diffusion

Time for tea?

Matcha, tea in Japan, frothy tea
A Matcha tea in Japan. A lot to contemplate here.

A recent article in Caffeine magazine caught my attention. Emilie Holmes of Good and Proper Tea was writing about the joys of appreciating loose leaf tea. While tea is a little diversion from coffee, January is traditionally a time to look forward as well as back and maybe, BeanThinking should occasionally cross over to the tea side. It was one line in particular of that article that puzzled me. Writing about the ‘naturally “slow” nature of the tea ritual’, Holmes observed that while brewing loose leaf tea you would be able to see “the leaves in a glass pot emit wisps of colour as they infuse…”

It was great to read someone who clearly had spent time carefully observing their tea. And yet that sentence prompted a series of questions in my mind. It was not that I doubted the observation, indeed, thinking back to teas I have made and enjoyed, I realise that I have seen these wisps before. It was more a question of why would it happen, why would the brewing tea emit lines of colour from the leaves? These lines must be telling us something.

diffusion, convection, tea brewing
A tea bag in hot water. The lines of tea are difficult to see in the photo, you’ll just have to do your own experiments, but, streaming from the bottom of the bag, you can see wisps of darker tea-water.

We need to think about how tea brews. A first mechanism would be through turbulence. Hot water poured onto a bed of tea leaves would stir them up and the resulting movement within the pot would mix the leaves with the water leading to a properly brewed cup of tea. This is very much the lazy tea brewers bag-and-cup method (which I can share). It would lead to a brewed tea, but it could not lead to a situation in which you could sit back and see wisps of colour. That requires calm and the quiet moments of a pot of tea brewing while you can enjoy the process.

A second mechanism would be through diffusion. Ultimately the same mechanism as the principle behind how LEDs work, diffusion is where the soluble parts of the tea leaves would travel, through the process of a random walk, throughout the water of the pot. This is a very slow process and we would expect that the concentration of colour would be most intense around the leaves and then fade out gradually with distance from the leaves. We would not expect ‘wisps’ nor lines of tea, that suggests something else.

It suggests the third mechanism of the tea brewing: a mix of diffusion and then convection within the hot water of the pot. The lines of tea are indicating that within the cup, regions of the hot water are at slightly different temperatures. Owing to the hot water being in contact with cooler air surrounding it, the surface of the water is cooling down and sinking, leading to a convective motion within the water inside. As the water moves it carries the diffused tea with it into new areas of the water, a movement of hot water to cooler water and back again. The tea is carried in a line because the convection patterns are occurring in small cells within the tea pot, small regions where hot tea is moving towards cooler tea which is warmed and itself moves. The convection does not happen as if the hot water is one big mass but a series of smaller ‘cells’. We see similar cells on the surface of the Sun. The lines are telling us of the movement in the tea pot and the amount and speed of their movement reveals more about how hot the water is relative to the air outside the pot.

diffusion only
A tea bag in cold water: This time, there are no wisps of tea as the drink brews. Instead, there is a slow diffusion of tea infused water from the bag outwards.

Testing this idea I required tea bags. My tea pots are opaque and so would not help me to appreciate this detail of brewing a cup of tea and so it was back to the bag-in-cup method. However, in order to avoid turbulence, I poured the water (hot or cool) into the mug before adding the tea bag. It was not the best way to make a tea, apologies to tea lovers, but it was a tea that I do not enjoy anyway, so it was good to use it up. Sure enough, when the tea bag was put into the hot water, within a very short time, wisps of coloured water formed lines curling underneath the bag. Why did they flow down? Was it because the tea in the bag was slightly cooler than the hot water and so, as the tea diffused out of the leaves it moved with convection downwards because of gravity and the fact that cooler water is denser? A tea bag in cool water however behaved differently. The water in the cup had been taken from the tap and then left in the cup for a couple of hours so that the water was definitely at the same temperature as the room. This time, the tea bag first floated and then sank to the bottom of the cup. There was no obvious infusion of the tea-coloured water into the plain water but slowly the region around the bottom of the tea cup at the bag turned browner with the tea. As time went on, this region expanded to give a tea layer and a water layer.

The wispy lines of tea only happened when using hot water. Which suggests a further experiment. How do these wisps change when brewing for black teas as opposed to green teas (which use a lower brewing water temperature)?

After about five minutes the tea brewed in hot water (left) was fairly evenly distributed throughout the cup whereas the tea brewed in cold water (right) showed a distinct layering between concentrated tea at the bottom of the cup and plain water above that layer.

One last observation with these tea bags in the hot water. Some of the tea floated within the bag, some sank, as time went on, more tea leaves fell towards the bottom of the bag (which was itself floating). What was happening there? Maybe if you experiment with your tea, you can let me know in the comments below, on Twitter or on Facebook. There are definite advantages to slowing down and brewing a proper cup of tea.

A cup of tea for a light bulb moment at Ginger and White, Hampstead

Coffee, Ginger and White, Hampstead

Coffee at Ginger and White, Hampstead

It was late afternoon by the time we stopped by Ginger and White in Hampstead. The warm weather meant that we could enjoy time spent sitting outdoors in the little alleyway in front of the café. We had been taking a friend around the various sights (and foodie places) of London and so stopped here before heading back home. The long black, cortado and soya latte were all very well done and, while the others had enjoyed a crepe at La Creperie de Hampstead just around the corner, I took the opportunity to try the excellent banana bread on offer at Ginger and White. There was a fairly good selection of cakes on offer, but sadly those that the staff could confidently affirm were nut free were far fewer. However, the moist and tasty banana bread was a good option anyway. Coffee was roasted by Square Mile and there were also Square Mile beans available for purchase should you wish to take some home with you. While the café was fairly busy, it was nevertheless a relaxing place to sit and watch the people of Hampstead go by.

The interior of Ginger and Whites

Everything is connected. From the lights to your cup of tea.

As I was looking around, wondering what the physics part of this cafe-physics review would be, I had what you could call a “light-bulb moment”. The walls of the building opposite were reflected in the windows of the café but looking inside, I noticed the lights which appeared to be LED lightbulbs set-back into the ceiling. Along with requiring less energy to power than conventional or halogen lightbulbs, LED lightbulbs in a café offer another, more poetic advantage for the café: they have a connection to the drinks being served and particularly tea. It’s all about diffusion.

At the heart of an LED light, there are two materials that form a junction. On one side of the junction is a semiconductor material that conducts electricity by means of electrons. Electrons conduct electricity in metals and are the ‘normal’ way that we consider electrical current to  be carried. On the other side of the junction is a different semiconductor, one that still conducts electricity but this time does so with carriers called ‘holes’. You can view the electrons as having a negative charge and the holes as having a positive charge.

tea bag, tea cup, diffusion, turbulence

What happens when you put a tea bag into a cup of cold water. How long until the water becomes ‘tea’?

But what happens at the junction? Is there really a sharp barrier between these two types of material? Think about putting a tea bag in a cup of cold water, does the tea bag just sit there or does it slowly, very slowly, start to diffuse tea into the cold water? It is a similar thing for the two materials. Slowly the electrons diffuse into the hole material and the holes into the electron material. In fact, mathematically, the same equations describe the process in the junction as in the tea cup. But unlike tea, in the LED, the holes and electrons have an electric charge associated with them and so, as they diffuse away from the junction, they set up an electric field across the junction. It is this electric field that eventually stops any further diffusion of electrons or holes across the junction and sets up the conditions necessary for LEDs to emit light. It would be like having a tea bag that diffuses tea into the cup until it is perfectly brewed and no further.

Of course, there is much more than this to understanding LEDs. If you’re interested, there is further information here. I find it fascinating however that what happens in your tea cup, is also happening on many different scales in many places in the universe. And of course, in the lighting of cafés and coffee houses around the world.

Ginger and Whites is at 4a-5a Perrins Court, NW3 1QS

 

On nuclear fusion and making tea

tea bag, tea cup, diffusion, turbulence

How not to prepare tea

Although largely a coffee drinker, occasionally I will order tea in a café. When I do so, one of my pet hates is being served a cup of hot water with an individually wrapped tea bag sitting on the saucer beside it. Quite apart from the unnecessary environmental cost of individually wrapping tea bags, there is the problem with the resultant cup of tea. Hot water poured onto tea (preferably in a pot) allows the tea to infuse by a mixture of turbulence, convection and diffusion as the hot water swirls around carrying the tea with it. A tea bag placed into hot water on the other hand relies on infusion by convection and diffusion only and so takes a lot longer to brew. Oddly enough, there is at this moment, a major scientific project being built in the south-west of France that has the opposite problem. The aim of the project is to generate electricity by nuclear fusion in extremely hot clouds of gas that are confined into the shape of a doughnut. To achieve this, they must reduce the turbulence within their doughnuts. Unlike the tea, nuclear fusion seems to require diffusion and convection to prevail over turbulence.

Supplying the growing energy demands of the planet is a major problem for us all. How can we simultaneously generate the electricity that we want while limiting our carbon dioxide emissions to levels that will cause minimal damage to our planet? Renewable energy is part of the solution, some have argued that nuclear fission could be another part of the solution (all of our current “nuclear” power plants run by nuclear fission). The “ITER” project in the Provence-Alpes-Côtes d’Azur region of France aims to demonstrate the feasibility of nuclear fusion to supply our energy needs instead.

Sun, heat, nuclear fusion

The Sun is powered by nuclear fusion. Could we generate electricity on Earth with a fusion generator? Image © NSO/AURA/NSF

Unlike nuclear fission which works by exploiting the decay of radioactive elements, nuclear fusion ‘fuses’ elements together to produce energy. Gazing up at the sky you can see thousands of nuclear fusion generators: Each star (including our Sun) produces light and heat, by nuclear fusion. First the stars fuse hydrogen into helium (as our Sun does now), then, as the star ages, the heavier elements combine until finally iron is formed in the core of the dying star. All the elements found on our planet and elsewhere in space have, ultimately been formed in the core of a star (or in reactions as the star dies in a final explosion). Every atom in us has been formed by such reactions in stars and so it is very true to say “from dust you came and to dust you will return”, the dust in question being star dust. If we can exploit it on Earth, nuclear fusion offers a method of providing energy with no long term radioactive by-products and limited carbon dioxide emissions. It is a possible, but very long term, route out of our quandary about energy generation.

doughnut tokamak

A photo to demonstrate “doughnut shaped” was probably unnecessary, but it did provide a good excuse for an unhealthy breakfast.

So why can’t we start using it immediately? A clue comes from the fact that the nuclear fusion reactors that we know of (stars) are very hot and relatively dense. It is not easy to smash two hydrogen atoms together such that they fuse, it requires them to have a lot of energy (ie. be very hot) and be quite close together. To build a nuclear fusion reactor requires us to heat a gas until it becomes a ‘plasma’ which means heating the gas to temperatures of around 150 million ºC. At this temperature we need to confine the plasma with very high magnetic fields so that it does not hit the walls of its container and it turns out that the best way to do this is to manipulate the plasma into a ring doughnut shape. This doughnut confinement, known as a ‘Tokamak’ has become the standard way of confining the plasma. At the moment, we cannot keep the plasmas hot enough for long enough (the current record is 6min30 sec confinement) for fusion to generate more energy than is required to form the plasma in the first place. One of the things limiting the lifetime of the plasma is the fact that the plasma cools down and one of the things that cools the plasma down is the turbulence in the plasma carrying the heat energy from the centre to the edge of the doughnut. Increasing the time it takes for the heat to escape from the centre of the doughnut to the outer edge is one of the challenges facing the ITER team. Just as with the pot of tea, were the cooling by diffusion and convection only, the plasma would take a lot longer to cool down. Understanding the turbulence inside the plasma is one of the challenges facing the team at ITER.

Our method of making tea can tell us a lot, not just about the problems for nuclear fusion generators, but also about diffusion and turbulence generally. It is worth pondering that brew a little more deeply next time you make your pot.