experiments with tea

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 tense moment for a coffee…

capillary bridge

A bridge formed by water between a cup and a cafetière.

Each and every coffee represents an opportunity to uncover an unusual bit of science. Sometimes the connections between what happens in your cup and the wider world are fairly obvious (e.g. the steam above your coffee and cloud formation), but sometimes the connections seem a little more obscure. On occasion, your observations may lead to philosophical speculations or stories from history. Every coffee is an opportunity to discover something, if you just slow down and ponder enough.

It was with this in mind that I looked at my freshly made French Press coffee a few weeks ago. I had positioned my cup very close to the cafetière such that a small water bridge had formed between the cup and the cafetière (see photo). Such “capillary bridges” have been studied for a couple of centuries and yet there is still more work to do. Caused by the surface tension of the water, understanding the way these bridges form and the shape of the surfaces produced is important for fields such as printing and powder processing. Yet it is only in the last 150 years or so that we have started to understand what surface tension is. Moreover, much of the pioneering work on this subject was done by an amateur scientist who just noticed things (and then designed some very clever experiments to discover more).

Agnes Pockels (1862-1935) is now regarded as a surface science pioneer but in 1891 she was a complete unknown. Although she had wanted to study physics, she was prevented from going to university because she was female. Consequently, all her study of the subject had to be through her brother Friedrich’s books and letters. It is not known what prompted her investigations but from 1880 she had been experimenting with a device to measure the surface tension of water. The device used a sliding weight to measure the force required to pull a 6mm diameter wooden disk off of the surface of a trough of water.¹ The design of this device was so successful that, a few years later, Irvine Langmuir adapted it slightly in order to study the surface of oils. He went on to receive the Nobel Prize for his work in 1932. Yet it is a device that could also be built in your kitchen, exactly as Agnes Pockels did².

reflections, surface tension

The effects of surface tension can be seen in the light reflected from a coffee

Pockels measured the surface tension of water contaminated by oil, alcohol, sugar, wax, soda crystals and salt (amongst other things)¹. She discovered how the surface tension of the water could be affected by pulling the surface or introducing metal objects onto it. She discovered the “compensating flows” that occurred between regions of different surface tension (you can see a similar effect with this soap boat). Yet all of this remained hidden from the wider world because Pockels was unable to publish. Not having access to the contemporary literature about surface tension and moreover unknown, unqualified and female, no journal would look at her work let alone publish it. Nonetheless, she was clearly a brilliant experimentalist and capable physicist.

Things changed when Pockels read a paper by John William Strutt (Lord Rayleigh) in about 1890. Rayleigh was quite the opposite of the unknown Pockels. As well as his work on sound, electricity and magnetism and the (co-) discovery of Argon, Rayleigh is known for his work on understanding why the sky is blue. (Which is another phenomenon that you can see while preparing your coffee if you drink your coffee with milk.) In his paper on surface tension, Rayleigh had come to similar conclusions as Pockels’ work but Pockels had gone further. Unable to publish herself, she instead wrote to Rayleigh, in German, detailing her experimental technique and results. Rayleigh responded by forwarding her letter to the scientific journal Nature together with an introductory paragraph:

“I shall be obliged if you can find space for the accompanying translation of an interesting letter which I have received from a German lady, who with very homely appliances has arrived at valuable results respecting the behaviour of contaminated water surfaces. The earlier part of Miss Pockels’ letter covers nearly the same ground as some of my own recent work, and in the main harmonizes with it. The later sections seem to me very suggestive, raising, if they do not fully answer, many important questions. I hope soon to find opportunity for repeating some of Miss Pockels’ experiments.”¹

Coffee Corona

You may have seen white mists form over the surface of your coffee (seen here by the rainbow effect around the light reflection). But what are they and how do they form? This is still not really known.

Rayleigh’s introduction and Agnes Pockels’ letter were published in Nature on 12 March 1891. The paper enabled Pockels to publish further results in both Science and Nature as well as in other journals. In 1932 she received an honorary doctorate in recognition of her work.

It seems that this coffee-science story has two main messages. The first is to emphasise how much we gain by ensuring everyone has access (and encouragement) to study physics (or indeed whatever subject they are motivated by). What would we have lost if Agnes Pockels had not had the books of her brother and made the decision to write to Rayleigh? But the second message is that Agnes Pockels managed all this, at least initially, by merely noticing what was going on in the liquids around her. Being curious she designed and built a piece of equipment that enabled her to measure what she was intrigued by and by taking a systematic series of data she discovered physics that was unknown to the wider community at the time. So the question is, what do you notice when you look at your coffee? How does it work, what can you discover?

Please do share any interesting physics that you see in (or around) your coffee either here in the comments section below, on Facebook or on Twitter. Tea comments would also be welcome, but whatever you do, slow down and notice it.


¹Rayleigh, Nature 1891, 43, 437-439, 12 March 1891 (full text here)

²Reference to the kitchen is here.