soap boat

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.