Month: August 2017

Categories
Coffee cup science General Home experiments Observations Science history

Coffee Rings: Cultivating a healthy respect for bacteria

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

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

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

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

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

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

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

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

Categories
Coffee review Observations Sustainability/environmental Tea

Breathing underwater at the London Particular

table and inside of the LP
Inside the London Particular

Tucked out of the way in New Cross, the London Particular has always been just that little bit far away to travel to, but always so tempting, a siren calling towards New Cross. The reviews of the food and the place were intriguing, while the coffee is roasted by HR Higgins, a roaster with a café that always seems closed when I get the opportunity to pass by (which is usually Sundays). So it was with some relief that I finally managed to get to the “LP” a couple of weeks ago. Towards the end of a row of shops, the space outside the café has plenty of seats where you can enjoy a spot of lunch and/or a coffee on a warm day. Inside feels more cosy. A bar on the left of the entrance forms a corridor with the wall that you walk through to get to a room with communal table at the back. In addition to the communal table, there are a series of individual high chairs along the wall. At the back of the café is a window with an old device sitting on it. “An old digital multi-meter” I said before being corrected by my sometime companion in these reviews, it has a dial, it must be an “analogue multi-meter” then! It did seem to be able to measure current and resistance and it did have a dial to indicate the value measured. Quite why it was sitting, unconnected, on the windowsill is anyone’s guess.

AMM, LP, NC
An Analogue multi-meter. But why was this sitting on the windowsill at the back of the cafe?

The lunch menu is good. Enough items there to provide choice, few enough that each can be done well. Significantly, the true London Particular, the pea soup, was not on the menu on the day we were there. We had a light bite of lunch, a black coffee and shared the jug of mint infused tap water that was placed on our section of the table. At the other end of the table, another customer was enjoying her lunch. So although communal, the table gave us enough room to be private and have our own conversation. A mirror along the wall above the table reflected the blackboard menu between the table and the bar. Thinking about mirror writing reminded me of Dr Florence Hensey and his letters of lemon juice ink. Back in the eighteenth century he had operated as a spy out of coffee houses on the Strand and in St Martin’s Lane¹. Spying on England for France, his letters, written in lemon juice (invisible ink) passed without detection before the frequency of correspondence drew suspicions. Times move on. Spies would surely no longer write in lemon juice or even mirror writing to avoid detection.

Lunch on a week day was a very good time to experience this café. It must get quite crowded at weekends or brunch times. So it was good to be able to sit back and contemplate our surroundings from the back of the café. In the foreground of our view though was the water jug. With fresh mint leaves stacked inside, it was evident that air had become trapped under some of the leaves forming tiny bubbles. How had the air got stuck there? Was it merely that the leaf was blocking the air bubble from rising through the water? Could there be slightly more to it?

Coffee and mint water in New Cross
Coffee and mint water at the LP

There is a popular expression “like water off a duck’s back”. Perhaps it arose because the duck’s back is often thought one of the most waterproof surfaces we know. But what makes the duck so waterproof? Why does water just form drops and then fall off the back of the duck? It is not because the feathers are oily. We sometimes ‘wax’ our waterproofs with a grease to make them resistant to getting wet and so perhaps we have thought that the duck’s back was just a bit greasy? And yet a study done back in 1944 showed that mere oil could not account for the waterproofing of the duck’s back.

Before delving into why the duck’s back is such a waterproof surface, it’s helpful to know how to quantify ‘waterproof-ness’ in the first place. To measure how waterproof something is, we use what is known as the contact angle, which is the angle that the drop makes with the surface on which it is sitting. Surfaces that are not waterproof (technically we call them “wettable” or hydrophilic), have very low contact angles, the ‘droplets’ of water on the surface are flattened. Waterproof surfaces on the other hand (imaginatively called hydrophobic), have contact angles which are much greater than 90º (it may be helpful here to have a look at the cartoon illustrating this point). Droplets that formed on a duck’s back had contact angles much greater than 90º, indeed, they formed almost spherical drops of water. What could be going on?

artemisdraws cartoon, contact angle, wettability
How ‘wettable’ a surface is can be defined by the contact angle that the drop makes with the surface. Image thanks to artemisworks.

The answer is in the details of the feather. The feather is not a flat surface but a material that has irregular protrusions and structure at the micro and nano-scale (one thousand and one million times smaller than mm scale respectively). These protrusions trap air within the feather and so effectively suspend the drop above the feather surface. The droplet does not have a flat surface on which to spread out. The structure means that the contact angles of the drops of water on a feather can be even higher than 150º; the droplets are held up almost as if they are spheres of water.

mint infused water at the LP New Cross
A breath of fresh air under water. Air bubbles trapped under mint leaves.

Another creature that uses the irregular protrusions on the hairs on its legs for waterproofing is the spider. The hairs on the legs of a spider mean that, just as the duck’s back, the spider’s legs are extremely waterproof. But it also means that air is trapped under the droplets. Consequently, if a spider finds itself submerged under water, the air under the droplets forms little bubbles similar to those under the mint leaf in the London Particular. And this allows a drowning spider the air it needs to breathe. Nanostructure helping the duck to dive and the spider to survive. And the mint water to be particularly refreshing on a warm day in a very pleasant place for a spot of lunch and a coffee.

 

 

 

The London Particular can be found at 399 New Cross Road, SE14 6LA

¹London Coffee Houses, Bryant Lillywhite, Pub 1963

Categories
Coffee cup science General Home experiments Observations slow

On rings, knots, myths and coffee

vortices in coffee
Vortices behind a spoon dragged through coffee.

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

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

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

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

chimney, coffeecupscience, everydayphysics, coffee cup science, vortex
In high winds, vortices around chimneys can cause them to collapse. The spiral around the chimney helps to reduce these problem vortices.

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

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

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

 

Categories
Coffee review Observations Science history Sustainability/environmental Tea

Looking under the surface at Mughead coffee

Mughead Coffee, Coffee in New Cross
Set back from the busy A2, Mughead Coffee offers a space to unwind.

A new café has just opened in New Cross. Mughead Coffee opened in July 2017 and sits fronting the A2, part of an old Roman road connecting London to Dover. The large pedestrianised space outside the café provides plenty of room for a few tables together with some further chairs arranged along the café window. It also means that the cafe is set-back far enough from the road that it is possible to sit outside and enjoy the surroundings. Inside, there were plentiful seats but, sadly equally plentiful numbers of occupants relaxing in this new cafe. Clearly this new coffee place in New Cross is proving popular. And why not! Just down the road from the London Particular, Mughead Coffee serves Square Mile in a friendly atmosphere. It is easy to see this becoming a popular local haunt. The usual array of coffees were on offer together with a filter option but as we arrived shortly after lunch, the cake/edible option appeared a little depleted. The interior of the café is quite light and airy with comfortable chairs at the back and more regular seating towards the front. We ordered a long black and a ginger beer and then adjourned to a table outside to await our drinks.

The tables outside are arranged on a sloping pavement. This is not really a big deal, but did remind me of a comment made by the lecturer who was trying to instil experimental design into us as undergraduates: The only stable table is a three legged one. However there was not much time to reflect on that as very soon both coffee and ginger beer arrived with a glass of ice. The natural light revealed the oils on the surface of the coffee as they moved with convection. Different convection zones moving in the coffee just as air parcels do in the sky to form mackerel skies or hot lava moves to form different rock formations, both on Earth and elsewhere.

coffee and ice in New Cross on a wooden table
Coffee and ice at Mughead Coffee. Note the reflections on the coffee surface.

Once the ginger beer was poured into the glass, the ice cubes floated upwards with just a fraction of them bobbing above the surface, the majority of the ice cube beneath. A glance around our surroundings revealed other hints of sub-surface structures. A drain cover nearby indicated, together with some tiling along the pedestrianised zone, the line of the rain sewer running along the road. A public telephone box had no wires obviously leading from it meaning that all the wiring for the communication had to be subterranean. And a raised flower bed, full of thriving plants, had a little drainage hole right at the bottom in order that heavy rain storms did not drown the plants.

This last feature reminded me of a documentary I’d recently seen concerning climate change. Often we tend to think of climate change as involving things that we can see: the melting of glaciers or the disappearance of sea-ice, or freaky rain storms that cause local flooding. However there is another aspect, a sub-surface aspect, that has perhaps been far more visually alarming than even the break-off of the Larson A, B and C ice shelves. If only we could see it. The problem is that, as it happens below the surface of the sea, few of us see it, it is hidden from view and therefore easily hidden from our conscience. It is the drastic effect that rising sea water temperatures are having on a particularly unusual plant-animal combination, the coral reefs. Coral reefs such as the Great Barrier Reef just off Australia, are animals that exist in a symbiotic relationship with a particular type of algae called zooxanthellae. Although the ‘mouths’ of the coral eat passing zoo plankton at night, during the day, they get other nutrients from the photosynthesis products produced by the zooxanthellae that live within their skeletons. These plants give the corals those amazing colours (as well as food). In return, the coral provides the plant life with shelter (they live within the coral itself) and extra carbon dioxide.

Outside Mughead Coffee New Cross
Indications of a hidden architecture. Can you see the drainage hole at the bottom of the planter at the back of the photo?

As the sea temperature rises, the zooxanthellae become less efficient at photosynthesising and so of less use to the coral. If the temperature stays high, the coral ejects the plant life from its body causing the coral to lose all its colour, it has bleached. What sort of high temperatures are needed? It seems that if the temperature of the water is about 1-2°C above the usual seasonal maximum, the coral are ok for a few weeks. But if the temperature rise is 3-4°C (or higher) above the usual seasonal maximum, the damage can occur in just 2 days¹. Coral bleaching does not necessarily lead to coral death but if the bleaching is sustained vast areas of coral reefs can die and get destroyed, with significant impact to the local ecosystem. As corals host “nearly one-third of the world’s marine fish species…”² this impact will be far reaching and affect the livelihoods of millions of people³.

Although small scale coral bleaching has been documented since 1979¹, the first global scale coral bleaching occurred in 1998. It was 12 years until the next global bleaching event occurred in 2010. Following that, we have just had the third global bleaching event in 2015-16. In the latest episode, it is estimated that 29% of the Great Barrier Reef’s coral died (as in actually died, not just bleached). These temperature increases can be associated with global warming caused by increased greenhouse gases in the atmosphere (for more info click here (opens as pdf) or refer to [4]).

The frequency of these events, together with the fact that there were no global bleaching events prior to 1998 should be a dramatic warning siren calling on us to do something to arrest climate change. But what can be done and is it already too late? Well, it is not yet too late to do something. The plants, thriving in the box in front of Mughead can emphasise to us the importance of maintaining our local environment and by extension our global one. Taking time to slow down and take stock of what is beautiful in our environment, and the habits we need to develop to keep this for future generations, these are things that we can do. If you eat fish, was it caught sustainably? Some fishing methods can kill the coral reefs, check before you eat. This is not going to be hard to do. After all, we already do this with coffee. Many coffee drinkers (and roasters) will check how the coffee is grown and processed for both environmental cost and the conditions experienced by the farmers. Many such small actions can cumulatively build to an effort to stop climate change.

Which brings us, in a sense, back to the surroundings at Mughead Coffee. Sitting down and taking time to enjoy that coffee while appreciating our surroundings, the visible and the hidden, the busy road and the mini-oasis of plants in the planter, may help us to see that connectedness that pushes us to accept our responsibility to our common home. Contemplating the history of the road in front of us, will our planet still be beautiful in another 2000 years? With an offer of “gourmet sandwiches” on the menu (if only we’d got there early enough), there’s plenty of reason to head along to the old road in New Cross and sample the coffee while pondering our own impact on this interesting location.

 

¹ Life and Death of Coral Reefs, Charles Birkeland (Ed), Chapman & Hall, 1997

² Coral Reef Conservation, Ed Isabelle M Côté and John D Reynolds, Cambridge University Press, 2006

³ Chasing Coral, Netflix Documentary, 2017 (see trailer below)

4 Climate and the Oceans, Geoffrey K Vallis, Princeton University Press, 2012

Chasing Coral Trailer:

 

 

Categories
General Observations slow

The impact of water on coffee

lilies on water, rain on a pond, droplets
What is the crater shape produced by falling droplets of water on freshly ground coffee?

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

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

bloom on a v60
Bubbles in a V60 filter – but what is the impact of individual drops of water on the dry grains of coffee? The ultimate in slow coffee.

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

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

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

coffee ground in a candle holder
Early experiments with coffee grind craters: There are advantages to working with glass beads and high speed cameras.

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

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