hydrophobic – Bean Thinking

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

A drop in the Chemex?

Chemex, 30g, coffee — How do you prepare your coffee?

How do you prepare your coffee? Generally I’ll either use the Chemex or a French press. Often it will be the French press purely because it is, sadly, quicker. However, on those mornings that I do slow down to prepare a Chemex, I generally feel better for it. Not only does the coffee taste better, but those 5 minutes of preparing the coffee pay off as time for the mind to wander rather than just time spent waiting for the caffeine. When the Chemex is nearly ready, the fresh brew drips slowly from the filter onto the liquid below. Each drop produces a ripple pattern. At the start of the UN conference on climate change in Paris (COP21), we may well hear talk of some of our efforts being mere “drops in the ocean”. So it seems a good time to reflect on those “drops in the Chemex”. Just how much influence can a drop have?

It is worth stopping for one moment to consider what is going on around us at this moment. As I write this, it is late November in the Northern Hemisphere. Taking a walk outside, I can see the last of the yellow leaves falling off the trees. In just a couple of weeks time, many of the trees will be bare. Why do the leaves fall from the trees? We could answer this question in a number of different ways. Biologically, the tree is forming cells at the joint between the leaf and the tree that will eventually enable the leaf to tear from the tree. As these cells are, in some way, responsible for the leaf falling off, they are called “abscission” cells. But even with these abscission cells, the leaf still needs something to force the leaf off. Often this is the wind which is why we get such an abundance of leaf fall on windy days. However there is another mechanism that can help a leaf to drop, and that is a curious interplay between the leaf and rain.

autumnal scene, red leaves, hydrophilic — The surface of the leaf changes from waterproof to ‘wettable’ over the course of the summer

In the spring, many species of tree, including Oak, develop a wax layer on the leaf. Perhaps you have been walking in the country and have needed to wax your walking boots before you go? The wax on the boots acts as a waterproofing for the boot, ensuring that your feet don’t get soggy. The wax on an oak leaf performs the same function for the leaf, it makes the leaf waterproof. Although this is not the only function of the wax. It seems that a waxy surface also slows the processes that dry out the leaf, prevents insects and pathogens attacking the leaves and may even play a role in affecting the way that the light is concentrated into the leaves for photosynthesis. Nonetheless, from the tree’s perspective, it is a significant advantage to have waterproof leaves. Imagine rain falling onto a waterproof surface. The drops of rain do not ‘wet’ the leaves but instead roll off. As the raindrops roll off, they take particles of dust and dirt with them. It is a tree’s way of cleaning itself. Waterproof surfaces are self-cleaning surfaces. Something that some scientists are now trying to replicate for man-made products.

hydrophobic leaves — Some leaves are more waterproof than others.

As the summer continues and the leaf gets older, the wax layer changes. The structure of the wax changes and erodes as the wind, weather and even pollution batter the wax layer. Just as with the hiking boots, the damaged wax layer results in a less waterproof leaf. The leaf becomes “wettable”. When a drop falls on a surface, the shape of the droplet is determined by how waterproof the surface is (more details here). A surface is termed “wettable” when the droplet becomes significantly flatter and coats the surface rather than forming a spherical drop that can roll off. Now consider each raindrop as it hits the different types of leaf. In the spring, the leaf is waterproof and the raindrops will roll off them. A drop of rain will cause the leaf to shake on its stem but then to return to its original position. It is ultimately not affected by a light rain shower. In the autumn when the leaves are no longer waterproof, the rain will start to stick to the leaf surface. Now when the leaf shakes, the wet leaf will not return to its original position but will bend slightly further downwards. As it continues to rain, the leaf will experience a greater torque and this means that it is more likely to fall off the tree. As each rain drop hits the leaf, the likelihood that the leaf will tear away from the abscission cells at the base of the leaf increases. Each drop has an effect.

This also has an important consequence for some of our technology. One renewable energy source that has been proposed for self-powering electronic devices harnesses the energy of rain. When rain falls on an array of cantilevers, it forces the cantilever to bend and to oscillate. This energy can be harvested ( that is, changed into a form that is useful to us) by using small piezo-electric devices (that convert movement into electricity or vice versa) at the base of the cantilever. When a tree leaf is wet, the leaf joint experiences a greater torque which causes the leaf to ultimately tear from the tree. For the rain-energy harvesters, this is exactly what we want. The greatest energy obtainable from the cantilever system will be from cantilevers that can be made wet. Waterproof cantilevers would be a bad idea. A renewable energy that comes from rain would definitely be a positive development for UK energy production!

It seems that one coffee drop does indeed go a long way.