal and wouldn’t necessarily burst. If however you pierced the black region of the film, the film always burst entirely.

It seems that we could play endlessly with soap films, perhaps while watching the bubbles in our coffee. However you enjoy your coffee, have fun experimenting.

A couple more soap films showing reflected coloured interference bands. At the top, the film has become so thin that no light is reflected (clearly seen in the image on the right, where the lamp in the top left should be a circular reflection but is not reflected in the region above the coloured bands). In the image on the left, you can see what looks like turbulence or mixing just above the uppermost band.

To err is human…

Press Room coffee Twickenham
A smaller V60. For one cup you would use less coffee, but the errors on the measurement will always be there.

Preparing a good V60 requires 30g of coffee (for 500 ml of water)*. This can be measured using a set of kitchen scales, but a first estimate can also be obtained, if you are using whole coffee beans, by timing the passage of the grind through the grinder. Using an Ascaso burr grinder, my coffee used to come through at an approximate rate of 1g/s, so that, after 30 seconds, I’d have the perfect amount of coffee. Recently however this has changed, depending on the bean, sometimes 30g is 40 seconds, sometimes just less than 30 seconds.

Clearly there is an error on my estimate of the rate of coffee grinds going through the grinder. This may be influenced by factors such as the hardness of the bean (itself influenced by the degree of roast), the temperature of the kitchen, the cleanliness of the grinder and, the small detail that the ‘seconds’ measured here refers to my counting to 30 in my head. Nonetheless, the error is significant enough that I need to confirm the measurement with the kitchen scales. But are the scales free of error?

Clearly in asking the question, we know the answer will be ‘no’. Errors could be introduced by improper zero-ing of the scales (which is correct-able), or differences in the day to day temperature of the kitchen (not so correct-able). The scales will also have a tolerance on them meaning that the measured mass is, for example, only correct to +/- 5 % Depending on your scales, they may also only display the mass to the nearest gramme. This means that 29.6g of coffee would be the same, according to the scales, as 30.4g of coffee. Which in turn means that we should be using 493 – 507 ml of water rather than our expected 500 ml (the measurement of which also contains an intrinsic error of course).

Turkish coffee
A Turkish coffee provides a brilliant illustration of the type of particle distribution with depth that Jean Perrin used to measure Avogadro’s constant. For more information see here.

The point of all of this is that errors are an inescapable aspect of experimental science. They can also be an incredibly helpful part. Back in 1910, Jean Perrin used a phenomenon that you can see in your coffee cup in order to measure Avogadro’s constant (the number of molecules in a mole of material). Although he used varnish suspended in water rather than coffee, he was able to experimentally verify a theory that liquids were made up of molecules, by the fact that his value for Avogadro’s constant was, within error, the same as that found by other, independent, techniques. Errors also give us an indication of how confident we can be in our determination of a value. For example, if the mass of my coffee is 30 +/- 0.4 g, I am more confident that the value is approximately 30 g than if the error was +/- 10 g. In the latter case, I would get new scales.

But errors can also help us in more subtle ways. Experimental results can be fairly easily faked, but it turns out that the random error on that data is far harder to invent. A simple example of this was seen in the case of Jan Hendrik Schön and the scientific fraud that was discovered in 2002. Schön had shown fantastic experimental results in the field of organic electronics (electronic devices made of carbon based materials). The problem came when it was shown that some these results, despite being on different materials, were the same right down to the “random” noise on the data. Two data sets were identical even to the point of the errors on them, despite their being measurements of two different things.

A more recent case is a little more subtle but crucial for our understanding of how to treat Covid-19. A large study of Covid-19 patients apparently showed that the drug “Ivermectin” reduced mortality rates enormously and improved patient outcomes. Recently it has been shown that there are serious problems with some of the data in the paper, including the fact that some of the patient records have been duplicated and the paper has now been withdrawn due to “ethical considerations”. A good summary of the problems can be found in this Guardian article. However, some of the more worrying problems were a little deeper in the maths behind the data. There were sets of data where supposedly random variables were identical across several patients which suggested “that ranges of cells or even entire rows of data have been copied and pasted“. There were also cases where 82% of a supposedly random variable ended in the digits 2-5. The likelihood of this occurring for random variables can be calculated (it is not very high). Indeed, analysis of the paper showed that it was likely that these values too were either copy and pasted or “invented” because humans are not terribly good at generating properly random numbers.

A gratuitous image of some interesting physics in a V60. If anyone would like to hire a physicist for a cafe, in a 21st century (physics) recreation of de Moivre’s antics at Old Slaughters, you know how to contact me…

Interestingly, a further problem both for the Ivermectin study and for the Schön data comes when you look at the standard deviation of the data. Standard deviation is a measure of how variable is the measured outcome (e.g. duration of time a patient spent in hospital). For the ivermectin study, analysis of the standard deviations quoted on the patient data indicated a peculiar distribution of the length of hospital stay, which, in itself would probably just be a puzzle but in combination with the other problems in the paper becomes a suggestion of scientific fraud. In Schön’s data on the other hand, it was calculated that the precision given in the papers would have required thousands of measurements. In the field in which Schön worked this would have been a physical impossibility and so again, suggestive of fraud. In both cases, it is by looking at the smaller errors that we find a bigger error.

This last detail would have been appreciated by Abraham de Moivre, (1667-1754). As a mathematician, de Moivre was known for his work with probability distribution, which is the mathematics behind the standard deviation of a data set. He was also a well known regular (the ‘resident’ mathematician) at Old Slaughters Coffee House on St Martin’s Lane in London[1]. It is recorded that between 1750 and 1754, de Moivre earned “a pittance” at Old Slaughters providing solutions to games of chance to people who came along for the coffee. I wonder if there are any opportunities in contemporary London cafes for a resident physicist? I may be able to recommend one.

*You can find recipes suggesting this dosage here or here. Some recipes recommend a slightly stronger coffee amount, personally, I prefer a slightly weaker dosage. You will need to experiment to find your preferred value.

[1] “London Coffee Houses”, Bryant Lillywhite, 1963

Up in the air with a Pure Over Brewer

The diffuser sitting on top of the Pure Over coffee brewer. The holes are to ensure that the water falls evenly and slowly onto the grounds below.

The Pure Over is a new type of coffee brewer that is designed to brew filter coffee without the need for disposable paper filters. The brewer, which is completely made of glass, is a perfect size for brewing one cup of coffee and, as promised, makes a lovely cup without the need for wasteful paper filters. Generally, for 1-cup filter coffees, the Pure Over has become my go-to brewing method, although it does have a few idiosyncrasies to it that are helpful to be aware of while brewing.

An advantage of this brewing device is that it provides a large number of opportunities for physics-watching, including a peculiar effect that connects brewing coffee to an air balloon crash into the garden of a London Coffee House. It concerns a feature of the Pure Over that is specific to this particular brewing device: the ‘diffuser’ that sits on top of it.

The glass diffuser has five small holes at the bottom of it which are designed to reduce the flow of the water onto the coffee bed so that it is slower and more gentle. In order to avoid the paper filters, the Pure Over features a filter made of holes in the glass at its base. This filter does surprisingly well at keeping the coffee grounds out of the final brew, but it works best if the coffee bed just above it is not continuously agitated. The idea of the diffuser is that the coffee grounds are more evenly exposed to the water, with the grounds closest to the filter being least disturbed and so the coffee is extracted properly.

As water is poured from a kettle through the diffuser, the water builds up in the diffuser forming a pool that slowly trickles through the holes. Initially this process proceeds steadily, the water is poured from the kettle into the diffuser and then gently flows through and lands on the coffee. At one point however, the pressure of the steam within the main body of the brewer builds until it is enough to push the glass diffuser up a bit, the steam escapes and the diffuser ‘clunks’ back onto its base on top of the pure over. Then, this happens again, and again, until there is a continuous rattle as the steam pressure builds, escapes and builds once more.

The ideal gas laws, such as that found by Jacques Charles, relate the volume and pressure of a gas to its temperature. The application of the laws helped to improve the design of steam engines such as this Aveling and Porter Steam Roller that has been preserved in central Kuala Lumpur, Malaysia.

The pressure of the steam builds until the force exerted upwards by the rising steam is greater than the weight of gravity pulling the diffuser down. Once enough gas escapes, the pressure is reduced and so the steam no longer keeps the diffuser aloft which consequently drops with a clunk. The motion could take our thoughts to pistons, steam engines and the way that this steam movement was once exploited to drive our industrial revolution. Or you could go one stage earlier, and think about the gas laws that were being developed shortly before. There’s Boyle’s Law which relates the pressure of a gas to its volume (at constant temperature). That would perhaps partially explain the behaviour of the pure over. But then there’s also Jacques Charles and his observation that the volume of a gas is proportional to its temperature (at constant pressure). This too has relevance for the pure over because as we pour more water in from the kettle, we warm the entire pure-over body and so the temperature of the gas inside will increase. Consequently, as the amount of hot water in the pure over increases, the temperature goes up, the volume of that gas would increase but is stopped by the diffuser acting as a lid. This leads to the pressure of the gas increasing (Boyle) until the force upwards is high enough, the diffuser lid rises upwards on the steam which escapes leading the pressure to once again drop and the diffuser top to go clunk and the whole cycle begins again.

Of course, we know that Boyle’s law is appropriate for constant temperature and Charles’s law is appropriate for constant pressure and so the laws are combined together with the Gay-Lussac/Amonton law into the ideal gas laws which explain all manner of things from cooling aerosols to steam engine pistons. And yet, they have another connection, which also links back to our pure over, which is the history of hot air balloons.

Charles discovered his law in around 1787, a few years after the first non-tethered hot air balloon ascent, in Paris, in June of 1783. The hot air balloon is a good example of the physics that we can see in the pure over. Although Charles must have suspected some of the physics of the hot air balloon in June, he initially decided to invent his own, hydrogen filled balloon which he used to ascend 500 m in December of 1783. Hydrogen achieves its lift because hydrogen is less dense than air at the same temperature. However, it is the hydrogen balloon that links back to coffee and coffee in London.

hot air balloon
The ideal gas laws also contribute to our understanding of the operation of hot air balloons. We are familiar with them now, but how would such an object have been perceived by observers at the time of the first flights?

The first balloon flight in England took place using a hydrogen, not a hot-air, balloon in 1785. The balloon was piloted by Vincenzo Lunardi who was accompanied by a cat, a dog and, for a short while, a pigeon (before it decided to fly away). But it was not this successful flight that connects back to coffee, it was his maiden flight on 13 May 1785. On that day, Lunardi took off from the Honourable Artillery Company grounds in Moorgate, flew for about 20 minutes and then crashed, or as they said at the time “fell with his burst balloon, and was but slightly injured”(1) into the gardens of the Adam and Eve Coffee House on the junction of Hampstead Road and, what is now, Euston Road. In the 1780s the Adam and Eve coffee house had a large garden that was the starting point for walks in the country (in the area now known as Somers Town)(2). Imagine the scene as, quietly appreciating your tea or coffee, a large flying balloon crashes into the garden behind you.

The Adam and Eve is no longer there, in fact, its original location now seems to be the underpass at that busy junction, and the closest coffee house is a branch of Beany Green. However there is one, last coffee connection and it brings us back to the pure over. The pressure of the steam under the diffuser needs to build until the upwards force of the steam can overcome the gravitational force down of the weight of the glass diffuser. In the same way Lunardi had to have enough lift from the hydrogen balloon to compensate for the weight of the balloon and its passengers. Lunardi had wanted to be accompanied by another human on the day of his successful flight. Unfortunately, the mass of two humans in a balloon was too much for the balloon to accommodate which is why, the human was replaced by the dog, the cat and the pigeon.

Which may go some way to illustrate how far the mind can travel while brewing a cup of coffee, particularly with a brew device as full of physics as the Pure Over.

1 London Coffee Houses, Bryant Lillywhite, George Allen and Unwin publishers, 1963

2 The London Encyclopaedia (3rd edition), Weinreb, Hibbert, Keay and Keay, MacMillan, 2008

Viewing an eclipse, the coffee way

NASA image of annular eclipse from space
A different perspective? This is the view looking towards Earth of the 2017 Annular solar eclipse over South America. Taken by the EPIC DSCOVR project of NASA.

This week, on Thursday, June 10th, 2021, there will be a solar eclipse. If you are at high latitudes in the Northern Hemisphere including parts of Canada, Greenland and Siberia, you will see a so-called ring of fire as the moon moves in front of the Sun. At lower latitudes the eclipse will be much more partial and in London we are expecting to see 20% of the Sun obscured by the Moon.

You can read more about solar eclipses on other websites such as here or here, on Bean Thinking, we are going to focus on the coffee links to the eclipse.

The first coffee link comes in how to view it. This website suggested a number of ways of viewing the eclipse, one of which was to use a colander. This suggests a perfect adaptation to a view via coffee: the Aeropress filter cap. The idea behind the method is that each of the holes provides a type of pin-hole camera to image the Sun. Knowing roughly where the Sun will be at 10.06am (BST = UTC+1), we can construct a device to hold the aeropress filter cap so that we can see 97 images of the Sun projected onto a piece of paper: 97 images of the Sun to be eclipsed over the following 2hours 18 minutes. The maximum eclipse is around 20% of the solar disc and occurs at approximately 11.15 (although the exact fraction obscured and timing depends on your location). The Aeropress Eclipse viewing device shown in the photo here has an added (smaller) pin hole which should provide a more focussed image of the Sun and so will provide a second way of imaging the eclipse.

A second coffee link comes with thinking about why this particular solar eclipse is not ‘total’ anywhere on earth but is instead described as annular. And to do this, we’ll think about a coffee bean. The amazing visual spectacle of a total solar eclipse occurs because the moon is 400 times smaller than the Sun but is (on average) about 400 times closer to the Earth. So when we think about looking at a coffee bean, held at arms length from our eye (about 60cm), it would totally obscure (eclipse) an object 3.2 m tall, 233.5 m away*.

Eclipse viewer
An aeropress based device for viewing the eclipse. The strings attached to the cardboard flap at the top allow the angle of the aeropress filter cap to be fixed at different points. The camera is at the approximate point where the images will be projected onto paper.

The word “average” though hides an important detail that neither the Moon’s orbit around the Earth, nor the Earth’s orbit around the Sun are completely circular. On the 10th June 2021, the Moon will be two days past its maximum distance (apogee) from the Earth, and while the Sun is also nearly at its maximum distance, the distance ratio will mean that the Moon does not entirely obscure the Sun. Instead, if we return to our coffee bean analogy, it is the equivalent of stretching our arm 2 more centimetres and noticing that the object that was obscured is no longer completely obscured.

This will still make for a fantastic view if you are in Greenland, Siberia or happen to be at the North Pole where you will see a dark disc surrounded by a ring of Sun. For those of us further south, we will only see the Sun partially obscured by the Moon. Nonetheless, such an opportunity in any one particular location doesn’t come super-often (although worldwide there are often several eclipses per year, in London there will only be 42 partial eclipses in this current century). And in London, we have to worry about the weather too. So, if the weather is good for you, why not have a go viewing it, particularly if you adapt a piece of coffee brewing equipment to do so, and post your pictures of the effect here, or to Bean Thinking on Twitter or Facebook.

Finally, the timing of the eclipse is perfect for a mid-morning coffee, though maybe you’ll have to brew with something other than the Aeropress. Have fun.

*These figures have been calculated using a ratio of the size of the Moon to the Sun as 1:400.8 and an average distance of 1:389.2 (calculated from the average values). The distances on June 10 2021 mean that the distance ratio is closer to 1:377

Update to post, the day before (9 June 2021): This is the Aeropress viewing device in action, but 24h before the eclipse. Will the clouds stay away tomorrow?

The Aeropress Eclipse viewer in action. The images of the Sun are projected onto the cardboard behind the filter cap.

Update 10 June 2021: It was cloudy in London and I couldn’t get the Aeropress filter cap method to work in the brief periods of sunshine during the eclipse. Suspect it was a problem with focus-distance/angle/remaining cloud cover at points. However, the smaller pinhole did work (see the blurry image below) and the clouds did mean that there was a natural filter that made a direct photograph possible (see below). Do share your images here if you managed to view it.

Although there were brief periods without cloud, focussing issues etc. meant that I couldn’t get the Aeropress filter cap viewing method to work. Maybe for the next one!
A smaller pinhole did give an image of the Sun being eclipsed (lower blurry bright image)
The fact that it was cloudy did mean however that I could take a photograph of the eclipsed Sun directly. This was at about 11.10am (5 minutes or so before the maximum point of eclipse)


When you prepare a filter coffee with a paper filter, you typically rinse the filter before starting the brewing process. As you do so the paper swells and can absorb several ml of water.

The other morning while preparing a V60, I noticed that the filter paper absorbed between 3-6g of water (3-6ml) each time I rinsed the filter before making a new coffee. My mind wandered to re-hydrating space food and the importance of water in the texture of the food we eat (and coffee we drink). And then I was reminded of a question I had been asked during these Covid-19 times: would a face mask that is damp work better, or worse, than a dry one for reducing the transmission of SARS-CoV-2, the virus that causes Covid-19?

The answer did not seem obvious. On the one hand, when we wet the paper filter while brewing coffee, the fibres within the paper swell and reduce the pore size of the filter. It seems likely that cotton fibres in a mask would behave similarly. This would have the effect of slowing and reducing the transmission of particulates through the mask. But on the other hand, we’re not thinking about particulates but about small amounts of viral material hosted in water droplets that are somehow exhaled. I decided on the “no idea” response at the time and put the question aside. Until the other morning while preparing coffee.

Unsurprisingly this question, and many like it are now the subject of intense research. I say unsurprisingly because a few years ago a new family of superconductors was discovered with (relatively) very high transition temperatures*. I was on holiday at the time but when I returned, it was to a large number of emails and ideas for experiments on these new materials that became known as the iron based superconductors. We had our first paper on these materials within a couple of months which, like all papers on this at the time, was uploaded, without peer review, to a pre-print server. Eventually most of the papers on the pre-print server got published in peer-reviewed journals, but this process was slow because it relied (and still does) on other scientists reading and taking the time to carefully respond to the points in your manuscript, then for you to address these points, for them to read it again and then, hopefully, ok the paper for publication. If you wanted to get the paper out and for a discussion to start, it had to be uploaded to the pre-print server.

canali Curators Coffee
Iron is a magnetic element. It was puzzling how a magnetic element could exist in a superconducting material and, moreover, seemed to make these materials even better superconductors than their non-magnetic counterparts.

Clearly, in order to keep up with scientists worldwide, we were looking at the pre-print server every morning looking for new ideas and new observations (and if anyone had done the same as we were trying to do at that precise moment but ‘beaten’ us to it). We had to be careful while assessing the claims in the pre-print papers. Some of the pre-prints were eventually withdrawn as they had made overblown claims (admittedly very few). Many were revised and had their claims either subtly altered or brought down a bit from hyperbole before being published in the journals. But none of this mattered to the world outside the lab because while exciting to us, and while the temperature of the transition was, from a physics perspective, very high, for the general public it would have been hard to get excited about materials that went superconducting below about 50 K or, in more common units, -223 C.

This side-story matters because, like our superconductors, the pandemic is the subject of intense research with much of it being uploaded to pre-print servers first so that scientists world wide can get into a conversation about the latest results. However, unlike our superconductors, the general public cares a great deal about a pandemic that is affecting us all and about the scientific rationale for measures such as mask-wearing, social distancing etc. While it is tempting to read the pre-prints, as I am not working in the field, it is not possible for me to read the papers on pre-print servers and be able to have a good guess as to whether the claims are reasonable, over blown or under-evidenced. So, I try to rely only on papers that are past the point of peer review and published in scientific journals. There is something very disheartening about reading an interesting newspaper report that near the end says “the study, which has not yet been peer-reviewed…”. Will the interesting study hold up? It is difficult, from outside the research area, to tell.

However, we need to get back to the masks and the filters. Was there a study, in the peer-reviewed and published literature, that looked at whether moistened masks performed better than non-moistened masks?

Masks: can we set up an experiment to see how effective ours are relative to the fitted N95s that are not available to most of us?
Masks: can we set up an experiment to see how effective ours are relative to the fitted N95s that are not available to most of us?

In fact, there is a lot of research on the effectiveness of masks. The research includes computer modelling, imaging of real people breathing/talking/coughing with and without masks and more reproducible tests where the mask material is tested using the conditions of a simulated sneeze. This last study also tested whether that simulated sneeze is contained better by a cloth mask (with filtration down to PM 2.5) or a damp cloth mask (with the same nominal filtration).

The different types of research are needed because they answer different types of question. How effective each type of mask is will depend on the type of material (tested with the simulated sneeze) and the way that people wear them (tested by the imaging of people wearing masks). While the computer modelling suggests what may happen in more ‘real life’ environments such as being outdoors with a gentle wind blowing.

In terms of the initial question about the damp masks, it turns out that the fact that the fibres in the mask swell with the water does indeed help reduce the droplet transmission through the mask material. But the authors caution that if the mask is worn for a longer period of time, the damp mask may get saturated with virus loaded droplets and so the mask would need to be changed (and refreshed with fresh water) frequently in order for it to be effective against transmission of the virus loaded droplets. (It’s also noteworthy that the effect of the damp mask was only tested for one mask type that may not be typical of what the general public wears). However, for most of us it would not be practical anyway to wear a damp mask. Moreover, if we were having to change the mask frequently, it may not be helpful for us at all. But the good news is that the imaging studies show that we don’t have to do either.

A fantastic report in Scientific Advances showed two things. First, that most masks that we wear properly give a significant benefit for the people around us. And secondly, they provided an experimental set up that can easily and relatively cheaply be replicated by people with a little technical knowledge and a mobile phone. However, given that ‘relatively cheaply’ still means about $200, I’ll take their results instead, if you don’t mind spending the money on a laser and some lenses (or happen to have some lying around), please do let me know how you get on.

Press Room coffee Twickenham
Another paper filter, this time at the Press Room, Twickenham. When we add water to a (dry) paper filter, the fibres within it swell and expand making it a better filter. Would the same happen with masks?

The authors took several of the types of face mask being worn by the public and imaged the droplets coming from a person speaking through each of them. The masks tested included surgical masks, N95 masks, and hand-made masks with 2-layers of cotton or 2-layers of cotton with an extra polypropylene layer in the middle. All of these masks reduced the droplets transmitted through the mask significantly. Indeed, relative to no-mask, some home-made multiple cotton layer masks cut the droplets by nearly a factor of 10. The exceptions were bandanas and neck gaiters. The bandanas that were tested only cut the droplets getting through by a factor of 2, but the gaiters were worse. Speaking through the neck gaiter that they tested, the authors observed that the number of droplets getting through the gaiter actually increased relative to speaking wearing no mask. While this seems counter-intuitive, they suggested that this was likely because the gaiter was breaking up the larger droplets into multiple smaller droplets and so their equipment, which just measured the number of droplets, measured an increase relative to someone wearing no mask.

The problem here of course is, as the computer simulations showed, smaller droplets stay in the air for longer, larger droplets tend to fall with gravity. Something else that we know by thinking about our coffee.

So the final conclusion? Yes, it is possible that a damp mask may be better than a dry one though there are caveats on that result. But in actual fact, most masks that we wear in an indoor environment will help to protect other people (though maybe be careful with the gaiter materials). And a second conclusion? Perhaps preparing a coffee should be a time of escape from the concerns of coronavirus and really, next time, I should just enjoy the moment and think about re-hydrating space food.

*Actually, the iron-based superconductors had been discovered a couple of years previous to the excitement. But at that point, the reported transition temperatures were low enough that even the superconducting field was curious but not excited.

Missing matter

soya latte at the coffee jar camden
Not one made by me! But instead a soya-latte at the Coffee Jar a couple of years ago.

During these strange times of working from home, perhaps you, like me, have been preparing a lot more coffee. For me this has included, not just my regular V60s, but a type of cafe-au-lait for someone who used to regularly drink lattes outside. My previous-latte-drinker turns out to be a little bit discerning (the polite way of phrasing it) and so prefers the coffee made in a similar way each day. Which is why I’ve been weighing the (oat) milk I’ve been using.

So, each morning to prepare a coffee, I’ve been using a V60 recipe from The Barn and then, separately, weighing out 220g of refrigerated oat milk into a pan that I then heat on the stove. Generally I heat the milk for just over 5 minutes until it is almost simmering whereupon I pour it into a mug (with 110 – 130g of coffee inside – depending on the coffee). Being naturally lazy, I keep the cup on the scales so that it is easier to pour the milk in and then, completely emptying the pan into the coffee, the scales register an increase of mass (of milk) in the cup of 205-210g. Which means about 10-15g of milk goes missing each morning.

Now clearly it is not missing as such, it has just evaporated, but it does prompt a question: can this tell us anything about the physics of our world? And to pre-empt the answer, it actually tells us a great deal. But to see how, we need to go on an historical diversion to just over three hundred years ago, when Edmond Halley was presenting an experiment to the Royal Society in London. The experiment shares a number of similarities with my heated oat milk pan. It was later written into a paper which you can read online: “An estimate of the quantity of vapour raised out of the sea by the warmth of the Sun; derived from an experiment shown before the Royal Society at one of their late meetings: by E Halley“.

lilies on water, rain on a pond, droplets
Coffee, evaporation, clouds, rain, rivers, seas, evaporation. Imagining the water cycle by making coffee.

Halley heated a pan of water to the temperature of “the Air in our hottest summers” and then, keeping the temperature constant, placed the pan on a set of scales to see how much water was lost over 2 hours. The temperature of the air in “our hottest summers” cannot have been very high, perhaps 25-30C and there was no evaporation actually seen in the form of steam coming from the pan (unlike with my milk pan). Nonetheless, Halley’s pan lost a total of 13.4g (in today’s units) of water over those two hours.

Halley used this amount to estimate, by extrapolation, how much water evaporated from the Mediterranean Sea each day. Arguing that the temperature of the water heated that evening at the Royal Society was similar to that of the Mediterranean Sea and that you could just treat the sea as one huge pan of water, Halley calculated that enough water evaporated to explain the rains that fell. This is a key part of the water cycle that drives the weather patterns in our world. But Halley took one further step. If the sea could produce the water for the rain, and the rain fed the rivers, was the flow of the rivers enough to account for the water in the Mediterranean Sea and, specifically, how much water was supplied to the sea compared to that lost through the evaporation? Halley estimated this by calculating the flow of water underneath Kingston Bridge over the Thames. As he knew how many (large) rivers flowed into the Mediterranean, Halley could calculate a very rough estimate of the total flow from the rivers into the Mediterranean.

Grecian, Devereux, Coffee house London
A plaque outside the (old) Devereux pub, since refurbished. The Devereux pub is on the site of the Grecian Coffee House which was one of the places that Halley and co used to ‘retire’ to after meetings at the Royal Society.

The estimates may seem very rough, but they were necessary in order to know if it was feasible that there could be a great water cycle of rain, rivers, evaporation, rain. And although Halley was not the first to discuss this idea (it had been considered by Bernard Palissy and Pierre Perrault before him), this idea of a quantitative “back of the envelope” calculation to prompt more thorough research into an idea, is one that is still used in science today: we have an idea, can we work out, very roughly, on the back of an envelope (or more often on a serviette over a coffee) if the idea is plausible before we write the research grant proposal to study it properly.

So, to return to my pan of oat milk simmering on the stove. 15g over 5 minutes at approaching 100C is a reasonable amount to expect to lose. Only, we can go further than this now because we can take the extra data (from the thermostats we have in our house and the Met Office observations for the weather) of the temperature of your kitchen and the relative humidity that day and use this to discover how these factors affect the evaporative loss. Just as for Halley, it may be an extremely rough estimate. However, just as for Halley, these estimates may help to give us an understanding that is “one of the most necessary ingredients of a real and Philosophical Meteorology” as Halley may have said before he enjoyed a coffee at one of the Coffee Houses that he, Newton and others would retire to after a busy evening watching water evaporate at the Royal Society.

Telling the time with an Aeropress?

Aeropress bloom, coffee in an Aeropress

The first stage of making coffee with an Aeropress is to immerse the coffee grind in the water. Here, the plunger is at the bottom of the coffee.

On occasion, it takes a change in our routine for us to re-see our world in a slightly different way. And so it was that when there was an opportunity to borrow an Aeropress together with a hand grinder, I jumped at it. Each morning presented a meditative time for grinding the beans before the ritual of preparing the coffee by a different brew method. Each day became an opportunity to think about something new.

Perhaps it is not as immediately eye catching as the method of a slow pour of water from a swan necked kettle of a V60, and yet making coffee using the Aeropress offers a tremendously rich set of connections that we could ponder and contemplate if we would but notice them. And it starts with the seal. For those who may not be familiar with the Aeropress, a cylindrical ‘plunger’ with a seal tightly fits into a plastic cylinder (brew guide here). The first stage of making a coffee with the Aeropress is to use the cylinder to brew an ‘immersion’ type coffee, exactly as with the French Press (but here, the plunger is on the floor of the coffee maker). Then, after screwing a filter paper and plastic colander to the top of the cylinder and leaving the coffee to brew for a certain amount of time, the whole system is ‘inverted’ onto a mug where some coffee drips through the filter before the rest is forced out using the plunger to push the liquid through the coffee grind.

clepsydra creative commons license British Museum

A 4th century BC Ptolemaic clepsydra in the British Museum collection. Image © Trustees of the British Museum

Immediately perhaps your mind could jump to water clocks where water was allowed to drip out of two holes at the bottom of a device at a rate that allowed people to time certain intervals. It is even suggested that Galileo used such a “clepsydra” to time falling bodies (though I prefer the idea that he sang in order to time his pendulums). With many holes in the bottom of the device and an uneven coffee grind through which the water (coffee) flows, the Aeropress is perhaps not the best clock available to us now. However there is another connection between the Aeropress and the clepsydra that would take us to a whole new area of physics and speculation.

When the medieval thinker Adelard of Bath was considering the issue of whether nature could sustain a vacuum, he thought about the issue of the clepsydra¹. With two holes at the bottom and holes at the top for air, the clepsydra would drip the water through the clock at an even rate. Unless of course the holes at the top were blocked, in which case the water stopped dripping, (a similar thing can be observed when sealing the top of a straw). What kept the water in the jar when the top hole was blocked? What kept it from following its natural path of flowing downwards? (gravity was not understood at that point either). Adelard argued that it was not ‘magic’ that kept the water in when no air could go through, something else was at work.

What could be the explanation? Adelard argued that the universe was full of the four elements (air, water, fire, earth) which are “so closely bound together by natural affection, that just as none of them would exist without the other, so no place is empty of them. Hence it happens, that as soon as one of them leaves its position, another immediately takes its place… When, therefore, the entrance is closed to that which is to come in, it will be all in vain that you open an exit for the water, unless you give an entrance to the air….”²

inverted Aeropress and coffee stain

The Aeropress inverted onto a coffee cup before the plunger is pushed down. Complete with coffee stain behind the cup where the inversion process went awry.

Now, we would argue that whether the water flows down and out of the Aeropress, or not, depends on the balance of forces pushing the water down and those pushing it up. The forces pushing the water down and out of the clepsydra, or Aeropress, are gravity and the air pressure above the water in the cylinder. Pushing it up, it is only the air pressure from below. Ordinarily, the air pressure above and that below the water in the Aeropress are quite similar, gravity wins the tug of war and the water flows out. In an enclosed system however (if the holes at the top are blocked), were the water to flow out of the bottom, the air pressure above the coffee space would reduce. This makes sense because, if no new air gets in, the same amount of air that we had before now occupies a larger volume as the water has left it, the pressure exerted by that air will have to be less than before. A reduced air pressure means a reduced force on the water pushing it down through the filter and so the force pushing the water down can now be perfectly balanced by the force (from the surrounding air) pushing the water up: the water remains in the Aeropress. The only way we get the coffee out is to change the balance of forces on the water which means pushing down the plunger.

But perhaps it is worth stepping back and imagining what the consequences could be of having the idea that the universe was just full of something that had to be continuous. You may find it quite reasonable for example to consider that vortices would form behind and around the planets as they travelled in their circular orbits through this ‘something’*. Such vortices could explain some of the effects of gravity that we observe and so there would perhaps be no urgency to develop a gravitational theory such as the one we have. There would be other consequences, the world of vacuum physics and consequently of electronics would be significantly set back. In his lecture for the Carl Sagan Prize for Excellence in Public Communication in Planetary Science, The Director of the Vatican Observatory, Br Guy Consolmagno SJ explored previous scientific ideas that were almost right, which “is to say wrong” (You can see his lecture “Discarded Worlds: Astronomical Worlds that were almost correct” here) If it is true that so many scientific theories lasted so long (because they were almost correct) but were in fact wrong, how many of our scientific ideas today are ‘almost correct’ too?

It makes you wonder how our preconceptions of the world affect our ability to investigate it. And for that matter, how our ability to contemplate the world is affected by our practise of doing so. They say that beauty is in the eye of the beholder. For that to be true, the beholder has to open their eyes, look, contemplate and be prepared to be shown wrong in their preconceptions.

What connections do you make to your coffee brew each morning? I’d love to know, here in the comments, on Twitter or over on Facebook.


* Does a connection between this and stirring your freshly brewed Aeropress coffee with a teaspoon trailing vortices stretch the connectivity a bit too far?

¹ “Much Ado about Nothing: Theories of space and vacuum from the Middle Ages to the Scientific Revolution”, Edward Grant, Cambridge University Press, (1981)

² Quoted from Adelard of Bath’s “Quaestiones Naturales” taken from Much Ado about nothing, page 67.

Corona gazing in cafes

interference patterns on coffee

There are many ways in which rainbows of colour are produced as light interacts with our coffee or in a cafe. Looking around yourself now, how many do you see? What physics underlies each?

As the nights grow longer and the days colder, we notice that windows steam up as the water vapour in the café condenses onto the cooler glass. Perhaps we see a similar thing on our glasses while we are drinking tea or on the windows of a bus. Initially we perhaps become frustrated at our inability to see what is going on outside but then we notice the colourful patterns around the lights of passing cars and of street lights. Haloes of coloured light around a central bright spot. What does this tell us and where else can we see it, either in a café or in life generally?

On a window pane, a large number of small droplets of water have condensed into what appears to us as a fog on the glass. As the light shines through from the car headlights, each droplet acts as an obstacle to the light and so bends it. You could see a similar effect with the waves on the sea going around stones or perhaps if you brew a large cup of coffee with the surface waves going around a spoon (let me know if you manage to see this bending in a coffee cup). The amount that the light bends is dependent on the wavelength of the light (look carefully at the waves going around obstacles in ponds to see this) and so different wavelengths (different colours) get bent by different amounts and interfere with each other at different points – a spectrum is produced. It is a phenomenon known as diffraction.

Not all beans are equal! How could you quickly distinguish between arabica and robusta beans?

This phenomenon means that we have a way of separating the frequencies (or wavelengths) of light. And so this means that we have a way of measuring the chemical composition of some substances as different chemicals absorb different frequencies and so have ‘fingerprints’ in the light they scatter. By passing the light scattered from a substance (such as arabica coffee beans compared to robusta) through a diffraction grating (which is an obstacle with a pattern of fixed size), we can separate the frequencies being scattered and see if any of them are ‘missing’ (ie. they have been absorbed by the material we’re studying). It would be  a bit like looking at that rainbow pattern in the café window and not seeing blue, its absence tells you something. This is one of the ways that robusta beans can be quickly found if they have been substituted for arabica beans in coffee trading.

Coffee Corona

Look carefully: Sometimes you can infer the existence of a thin (white) mist over your coffee by the corona pattern around reflected light fittings.

But it is not just its technological aspect that has interest for us surely? When gazing at the moon on a misty evening, the halo around the moon suggests the clouds between us and it. It is something that poets have remarked upon to evoke atmosphere, it is something that we can gaze at as we imagine the giant café window of our atmosphere. But the size, and distinctness of the lunar corona actually give us clues about the droplets making up the cloud. And then we look closer to home and to our own coffee and we see the same diffraction pattern again looking back at us from our coffee’s surface. Occasionally it is possible to see haloes on the coffee surface around the reflection of overhead lights in the café. A coffee corona! This reveals to us the fact that there are droplets of water above the surface of our coffee; an extra layer of hovering droplets. Something that we can sometimes see more directly in the dancing white mists.

Diffraction is a beautiful phenomenon that allows us to gaze and to contemplate how much we are able to deduce and how much we have yet to understand. How atmospheric our coffees and cafés are and the journey of understanding that we have taken to get to this point. Coffee gazing is a hobby that should be taken up by far more of us.

Bean Thinking noticing afternoons are going to start in London in early 2019. To find out more information, sign up to the Bean Thinking events list here:

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Noticing at Artisan, Ealing

coffee Artisan Ealing

A good coffee is a solid foundation for any afternoon’s noticing.

A cafe-physics review with a difference. In that, it’s not so much a review as an invitation. What do you notice in a café?

Last week, I had the opportunity to try Artisan’s Ealing branch. Although I had found a lot to notice on my previous visit to the East Sheen branch, I had a very specific reason for visiting the Ealing location of this small chain of four cafés. The coffee (espresso) was reliably good. Smooth and drinkable in a friendly atmosphere. Just as with the café in East Sheen, there were a good selection of edibles at the counter and plenty to notice. The light shades were immediately outstanding as something to notice while a framed ‘hole in the wall’ provided a conversation point. The café was very busy and while there was plenty of seating with many tables, we were still lucky to have got a table for two near the back. Behind us there was a lesson going on in the coffee school while on the wall was the calendar for the space booking downstairs. And it was this that I had come here for.

A couple of months ago, Artisan announced that this space would be available to rent to provide a friendly space (with coffee) for the meetings of local small businesses or charities. This stayed in the back of my mind for a while as it came about at roughly the same time as an idea for Bean Thinking.

Lampshades at Artisan Ealing

First the obvious. Immediately striking, these lampshades could provide several avenues for thought.

There are a couple of us who are interested in meeting, about once a month, to discuss science. As ‘science’ is quite a big subject, we thought we would limit it to science that is associated with coffee or with the café at which we are meeting. Perhaps readers of this website may realise that this is not such a restriction, it is quite easy to connect coffee to the cosmic microwave background radiation of the Universe or to chromatography and analytical chemistry. If we were to meet in a location such as Artisan, there should be plenty more food for thoughts. The lampshades prompted me to consider what made substances opaque or transparent? Where is the link to coffee and methods for measuring the coffee extraction? The hole in the wall suggested thoughts about the algorithms behind cash machines. I’m sure that there is plenty more to notice if we take the time to see it.

And so this is an invitation. Would you like to join us in exploring what we each notice about the science of our surroundings? The plan would be to meet once a month, probably starting late January 2019 or early February (date and location to be confirmed). An afternoon on the weekend is probably better than an evening and we’d probably stay for an hour or two. You do not have to be a practising scientist to come along indeed, it would be great if we could have people from a variety of walks of life. The idea is not (necessarily) to answer scientific questions that we each may have but instead to explore the science behind the questions, to find the connections that form our ideas of the universe. To really notice our surroundings and our coffees (tea drinkers would also be welcome). As a consequence of this, mobile phones/laptops etc. will be discouraged during the afternoon. We’d like to notice things around us and not be distracted by what a search engine suggests about it; if we think a search engine could help us, we’ll use it after we’ve left and come back the following month to discuss the issues further. So, if you are curious, would like to explore what you notice and can tolerate keeping your phone on silent and in your pocket for an afternoon, please do come along, it would be great to meet some of you.

menus and lampshades in Artisan

You may like to look more closely at this photo. How are the menus supported? What does that tell us about the history of science?

In order to understand whether there would be any interest in this idea and to hear your input about the format, content, location, time etc. I have set up a mailing list for these cafe-science-spaces. Please do sign up to the mailing list to hear the latest announcements concerning these events and also to email me back to contribute your opinion. You can sign up to the mailing list using the sign up form below. Alternatively, if you don’t want to sign up to the mailing list but do want to hear more, I will be advertising the events on Twitter and Facebook so please do feel free to follow me there.


Please enter your email address here if you would like to hear about future Bean Thinking events.


21 years of the coffee stain

dried coffee stains, alcohol and coffee

Happy 21st birthday to the coffee stain. But there is still much for us to learn 21 years after the first paper on the coffee stain was published.

On the 23rd October, 1997, a paper was published in the journal Nature titled “Capillary flow as the cause of ring stains from dried liquid drops.” The title is in the dry style that scientific papers can be written. An alternative title could have been “How coffee stains form”*. Perhaps you would think, surely someone had known how coffee stains formed before 1997? And maybe you would go on to think: certainly 21 years later in 2018, we’d know all there was to know about the coffee stain? I hope that readers of Bean Thinking would not think “who cares about coffee stains?”, but I wonder whether it was the combination of disinterest and assuming that someone somewhere surely knew how they formed that meant it took until 1997 for anyone to ask the question: well how do they form?

Coffee is a very popular drink among scientists, though even this does not explain how popular this paper has become. A paper’s popularity can be measured in ‘number of citations’ which tells you how many times other authors have found this piece of work important enough to reference it in their own published paper. As of early November 2018, this paper has been cited nearly 3300 times. Why? Well, there seem to be at least two reasons. Firstly, it turns out that the coffee stain effect is of enormous technological relevance; it may even have been used in the manufacture of the device you are using to read this website. But secondly even now, 21 years later, we still don’t understand what is going on, there is still much to learn and some of it is some very subtle and very beautiful physics.

the droplets ready to dry

What happens when you form coffee stains using drops containing two liquids (alcohol and water) compared to just one (water)?

Very recently for example, a new paper was published in Physical Review Letters. This one was titled “Density-driven flows in evaporating binary liquid droplets“. Another exciting title, another time we’ll retitle it for the purposes of this post: “what happens when you mix alcohol with a coffee type suspension, dry it at different angles and film it drying.” Arguably this time the given title is more succinct. Why does it make a difference if you add alcohol to your coffee rather than just drink it straight (the coffee, not the alcohol)? And what happens to the resulting coffee stain?

Maybe of an evening you’ve been relaxing with a glass of wine, or something stronger, and noticed the “legs” rising up the glass. Their formation and appearance is due to the differing surface tensions between alcohol and water and the fact that alcohol evaporates more easily than water, you can read more about that effect here. The point is that because of the difference in surface tension between alcohol and water, you get a flow of liquid from areas of low surface tension (higher alcohol content) to high surface tension (high water content). And it was this that had been thought to drive coffee stain formation in droplets which were a mix of liquids, water and alcohol for example. But how do you isolate this effect from the other effect in which alcohol evaporates more quickly than water and so there are changes in density and buoyancy of the droplet?

pendulant droplets

Drying droplets upside down. The things we do for coffee science.

To answer this you could add n-butanol to the water (or coffee) rather than alcohol. Just like ethanol based alcohol (the sort you may get in gin), n-butanol has a much lower surface tension and lower density than water but unlike alcohol, it evaporates much less readily than water. So, in a water-butanol mix it will be the water that goes first, while exactly the opposite will happen for an alcohol-water mix. In a drying droplet, the liquid evaporates most quickly from the edge of the drop. Therefore, after an initial, chaotic stage (imaginatively called stage I), you will end up with a droplet that is water rich around its rim in the alcohol-water mix but n-butanol rich around the droplet edge in an n-butanol-water mix (stage II). This suggests a way that you can distinguish the flows in the drop due to surface tension effects from those due to the differences in density between water and alcohol/n-butanol.

How would you test it? One way would be to compare the droplets evaporating as if you had spilled them on the table top with droplets evaporating ‘upside-down’, as if you had tipped the table by 180° after spilling your coffee. You can then watch the flow by taking many photographs with a camera. In this way you would be able to test whether it was surface tension flow (which should be in the same direction within the drop whether the droplet is upright or suspended) with gravity driven flow which should be opposite (the drop is upside down after all).

schematic drops upright and upside down

A cartoon of the flow found in droplets of alcohol and water mix. When upright, the flow is up through the centre of the drop and down the sides. This is expected for both surface tension based flows and flows due to gravity. When upside down, the flow is still upwards through the centre of the drop but this time the drop is upside down. So this is what you’d expect if the dense water at the edge of the drop flowed downwards (gravity based) but not if the flow were dominated by surface tension effects which should be the same, relative to the drop-interface as if the drop were upright.

The authors of the study did this and found that the flow in upright drops of alcohol-water was opposite to that in n-butanol-water drops. This is what is expected both in surface tension dominated flow and in gravity dominated flow. But, when the drops were inverted, the flow within the droplet did not change absolute direction, instead it changed direction relative to the substrate (it may be helpful to see the cartoon), in both droplet types. Expected for a gravity driven flow (dense liquids move downwards), this is exactly the opposite to what would be expected with surface tension driven flow. It is sensible to conclude that the flow in drying droplets containing two liquid types is dominated by gravity, or as the authors phrased it “density-driven flows in evaporating binary liquid droplets”.

dried upside down drops

The resultant coffee stains of drops that had been suspended upside down. They seem fairly similar to the upright ones with the exception of the central dot in many of the stains. The arrow shows some coffee that spilled down the surface as the tray was flipped over.

While the authors did a lovely job of watching the flows within the droplet, what happened to the the actual coffee stain? It could prompt us to do an experiment at home. How does adding alcohol affect the appearance of a coffee stain if the drop is upright compared to if you turned the drops all upside down? What happens if the droplet is not held upside down but instead at an angle to the vertical? There are many ways you could play with this result, see what happens, have a glass of wine and see if that gives you any insight into what you see with your coffee. As ever, have fun and if you do get any interesting results, please do let me know here, on twitter or over on FB.


*The dry scientific author in me wants to point out that although catchier, the title “how coffee stains form” does not actually capture the extent of the physics nor what the paper was about (the fact that this happens more often than just in coffee) and the given title was much better. The coffee drinker in me thinks yes, but, surely we could make it all about coffee anyway…