A couple of weeks ago we hosted a first “coffee and science” evening at Amoret Coffee in Notting Hill. Designed to explore a physics concept that you could notice in your coffee cup with people from a diverse range of backgrounds, in some ways, the evening itself was an experiment. Would anyone turn up? Would the experiments be interesting? Was I just making my coffee badly?
This last question referred to the fact that the connection for that particular evening had been the dancing drops that skirt across the surface of a V60 (or other pour over) as you prepare your coffee. I had noticed these a couple of years ago but at that point had not appreciated their significance. To answer that question, we were prepared two excellent pour overs by people who really knew what they were doing. And we were spoiled for the coffee which was a recently roasted Nicaraguan washed coffee grown by the Baltodano family who also came along for the evening. The two pour overs were prepared very slightly differently and produced drinks that highlighted different aspects of the flavour of the coffee (though sadly I only managed to try one). This led to a fair amount of discussion amongst those present, not just about which they preferred, but how the preparation affected and highlighted different flavour notes.
The pour overs showed that the dancing drops were there (in both techniques) when coffee was made properly. This was a relief for me! But did they also supply a clue as to how these drops were able to survive, as liquid drops, on the surface of the coffee?
Ordinarily, when a drop drips into a bath of liquid, you would expect it to quickly coalesce with the liquid bath. Once the drop gets close enough to the surface, the van der Waals forces in the drop and the liquid bath will overcome the surface tension effects and the drop will be subsumed into the liquid. If the drop does not coalesce, but instead appears to ‘float’ on the surface there must be a reason.
The first reason that the drop may survive for a while on the surface is because there is a temperature difference between the drop and the bath. This sets up stresses within the drop that pull air into the region between the drop and the bath and keep the drop ‘floating’ for a little while.
Secondly, if you increase the surface elasticity of the droplet, you can stabilise it on the liquid bath for longer. This is usually done by adding soap to the water, not something we did with the V60. But could there be an effect of the coffee oils or some other aspect of coffee chemistry that is keeping these droplets afloat?
Lastly, if you vibrate the surface of the liquid bath, you can create conditions whereby the droplet ‘bounces’ on a cushion of air on the bath. It was interesting, that in the preparation of both pour overs at Amoret that evening, the times that we observed the dancing drops coincided with those times that the pour over was dripping into the coffee bath, causing a noticeable ripple on the surface.
This last condition was the subject of an experiment in the corner of the upper room at Amoret where we used a loud speaker to generate vibrations to two different liquid baths (water and soap water for example) to see if we could obtain stable drops on the surface. Astonishingly, some of the participants on that Tuesday managed to keep a droplet stable for about 15 minutes, you can see their droplet in the photo. The photo is interesting because if you look closely, not only can you see the wave on the bath of water caused by the vibration of the speaker, but you can also see a circular ripple pattern around the droplet. Is that the ripple caused by the droplet’s bounce?
Conversations led on to the fact that these drops were not just seen in pour overs but could occasionally be seen in espressos too. I’m definitely looking forward to the video of that one. While we also got to discuss the importance of different parameters on the stability of the drops – it turns out droplet diameter, as well as the forcing amplitude (which translates to, how loud you have the volume on the loud speakers) are key parameters that affect the behaviour of the drop, something that has been pointed out elsewhere.
The evening also emphasised just how much we have to talk to each other about! One topic that kept coming up was fermentation, specifically with how the coffee cherries are processed. Hopefully this could become the subject of a future conversation.
Future events are planned (in theory but not yet in practise) and so if you’d like to make sure you hear about them, you can sign up to the Bean Thinking events list here. Also, if you didn’t get chance to take part in the evening but would like to continue the discussion and maybe add your videos & comments about the droplets, you can sign up to the Virtual Coffee House which will be discussing this topic (until the next coffee & science evening).