beautiful physics

From fried eggs to coffee boules via milk rings

Egg no pales, coffee, garden centre
We can often see the Leidenfrost effect when we cook a fried egg. But could we see it while simultaneously preparing a coffee cocktail?

We have probably all come across the Leidenfrost effect, the splash of water into a hot frying pan causing drops of water to skirt across the hot surface before evaporating. We may even be familiar with it in frying pans and cooking surfaces. But what would happen if you swapped the frying pan surface for a (very hot) liquid surface. What happens to the Leidenfrost effect then?

One of the first differences between a frying pan and a bath of hot liquid (we’re not quite yet to the coffee bit) is that the frying pan based Leidenfrost effect requires a lot of heat: the frying pan has to be many degrees hotter than the boiling point of the liquid being levitated. But for the Leidenfrost effect to happen on liquid surfaces requires nowhere near so much heat. In some cases levitation can even occur if the liquid bath is just one degree higher than the boiling point of the levitating liquid. What makes a hot liquid so much different from a hot solid?

The first explanation could be that a liquid surface is absolutely flat at the molecular level. Frying pans and other surfaces have scratches and dents and all sorts of bumps that mean that bubbles can form at the interface and disrupt the levitation of the drop. Could this be it? Probably not as a complete explanation because people can study the Leidenfrost effect over semiconductor wafers which are also atomically flat and even there, many more degrees are needed between the temperature of the surface and the boiling point of the drop than are observed in liquid substrates.

A second explanation is that a liquid surface is able to deform a bit to support the weight of the drop above it, this means that the drop has more of a chance of remaining levitating above the liquid surface. And yet, it turns out that there is more than that happening in liquids as a recent study in a prominent physics journal showed.

If you look carefully at the surface of the coffee in the V60 jar, you will see it is bent underneath the drop on top of it. While the drop on the coffee here is not ‘floating’ because of the Leidenfrost effect (it is stable due to other effects described here), the fact that liquids may be able to bend under drops has been thought to make the Leidenfrost effect more stable on some liquid surfaces.

That study used a bath of silicone oil as the heated surface. The drops that levitated were either of two different liquids: ethanol (ordinary alcohol) or HFE-7100 (an engineered fluid designed to replace ozone depleting chemicals in certain industrial applications). What made the study so interesting was that tiny fluorescent particles were mixed with the silicone oil that allowed the researchers to see how the liquid underneath the drop was moving.

A toroidal vortex formed in the silicone oil under both the ethanol and HFE-7100 drops. We can see similar toroidal vortices in our V60 or by dripping milk into a glass of water; they are doughnut shaped regions of moving fluid, like smoke rings, they could be considered ‘milk rings’. But in this case, there was no drop entering into the bath of liquid as with the milk rings. The drop and the bath were not mixing at all. And, perhaps more puzzling, the direction of the rotation of the vortex was different for the two types of drop. For the alcohol drops, the liquid directly underneath the drop plummeted into the silicone oil before moving under and then back up to the surface to be pulled down at the centre again. Under the HFE-7100 it was different. There, the liquid at the centre of the doughnut surged up, dragged along the surface before going under and returning back once more to be pulled up at the centre of the torus.

Why would the two liquid drops show such different behaviour in the silicone oil substrate? It comes down to a competition of three forces. The first thing that you will notice is that if the levitating drop is slowly evaporating and will eventually disappear (as is the case with the frying pan), this means that it is absorbing heat from its local atmosphere in order to gain the energy needed for evaporation to occur (think about your hand getting cold after sanitising it with an alcohol liquid as the alcohol evaporates off). This means that the silicone oil immediately under the drop gets colder. Cold liquids are generally more dense than warm liquids and so the cold liquid sinks pulling the surrounding liquid down with it.

Linked with this effect is that the surface tension of a liquid decreases as the temperature of the liquid increases. This results in a flow of liquid from regions of low surface tension to regions of higher surface tension called a “Marangoni flow”. This is again something that we may have seen during the Covid-19 lockdown restrictions as videos were circulated showing the effect of soap on a layer of pepper scattered on the surface of water. The pepper retreats away from the soap because of these Marangoni flows which can in fact be very fast.

Milk rings can be formed by dripping milk into a glass of water. But similar fluid rings can also form in liquids hot enough to support cold Leidenfrost drops levitating above them.

These two effects draw the liquid down at the centre of the torus and push the liquid up at the edges, this is what dominates when ethanol is levitating above the silicone oil. In contrast, a third effect dominates for the levitating drops of HFE-7100. Both ethanol and HFE-7100 drops are evaporating above the hot silicone oil surface and as they do so, the gas that evaporates out of them under the drop flows out from the centre of the levitating drop to the edge. Just as with a gentle breeze on a pond, this vapour flow leads to a shear force on the liquid underneath that pulls the liquid out from the centre of the torus towards the edges, down and then, to complete the circle, back up through the middle.

Remarkably, despite their different rotation directions, both types of vortex contributed to keeping the drop levitating. You can read more about the study in the summary here or in the journal here.

Given that water boils at 100C and that alcohol boils at 78C, it is feasible that by dripping vodka or another strong alcohol based drink onto our freshly prepared coffee we may see a similar effect. It may certainly be worth a try. I’ll leave this as an experiment that you can tell me about on Twitter, Facebook or in the comments section below, but it is an experiment with a positive result either way. Perhaps you will see levitating alcohol drops above your coffee. But even if you don’t, you can at least keep trying until you have made an interesting coffee based cocktail.

2 years in

3D hot chocolate art on an iced chocolate, Mace, Mace KL, dogs in a chocolate

Happy birthday to me

Last weekend, Bean Thinking turned 2. So I’ve been looking back at the cafés I’ve visited over the past two years. Bean Thinking started as a way to slow down and to try to see things in a (slightly) different way, to really enjoy the coffee but also to take time to explore the stories, and the science, that can be found in different cafés. I’ve enjoyed the coffee in each café that I have visited but, as always happens, some stick in the memory a little more than others.

So I decided to pull together five cafés which, for me, had an interesting story to tell or prompted an unexpected chain of thoughts. I have sadly had to leave out some great cafés and some really fun stories (for me to think about at least). However, these five stood out. Each café introduced an unexpected bit of science to me, or had something about them that meant that slowing down and enjoying the coffee provided a really special moment. Consequently, each café features for slightly different reasons, and so rather than create a top 5 (which would be impossible anyway), I have listed them alphabetically. I hope you’ll excuse this trip down memory lane.

Amoret, Hammersmith

Kettle drum at Amoret

Coffee on a drum at Amoret

It is not every day that a well made V60 can transport you to another planet. Yet that is what happened for me at Amoret in Hammersmith. The cylindrical design of the table reminded me of a drum but the question is, why do drums make the sounds that they do? The answer to this question took me on a journey into sounds. Just how different would Bach’s famous fugue sound if played on Venus rather than Earth? And then a surreal moment as a Dutch TV station decided to take Bananarama to Venus courtesy of research conducted at Southampton University. This was all accompanied by great coffee in a very pleasant cafe, the review can be found here.

Coffee Affair, Queenstown Road,

Contemplating the floor at Coffee Affair

Contemplating the floor at Coffee Affair

Where better to slow down and appreciate the moment than a place reminiscent of the geology of the South Downs that helped Charles Darwin to argue the case for his theory of evolution. Coffee Affair occupies the old ticket office at Queenstown Road station. The fixings and the floor of the café reveal evidence of the people who inhabited this space in times past. Watching the V60 being prepared, slowly, carefully, exactly, emphasises this sense of time. The result is great coffee in a place that almost forces you to step out of the speed of modern life and stop, put down the smart phone and take time to just notice. Coffee Affair was reviewed here.

Lumberjack, Camberwell,

Lumberjack coffee Camberwell

Exploring local connections at Lumberjack

There’s a strong emphasis on keeping it local at Lumberjack in Camberwell, as well as a preoccupation with all things wooden (this being an enterprise set up with London Reclaimed). So it was interesting to discover that there was a fairly local connection between Camberwell and the ultimate ‘local’ London tree, the London Plane. Not only that, but research that had been published a few weeks before I went to review Lumberjack had shown that, surprisingly, the wind speed needed to fell a tree was fairly constant at around 56 m/s, irrespective of the size or type of tree. This surprising finding was the cherry on the cake for this ultimate in local reviews (here).

Red Door, Greenwich,

vortices, turbulence, coffee cup physics, coffee cup science

Beautiful physics at Red Door

Just what would happen if you put a cup of coffee on a record player? A turntable in a corner at Red Door in Greenwich meant that not only did I start to think about this question, I decided to start some experiments to find out. The resulting physics was physically as well as scientifically beautiful. The experiments can be done by anybody with equipment that you can probably find at home (though I would recommend not using an actual turntable). It turned out to be an elegant experiment involving vortices, but as Helmholtz noticed, similar vortices form in organ pipes, the atmosphere and even in electromagnetism. Truly a beautiful piece of connected physics that I would have missed had I enjoyed my coffee ‘takeaway’. More here.

The Turkish Deli, Borough Market,

Turkish coffee

The universe in a cup of coffee at The Turkish Deli

“The universe is in a glass of wine” so said a Greek poet according to Richard Feynman, but at the Turkish Deli it is more obvious in a cup of coffee. When made properly, Turkish coffee requires at least four minutes of ‘settling time’ before it can be enjoyed. You could use this time to think about how the concentration of coffee particles changes as a function of the depth. Similar considerations led Jean Perrin to conduct experiments back in 1910 that he declared showed that “… it becomes very difficult to deny the objective reality of molecules” (which before that point had indeed been very much denied). Now that The Turkish Deli also roast and grind their own coffee on-site, there is even more reason to visit and ponder the connectedness of our coffee and our planet. The Turkish Deli was reviewed here.

With so many more cafés to explore, and things to discover, who knows what the next year or two will bring. And if you’ve got a recommendation or found a great café where you have stopped and noticed something intriguing, no matter how lateral, why not drop me an e-mail, I’d love to hear your experiences of slowing down and appreciating our coffees.


The importance of going slow

journals in a library

How can we assess the work of scientists? Should we count the number of papers that they write?

In the past few weeks there has been a bit of a media storm about the state of science. A paper that had been published in the journal Science, was retracted because it turned out that the study had, quite possibly, been faked. The retraction highlighted the problem of “publish or perish” which has been a concern for many scientists of late. A second article, this time an editorial in Nature, took a different and perhaps surprising perspective on things. Apparently the public trust scientists much more than scientists think that they do. Why would that be the case?

These two stories should concern us because they lie at the heart of a current problem in science. According to the dictionary, ‘science’ is “systematic and formulated knowledge”. Such knowledge takes time to develop, it takes us time to understand what goes on, both on an individual level and as a society. The ‘publish or perish’ culture acts in opposition to this. Within a ‘publish or perish’ culture, the way that science works is that the more papers that you have, especially those that get cited and are published in (apparently) good journals, the more successful you will be in your career and in your ability to get research funding. It is essential to publish “high impact” papers in order merely to survive in science. In more extreme cases this has led to data being faked and subsequent retractions of the papers (if it is ever discovered). Active faking of data though is only the tip of the iceberg. The pressure to publish high profile papers quickly, can lead to the original paper not having been investigated thoroughly enough. In fact, there are even motivations to publish too quickly. Firstly, if you are wrong, you just publish a second paper a few months later. Two papers, two sets of citations. Secondly, publishing early means getting there first, ie. more citations. It has got to the point where it is advantageous to quickly publish poor quality research with hyped key words than it is to do a thorough job and perhaps be beaten to the publication by a more incomplete work. This cannot be good for science or our society and it suggests that, in order to have a scientific career you must, to a greater or lesser extent, cease to behave scientifically. It is perhaps for this reason that scientists themselves have a doubt as to why the public would trust them, they no longer trust themselves.

lilies on water

Is there symbolism here? There’s certainly a lot of physics.

The ‘publish or perish’ culture has come about partly as a consequence of needing a metric by which to judge the worth of research. In itself this is understandable but it does suggest that we are no longer confident of our ability, as a society, to measure the ‘good’ of something. To judge something as ‘good for society’ necessarily involves many different inputs from many disciplines. Assessing something as good is a value judgement. To redefine ‘good’ purely into something that we measure (by profit, or by number of papers) is to artificially reduce what is good for society to an arbitrary, but on appearance scientific, method. Rather than admit that questions over what is ‘good science’ are, essentially, value judgements, we try to give a false ‘scientific’ measure of their worth, one based on citations and publications. We still have our biases but we have become less conscious of them and instead try to hide them with a false scientism.

How could we change this, how else can we assess who is a ‘good’ scientist or what research will benefit society? This is, I think where it is important for everyone to get involved and to slow down. It is open for everyone to investigate, for themselves, what they think would make a ‘good’ society. Clearly the quest for knowledge, and in particular scientific knowledge, will form part of that good but for us, as a society to realise what is good we need to stop and think about it. There is a need to encourage clear methods of thinking but at the same time everyone must feel eligible to be a part of this natural philosophy, purely as a consequence of their being a citizen of society. On a practical level, this can be achieved by our maintaining a sense of awe and wonder at the beauty of the world, and society, around us. In my own field of magnetism for example, to know the physics behind magnetic attraction is to make it more beautiful. And that is in essence what I am trying to communicate with Bean Thinking; just as an artist does with a painting, I am attempting to share the beauty that I see as a result of seeing physics all around me. The saying of Pierre Duhem that “Physical theory is a mathematical painting of reality” can be taken at many levels. As a scientist, I am to a certain extent, an artist.

rain drops on a tulip

A tulip in spring. The water droplets on the petals suggest some very deep physics. As the flower opens into the sunshine, each layer  (physical and metaphorical) of petals reveals a new level of beauty.

Of course, there is no immediate connection between appreciating the beauty of knowledge and allocating research funds. Yet if we, as a society, appreciate science and beauty where we see it, we are going to slowly move back to a more sustainable, scientific way of doing science. “By learning to see and appreciate beauty, we learn to reject self-interested pragmatism”¹. By allowing ourselves to assess the good of society across many measures, we recover science. Denying the fact that what qualifies as ‘good’ is ultimately a value judgement and instead covering it in false metrics, imperils science. It is in the history of humanity to ask ‘why’. Moving to a predominantly technology driven quasi-science does not enrich us as a species. Good art, good music, great science can. Great discoveries of the past have not been obtained by chasing the latest chimera of a device, they have been uncovered through an insatiable curiosity. A demand to know ‘why’ things are the way they are. We are destroying the very science we are so keen to promote if we conform to the key-word, hype and technology driven ‘publish or perish’ culture. It has got to the point where, in order to save science, it is imperative that we, as a society, recover our ability to appreciate the beauty in science.

I hope that Bean Thinking prompts at least some people to question the world around them. It is not important to agree with what is written in Bean Thinking indeed, perhaps with some things it is more important to disagree. The key thing is to notice the world around. The practise of slowing down and noticing things is the reasoning behind the cafe-physics reviews, as much as anything it enables me to practise slowing down and noticing too. To slow down and to appreciate what is there will mean that slowly, imperceptibly perhaps, we challenge the culture of ‘publish or perish’. To do so may not be too far short of a need to recover our humanity, to quote Laudato Si’ again, “[w]e need to see that what is at stake is our own dignity”².

¹ Pope Francis, Laudato Si’, (2015) #215

² ibid, #160

Further thoughts:

Michael Polanyi “Science, Faith and Society”, Sapientia Press, 1964

Michael Polanyi “Personal Knowledge: Towards a post-critical philosophy”, University of Chicago Press, 1974