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Coffee cup science General Home experiments Observations Tea

Making a splash

You spilled your coffee, a terrible accident or an opportunity to start noticing?

Why do some droplets splash  while others stay, well, drop like? It turns out that there is some surprising physics at play here. When a drop of water, or coffee, falls from a height and onto a flat surface (such as glass), we are accustomed to seeing the droplet fracture into a type of crown of smaller droplets that form a mess over the surface. Visually spectacular, these splashing droplets have even been made into an art form (here).

Fast frame-rate photography reveals how each micro-droplet breaks away from the splashing drop:

Video taken from Vimeo – “Drop impact on a solid surface”, a review by Josserand and Thoroddsen.

 

So it perhaps surprising to discover that there are many things about this process that we do not yet understand. Firstly, if you reduce the gas pressure that surrounds the drop as it falls, it does not make a splash. In the extreme, this means that if you were to spill your coffee in a vacuum, you would not see the crown-like splashing behaviour that we have come to expect of falling liquids. Rather than splash, a droplet falling in low pressure spreads out on impact as a flattening droplet. This counterintuitive result was first described in a 2005 study (here) that compared the effect on splashing of droplets with different viscosities (methanol, ethanol, 2-propanol) falling through different gasses.

cortado, Brunswick House, everyday physics, coffee cup science
Don’t spill it!
But would a latte splash more or less than a long black?

The authors of the study ruled out the effect of air entrapment surrounding the droplet as it falls as high speed photography had not indicated any air bubbles in the droplet just before impact. Instead they considered that whether a drop splashes on impact – or not – depended on the balance between the surface tension of the falling liquid and the stress on the drop created by the restraining pressure of the surrounding gas. Calculating these stresses led to a second surprising result. Whether a drop splashes on impact or not depends on its viscosity (as well as the gas pressure and the speed of impact). But the surprising bit is that the more viscous the liquid, the greater the splash.

From a common-sense perspective (that may or may not have any bearing on the reality of the situation), an extremely viscous liquid like honey should not splash as much as a less viscous liquid like coffee. This suggests that there is an upper-limit in viscosity to the relation predicted in the 2005 study. After all, although the authors did change the viscosity of the liquids, the range of viscosity they studied was not as great as the difference between coffee and honey. This sounds like a perfect experiment for some kitchen-top science and so if any reader can share the results of their experiments on the relative splashes formed by coffee and honey, I would love to hear of them.

 

Categories
Coffee review slow

Pulp fiction in KL?

Freshly roasted coffee, Pulp, Papa Palheta, KL
Coffee on the cutting machine at Pulp

There have been a few great cafés opening up recently in Kuala Lumpur, including Pulp by Papa Palheta in Lucky Gardens. However the space that Pulp occupies is unrivalled: The old cutting room of the Art Printing Works. It really is geek meets hipster in this café, with old electric fittings and the original paper cutting machine housed alongside a fantastic range of freshly roasted coffee.

There is a great range of coffee on offer too. From pour-overs to espresso based drinks and cold brew, Pulp is a great place to discover a wide range of coffees. I had a pour over Ethiopian (Suke Quto) that was beautifully presented with tasting notes ready for me to enjoy. A nice touch was that the cup had been pre-warmed so I got no condensation around the rim of the mug when I filled the cup with coffee. The coffee itself was very fruity, presumably very lightly roasted in order to retain the fruity notes of the beans. (On a second visit I enjoyed a long black which was also very fruity though less so than the pour-over).

pourover at Pulp, Papa Palheta, KL
Taking time with a beautifully presented pour-over

Although there are plenty of seats in this café, on both occasions we visited it was crowded and hard to find a seat. It seems that this is a very popular spot for good coffee in KL, so do be prepared to share a table! Indeed, one of these ‘tables’ is formed from the old cutting machine itself, the machine that used to prepare the paper used for newspapers and books. Sipping coffee here, in a place steeped in the history of printing, it seemed only natural to consider the role in our current society of fake news and whether there is anything that we can do about it.

The issue of fake news or of exaggerated or incomplete news stories is not just limited to issues surrounding the recent US election. Reporting our experimental results honestly and our theories thoroughly underpins all scientific research. However, as funding decisions and employment prospects increasingly depend on publications in prestigious journals, question marks can start to hover over each scientist’s paper (the “publish or perish” problem). Does reporting a result honestly include waiting for that last result (that could contradict or delay the ‘story’ thereby making publication in “high impact” journals such as Nature less probable)? Do we read the papers of others thinking that they have reported everything as truthfully and fully as possible or do we shrug as their next paper (in a lower impact journal) reveals the ‘caveats’ on their previous work? The chemist and scientific philosopher, Michael Polanyi wrote in 1946:

… Suppose scientists were in the habit of regarding most of their fellows as cranks or charlatans. Fruitful discussion between them would become impossible…. The process of publication, of compiling text books, of teaching juniors, of making appointments and establishing new scientific institutions would henceforth depend on the mere chance of who happened to make the decision. It would then become impossible to recognise any statement as a scientific proposition or to describe anyone as a scientist. Science would become practically extinct.“∗

Pulp, Papa Palheta KL
Where else could you see all these old electrical boxes?

Although we are hopefully still very far from that scenario, it is fairly clear that similar levels of trust are required for our society to function well too. For our society to flourish, these same standards of integrity are required of our press (and indeed of ourselves if we publish – or share – articles online). The perception that our society is moving into an era where fake news is as valid as proper investigative journalism has led to some calling ours a “post-truth” era. However, as Emmi Itäranta has argued, we should endeavour to avoid calling our times “post-truth”, in part because the term itself is not neutral. Our words and language matter and when we use the term we contribute to the idea that truth is no longer meaningful.

Such thoughts remind us of our own responsibility and contributions to society. If we don’t want fake news to influence politics, we need to be careful what we share or publish online. From our language to our values, we need to behave as if truth matters. And, to me at least, it seems that enjoying a coffee can help us with that. Stopping to appreciate the moment as we savour our well prepared coffee, we can step-back from the “retweet” or “share” button and think, is this evidence based and true or else, what is it that I gain by sharing this?

It strikes me that cafés such as Pulp, with their mix of great coffee and interesting surroundings are perfect spaces in which to slow down and think rather than react and retweet. Perhaps that is what we need for 2017, more time contemplating in cafés, less time on social media. Let’s hope for some quiet time ahead.

Pulp by Papa Palheta is at 29-01 Jalan Riong, Kuala Lumpur, Malaysia,

∗Michael Polanyi “Science, Faith and Society”, Oxford University Press, 1946

 

Categories
General Home experiments Observations Science history slow Tea

Reflections on physics and coffee

BeanThinking started as a way of slowing down and appreciating connections, often between a coffee and the physics of the wider world but also in terms of what can be noticed in any café. Perhaps, for this first post of 2017, it’s worth spending five minutes looking at your coffee while you drink it to see what you notice. Here are a few coffee connections that occurred to me recently:

reflections, surface tension
Reflections on a coffee.

Parallel lines and surface reflection: The parallel lines on the ceiling of a café were reflected in a long black. Surface tension effects on the coffee meant that the reflections were curved and not at all parallel. A piece of dust on the surface of the coffee was revealed in the reflection by the curved reflections of the ceiling. Astronomers can use similar effects (where images of a star appear in a different location to that expected) to infer the presence of dark objects between distant stars and their telescope. This gravitational lensing can be used to detect quasars or clusters of galaxies.

 

 

 

layering of coffee long black
Layers of coffee

Layering of crema as the coffee is consumed: The coffee stain effect and this layering of the crema suggests a connection between a coffee cup and geology. It used to be my habit to take a mug of tea with me when I taught small groups of undergraduates. In the course of one of these tutorials, a student (who had been observing similar layering in my tea mug) said, “You drink your tea faster when it is cooler than when it is hot”. Full marks for observation, but not sure what it said about his attention during my tutorials! Similar observations though can help geologists estimate the age of different fossils.

 

interference patterns on coffee
Bubbles in coffee

Bubble reflections: An old one but the interference patterns caused by bubbles on the surface of the coffee are full of fascinating physics. The fact that the bubbles are at the side of the cup and seem to be grouped into clusters of bubbles may also be connected with surface tension effects (although there is a piece of weather lore that connects the position of the bubbles to the weather. If anyone ever does any experiments to investigate this particular lore, I’d love to hear about them).

 

 

Coffee, Van Gogh
Art in a coffee cup

Van Gogh’s Starry Night: The effects of vortices and turbulence caused the crema of a black coffee to swirl into patterns reminiscent of this famous painting by Van Gogh. As a result of posting this image on Twitter, @imthursty sent me a link to this preprint of a paper submitted to the arxiv: the connections between Van Gogh’s work and turbulence. A great piece of coffee combining with art and science.

 

So many connections can be made between tea, coffee and science and the wider world, I’d love to see the connections that other people make. So, if you see some interesting physics, science or connections in your coffee cup, why not email me, or contact me via FB or Twitter.