The effects of surface tension can be seen in the light reflected from a coffee
UK Coffee week is once again upon us meaning that all week we can be justified in thinking about, drinking, appreciating and celebrating coffee. And of course, as soon as we start to do this, we realise we have to drink, appreciate and celebrate water which is, ultimately, what really makes most of the cup of coffee. So UK Coffee Week raises money for Project Waterfall which is a charity that brings clean water to coffee growing communities. Giving something back by enjoying something good.
In keeping with the water theme, this week The Daily Grind is all about water, including an experiment that enables you to make a hole in it. As this is also the week between Palm Sunday and Easter, perhaps we could call the post “Holey water for Holy Week”.
But moving quickly to the experiment. While drinking your coffee, you may have noticed how around the edge of the cup, the coffee appears lighter, not quite so dark, as in the interior. The coffee is being bent upwards at the edge of the cup by the surface tension of the water in the coffee. Now, what happens if you add alcohol to the coffee? If you do this in your coffee cup you may well end up with an Irish coffee which may provide even more of an excuse to celebrate your coffee drinking, but if you were to put your coffee on a plate first (I know, why? but bear with me) you will get a quite different result. You will be able to make a hole in the middle of your coffee. The reason is that the surface tension of alcohol is much weaker than that of water. Consequently, if you try to mix a very thin layer of coffee with a small amount of alcohol, something slightly unexpected happens as this video shows:
The addition of a small amount of alcohol into the middle of a thin layer of water (or coffee) causes the water to recede. As the alcohol evaporates off, you are left with a dry ‘hole’ in the coffee. Why is this? It is effectively a liquid-tug-of-war on your plate. The higher surface tension in the coffee (or water) pulls against the weaker surface tension of the alcohol which eventually means that the water breaks away, leaving the hole. As the water molecules are continually moving, eventually they start to meet again over the dry spot and close the hole.
You can’t see this in your mug of course because the mixing occurs throughout the liquid while the plate ensures that this is only a surface effect.
You will need a strong alcohol, perhaps gin or vodka but please do try this experiment, let me know how you get on and enjoy the coffee, water (and alcohol) in UK Coffee week. And if you want to donate to Project Waterfall, you could either find a participating café here or donate online here.
I was not initially going to do a cafe-physics review of Attendant. It wasn’t that I didn’t enjoy the coffee, I did. I had a very well prepared V60 which went very well with a lovely chocolate brownie. Nor was it that there was nothing to see at Attendant. No, it was quite the opposite. Part of the point of the Attendant seems to be its location. You see, if you were not aware of it already, Attendant is to be found in a (no-longer-used), underground, gentlemen’s toilet. Although they have been thoroughly cleaned, various fixtures (19th century urinals and cisterns) remain in place. Modern (deliberate) graffiti adorns the walls as you walk in. Understandably, there are no windows to gaze out of in this café. It is, in many ways, a very interesting place to visit and the coffee is certainly worth a visit too. However, it is difficult to do a review which is, after all, about noticing something unusual, when the former use of this space is almost shouting at you. I thought about doing a review based on how the shape of the coffee cup can influence the flavour of the coffee that you perceive. Yet somehow, writing a review on anything other than the fact that this is a re-use of an interesting space seemed, almost, perverse. So I left it. Until that is, UK Coffee Week came along.
UK Coffee Week raises awareness and money for Project Waterfall which in turn aims to help provide clean water and sanitation for coffee growing communities. Currently, Project Waterfall works in three countries, Tanzania, Rwanda and Ethiopia. In these countries a large number of the rural population lack basic access to drinking water while a greater number do not have access to sanitation facilities. Clearly this can lead to health problems. The World Health Organisation estimates that world wide, the drinking water of 1.8m people is contaminated with faeces, while 0.5 million people per year die from diarrhoeal diseases including cholera.
Interesting presentation. Coffee at the Attendant.
Perhaps, while sitting in cafés or having breakfast at home, we have a tendency to take water for granted. Certainly I will admit that I can. I’m sitting here writing this enjoying a great cup of coffee with a few biscuits both of which took water to produce. Beyond the obvious water in the kettle for the coffee and the water used for the dough for the biscuits, there is the ‘hidden’ water. The water used to irrigate the coffee crops and the wheat fields or to process the coffee cherry towards the green bean stage. The water used in generating the electricity used to bake the biscuits, or roast the coffee. The water used to clean the utensils between coffee roasts/biscuit batches so that we don’t get food poisoning. The list could go on. Indeed, the UN estimates that producing 1 cup of coffee requires 140 L of water. This figure though presumably cannot include the private water needs of the individuals who work on the coffee plantations. We all need water and we all need it to be clean.
So, in thinking about our water consumption (and the water consumption of those who help us to enjoy our coffee), we can do a few things during this coffee week 2016. Firstly, we could make a donation towards the work of Project Waterfall (here) or a similar charity that is working to provide clean water and adequate sanitation to those who don’t have it. Secondly, we could take the prompt from Attendant and start to think about where our water comes from. Why from Attendant? Well if you were living on the International Space Station or, to a lesser degree, in Singapore, this question may have an obvious answer. For the rest of us, we are often a little bit removed from direct water recycling, but it’s worth looking more closely at Singapore because they have developed a water strategy that may be of use for more of us in the future.
30% of water supplied in Singapore is ‘reclaimed’. Where does your drinking water come from?
Singapore has a population of just over 5.5m (London: 8.6m) with a land area of 719.1 km². As an island, it is surrounded by water and so you may think that water is not a problem for the inhabitants of the city-state. But the water surrounding Singapore is the salt water of the sea and so not easily converted into drinking water. While looking for a solution towards a self-sufficient water supply, Singapore decided to try the recycling route. Through a scheme called NEWater, currently 30% of Singapore’s water supply is from ‘reclaimed’ water (for reasons that may be obvious, they avoid the word ‘recycled’). The Singaporean authorities aim to make this 55% by 2060. Waste water produced in Singapore undergoes a process of micro-filtration (which takes out suspended particles), reverse osmosis and UV disinfection before being reintroduced to the water supply system. Although most of this reclaimed water is used for industrial processes, the reclaimed water can be added to Singapore’s reservoirs so that it will go into the drinking water supply.
A similar process is used on the International Space Station but there, as it is a closed environment, it is not just the waste water that goes down the drain that is ‘reclaimed’ but the water exhaled by the astronauts and the lab animals on the station. On Earth this would evaporate into the atmosphere, contribute to cloud formation and then rain back down closing the greater water cycle in that way. On the space station, the fact that it is a closed environment means that this moisture too can be ‘reclaimed’. By recycling the water in this way, the inhabitants of the space station avoid having to require too many costly water deliveries from the Earth.
Perhaps, while drinking our coffee (or tea, or even water) today, we can take five minutes to consider where our water comes from as well as considering whether those who contribute to our brew have adequate water supplies themselves. And as it is coffee week, here is that link again to Project Waterfall (Donation button at the bottom of the main Project Waterfall page). Enjoy your coffee.
Note added August 2017: It is with some regret that I have to say that Attendant is not a good place to go if you suffer from allergies. They have started serving almond milk and (according to a Twitter Direct Message received from the Attendant Team) do not adequately clean their steam wand between drinks so as to prevent cross contamination. Their advice to me was that I “should not have any hot drinks or food at our premises as we do not operate a nut free environment at our stores”. There are many good cafes to visit if you suffer from nut allergies, but please avoid this one (or just have a black coffee and enjoy the atmosphere).
Gazing into a coffee you can see the reflection of your face looming back at you.
Have you ever gazed into your coffee as you take a mouthful only to get disturbed to see a distorted view of your face looming back at you from the coffee? Has it struck you that while you often see such reflections, you rarely see shadows? Try it first with water and then coffee. Can you, perhaps, see a shadow on the coffee where you cannot see shadows on the water? Why would this be?
For a shadow to be visible on a surface, the surface must scatter enough light so that the contrast between shadow (where there is no light to scatter) and non-shadow (where the surface is illuminated) can be seen. Although a shadow (or at least the relative lack of light) is always going to be present behind any obstacle, it is whether or not it can be seen on the surface of the water/coffee that is at issue here. Pure water is of course quite transparent. Without anything in the water to scatter the light (such as mud for example), the light passes straight through the water to the other side. Overall, not enough light is scattered back from the surface of the water to generate the contrast required for seeing shadows. Seeing shadows on pure water is going to be hard.
The concentration of suspended particles will depend on how you make your brew
By contrast, coffee contains suspended particles, in fact they are part of the very essence of the drink. These particles offer a surface to scatter the light back towards the observer and so highlight the shadows formed by the object between the coffee and the light. It strikes me that different brew methods will result in different amounts of sediment and suspended particles in the coffee and therefore a greater or lesser tendency of the coffee to reveal shadows. Perhaps if anyone does notice that it is harder to form shadows on coffee prepared by a Chemex than a French Press (for example) they could let me know using the comments section below.
Shadows have been used by philosophers to illustrate by allegory how we perceive the world around us. In the tale of Plato’s cave a group of prisoners are held in a cave such that they can only ever see the shadows playing on the cave’s wall. The shadows are formed by a fire behind the prisoners that the prisoners cannot see. As they can only see the shadows, they start to think that it is the shadows themselves that are ‘real’. It is a tale questioning the reality of what we currently see and also our inability to adjust to the differences between looking directly at the Sun or discerning shadows in the dark. In the story of the cave, it is the fire, or the Sun that causes the shadows that deceive the prisoners. No consideration was given to the role played by the wall on which the shadows dance. Yet we can see from our coffee that to understand the world of shadows we do not merely need a light source. To understand shadows, we need a surface from which to reflect the shadows. Perhaps we need to spend some time contemplating our coffee, the shadows and what they can tell us about the world and how we see it.
For details about this and other phenomena involving light and its interaction with the world around us, see: “Color and Light in Nature”, David K. Lynch and William Livingston
