Anthropomorphising surface tension
Keith S. Taber
it seems good training for a scientist to always read accounts of science with a critical filter primed to notice figurative language and to check that the communication can be understood in a non-metaphorical way
When water is poured from a bottle or other container the stream of liquid can take up complex shapes. In particular, it has long been noted how the stream can appear to have the shape of a chain or string of beads, with the flow seeming to be wider in some places that others.
A stream of poured water does not form a perfect cylinder – something that physics should be able to explain.
(Image by tookapic from Pixabay)
This is just the kind of thing that physicists think they should be able to explain…using physics. An article in Physics World (Jarman, 2022) reports some recent work on just this outstanding problem,
"If you pour water out of a bottle, the liquid stream will often adopt a chain-like structure….At the heart of the effect is the non-cylindrical profile of the jet as it emerges. To minimize surface tension, the jet tries to become a cylinder, but this motion overshoots and results in an oscillation in the profile shape."
Article in Physics World
What intrigued me here was the choice of phrasing: "To minimize surface tension, the jet tries to become a cylinder…". This language could be considered to reflect teleology, and even anthropomorphism.
Teleology?
Teleological explanations are those that explain something in terms of some kind of endpoint. Something happens in order to bring about some specific state of affairs. The sun shines to allow us to find our way. Plants produce oxygen so we can breathe. That is, there is seen to be purpose in nature, something that is characteristic of mythical and supernatural thinking. In science, teleological explanations are strictly considered a kind of pseudo-explanation – something that has the form of an explanation, but does not really explain anything. Sometimes we find apparently teleological explanations in science because they are being used as a kind of shorthand. For example, if we know that science suggests entropy always increases in processes, we might interpret a scientist's comment that something happens 'in order to increase entropy' to be a loose (or lazy) way of saying that some suggested mechanism or action is considered likely because it is consistent with the assumption that entropy will increase.
Read about pseudo-explanations
Read about teleology in science
Here it is suggested that the odd shape is formed in order "to" minimise surface tension. Scientists have observed that many phenomena (such as rain forming roundish drops) can be explained in terms that surface tension tends to be minimised (cf. entropy tends to increase, objects tend to roll down hills, people tend to get older). But the language here might suggest minimising surface tension is an end that nature seeks – that would be a teleological explanation.
Although perhaps this is not simple teleology, as it is not that the water forms into the shape it does to minimise surface tension, but something more nuanced is going on – the jet of water is actively trying, but not quite managing, to minimise surface tension.
anthropo… (to do with humans, as in anthropology)
…morphism (to do with form, as in morphology, amorphous)
…and anthropomorphism?
Anthropomorphic language refers to non-human entities as if they have human experiences, perceptions, and motivations. Both non-living things and non-human organisms may be subjects of anthropomorphism. Anthropomorphism may be used deliberately as a kind of metaphorical language that will help the audience appreciate what is being described because of its similarly to some familiar human experience. In science teaching, and in public communication of science, anthropomorphic language may often be used in this way, giving technical accounts the flavour of a persuasive narrative that people will readily engage with. Anthropomorphism may therefore be useful in 'making the unfamiliar familiar', but sometimes the metaphorical nature of the language may not be recognised, and the listener/reader may think that the anthropomorphic description is meant to be taken at face value. This 'strong anthropomorphism' may be a source of alternative conceptions ('misconceptions') of science.
So, in our present case, we are told that "the "the jet tries to become a cylinder". This is anthropomorphic, as to try to do something means having a goal in mind and deliberately behaving in a way that it is believed, expected, or – at least – hoped, will lead to that goal. Human beings can try to achieve things. We can perceive our environment, have goals, conceptualise possibilities and means to reach them, and put in practice an intention.
Whether, and, if so, which, animals can try to do things rather than simply following evolved instincts is a debated issue.
- Does a dog try to please its human companion by bringing the newspaper?
- Does the dolphin try to earn a fish by jumping through a hoop? Perhaps.
- Does the salmon try to get to a suitable spawning site ('ground', sic) by swimming upstream?
- Does the spider try to make a symmetrical web?
- Does the bee try to collect nectar by visiting flowers. Probably not.
- Does she try to fertiliser those flowers with pollen to ensure there will be flowers for her to visit in future seasons? Almost certainly not!
Jets of water?
Do jets of water think that being cylindrical is desirable (perhaps because they recognise minimal surface tension as an inherent good?) , and so make efforts to bring this about? Clearly not. So, they do not try to do this. They do not try to do anything. They are not the kind of entities that can try.
So, this language is metaphorical. The reader is meant to read that "the jet tries to become a cylinder" to mean something other than "the jet tries to become a cylinder". Now, often figures of speech are used in science communication because the ideas being communicated are abstract and complex, and metaphorical language that describes the science in more familiar terms makes the text more accessible and increases engagement by the audience/readership.
A question here then, is what "the jet tries to become a cylinder" communicates that was more likely to be inaccessible to the reader. Physics World is the house magazine of the Institute of Physics, which means it is sent to all it members working across all areas of physics. So a broad readership, though largely a readership of physicists.
Tracing the stream back to the source
Another question that occurred to me was whether the reporter (Jarman) was simply reporting the original researchers' (Jordan, Ribe, Deblais and Bonn) ways of communicating their work. That report was in an academic journal, Physical Review Fluids, where formal, technical language would be expected. So, I looked up the paper, to see how the work was described there.
Under a heading of 'phenomenology', Jordan and colleagues explain
"Chain oscillations are most readily observed when the viscosities of the jet and the ambient fluid are low and the interface has a high surface tension. Water jets in air satisfy these criteria, and so it is no surprise that chain oscillations occur in many everyday situations. Deformation and vibration of a jet are capillary phenomena in which surface tension acts to reduce the jet's surface area. If the cross section is not circular, its highly curved portions are pulled inward and its weakly curved portions pushed outward relative to a circular section with the same area. But due to inertia the movement overshoots, with the result that the long and short axes of the section are interchanged. The shape of the section therefore evolves as it moves along the axis of the jet, producing a steady liquid chain when observed in the laboratory frame…"
Jordan, Ribe, Deblais & Bonn, 2022
"The shape of the section therefore evolves as it moves along the axis of the jet, producing a steady liquid chain when observed"
(Image by Kevin Phillips from Pixabay)
(This seemed to be a somewhat different meaning of 'phenomenology' to that sometimes used in science education or social science more generally. Phenomenology looks to explore how people directly experience and perceive the world. Jordan and colleagues include here a good deal of re-conceptualisation and interpretation of what is directly observed. 1 )
The effect Jordan and colleagues describe seems analogous to how a pendulum bob that is released and so accelerated (by gravity) towards the point directly beneath its support (where gravitational potential is minimised) acquires sufficient momentum to overshoot, and swing upwards, beginning an oscillatory motion. Something similar is seen in an ammeter where the needle often overshoots, and initially oscillates around the value of a steady current reading (unless the spring is 'critically damped'). The effect is also made use on in striking a tuning fork.
No need to try
There is no mention here of 'trying', so no clear anthropomorphism. So, this was a gloss added in the report in Physics World, perhaps because anthropomorphic narratives are especially engaging and readily accepted by audiences; perhaps because the reporter needed to rephrase so as not to borrow too much of the original text, or perhaps as part of preparing brief copy to an editorially assigned word length. Or, perhaps Sam Jarman was not even conscious of the anthropomorphism being used, as this seems such a natural way to communicate. 2
Surface tension acting up
Did the original authors avoid teleology? They do write about how "surface tension acts to reduce the jet's surface area?" This could be read as teleological – as there seems to be a purpose or goal in the 'action', even if it is not here presented as a premeditated action. Could any suggestions of such a purpose be avoided?
One response might be that, yes, a physicist might suggest the 'true' description is a mathematical formula (and there are plenty of formulae in Jordan et al's paper) and that a verbal description is necessarily the translation of an objective description into an inherently figurative medium (natural language).
And, of course, this is not some special case. We might read that gravity acts to pull something to the ground or air resistance acts to slow a projectile down and so forth. 'To' may just imply a cause of an outcome, not a purpose.
I think a rewording along the lines "the action of the surface tension reduces the jet's surface area"conveys the same meaning, but is more of a neutral description of a process, avoiding any suggestion that there is a purpose involved.
Reading and interpreting
But does this matter? In teaching young people such as school children, there is evidence that some figurative language that is anthropomorphic or teleological may be understood in those terms, and student thinking may later reflect this. Part of science education is offering learners an insight into how science does seek to (oh, science personified: sorry, scientists seek to) describe in neutral terms and not to rely on nature having inherent goals, or comprising of the actions of sentient and deliberate agents.
The readership of Physics World is however a professional audience of members of the community of inducted physicists who are well aware that, actually, surface tension does not try to do anything; and that minimising surface tension is a common observed pattern, not something set out as a target for physical systems to aim for. These physicists are unlikely to be led astray by the engaging prose of Sam Jarman and will fully appreciate the intended meaning.
That said, there is an intimate bidirectional relationship between our thinking and our speech – our speech reflects our thought pattens, but our language also channels our thinking. So, it seems good training for a scientist to always read accounts of science with a critical filter primed to notice figurative language and to check that the communication can be understood in a non-metaphorical way. That includes checking that our understanding of what we have read is in keeping with scientific commitments to exclude explanations that are framed in terms of nature's end goals, or the deliberate agency of non-sentient 'actors'.
- Jarman, S. (2022). Flowing liquid 'chains' are best described by Niels Bohr, not Lord Rayleigh. Physics World, 35(12).
- Jordan, D. T. A., Ribe, N. M., Deblais, A., & Bonn, D. (2022). Chain oscillations in liquid jets. Physical Review Fluids, 7(10), 104001. doi:10.1103/PhysRevFluids.7.104001
Notes
1 However, none of us are able to be completely naive observers of the world. As William James long ago pointed out, the un-mediated sensory experience of a newborn is a chaos of noise and shapes and colours and so on. Even recognising another person or the presence of a table is an act of interpretation that we learn.
So, experts in a field do see things others do not. A field palaeontologist sees a fossil fragment where the rest of us see undifferentiated dirt and stones. The biochemist sees a steroid structure in a patterns of lines. The football pundit sees a 4-4-2 formation where the occasional viewers just sees people running around. The experienced poker player sees a 'tell' that others would not notice. The professional musician hears a passage in E minor, when most of us just hear a tune.
2 This kind of language reflects a way of thinking and talking often called 'the natural attitude'. Science can be seen in part as a deliberate move to look beyond the common-sense world of the natural attitude to problematise phenomena that might be readily taken as given.
We may get used to, and simply accept, that ice is cold, fire burns, the Lord/King makes decisions and owns the land (and people!), rivers flow, things fall down, the heretic must die, the sun moves across the sky, etc. – and probably most people did for much of human history – where the critical (scientific) attitude is to always ask 'why?'
