coffee experiments

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.

Introducing Bean thinking

As this is the first true blog post, let’s do the introductions. What is Bean thinking and who is @thinking_bean?

Breakfast coffee, introductionThe human bean behind @thinking_bean has worked for a fair few years in university research centres, researching obscure but fascinating areas of physics where magnetism meets superconductivity. Such research fields can be very beautiful but perhaps not of immediate technological relevance. Understandably, this can cause some in our society to question the utility of investigating these phenomena. Part of the motivation behind Bean thinking is to explore this question, why do we do science?

A second motivation is to share the wonder of the world that today’s understanding of physics gives us. Some of these beautiful areas have not yet been fully understood even though they occur in something as apparently simple as a coffee cup. Through teaching, outreach, talking to friends and even in conversations with some colleagues, I became aware of the way that science, perhaps particularly physics, can be perceived as a very interesting, but perhaps very difficult subject, far removed from people’s everyday lives.

Yet this is not true! Slow down, put down your smart phone, e-book or tablet, observe the world. Physics is all around you. Warming your hands around a mug of hot coffee, you may not realise how it is related to the Big Bang. Looking at a glass of milk can illustrate the reasons that the sky is blue. Even the mere act of stirring coffee can be related the Heathrow minute (link, link2).

Hence Bean thinking, which hopefully will become a space where curious individuals can come and discuss interesting phenomena that they notice in the day to day. If this can be done with a cup of coffee, all the better. The point is to slow down and start noticing. Each Wednesday I will update the Bean thinking blog, the “Daily Grind” with things that I have noticed or that I find interesting. Who knows, if anyone starts to read this and shares their observations perhaps the Daily Grind can also include these. As this website develops, I may add a forum, but for the moment, please let me know what you think about the concept and what you observe around you in the comments section below.