Ventablack

The Dark Arts at Amar, Chelsea Green

Amar Cafe, Drinking coffee on Chelsea Green, Colombian Coffee
Amar Cafe on Chelsea Green. The small terrace area outside is on the spot of two car parking spaces.

Amar Cafe means to love coffee. Though this is in Spanish. In Bengali it apparently means “my cafe”. This could perhaps lead to a short meander onto a language inspired thinking trail about how a cafe that you love to frequent becomes “yours” in a certain sense. Though to return to the coffee, the cafe itself is a small space located on Chelsea Green. There are other branches of Amar cafe in telephone boxes around London and in Stratford upon Avon. A couple of parking spots just outside the cafe have been converted to an outside terrace complete with small olive trees at which you can enjoy your coffee in the fresh air of the Green. Although this is clearly a Covid-19 related temporary measure, it would be good if some of these outside places can remain on a more permanent basis. They do add to the character of the Green. Amar cafe specialises in Colombian coffees that they source themselves. All of the usual espresso based drinks are available as well as the option of a pour over. A small selection of pastries including empanadas are available for breakfast. There are about four tables outside and a couple of tables (along with a window bench) inside. As you enter the cafe, the bar is immediately in front of you. Outside, the cafe is painted a bright yellow colour which makes it stand out among the independent shops that are in this little quarter of Chelsea.

On the two occasions that we have visited, I have enjoyed a really well made pour over each time. Although I am not good at generating my own tasting notes, I would say that the coffee was sweet and syrupy, with a fruitiness and complexity that was very enjoyable. It was presented in a V60 jug together with a black, handle-less, porcelain cup.

V60 at Amar Cafe, Chelsea
Carafe of coffee and cup. The blackness of the coffee is similar to the blackness of the cup. On the carafe, condensation has formed on one side only. How did that happen?

Gazing into the filter coffee, there were patterns on the surface of the coffee that you could see reflected at different angles. But looking more deeply, it looked black, within a black porcelain cup. Where did the cup end and the coffee begin? How could you see black on black? Which was blacker?

A material appears black to us when it absorbs the majority of the visible light incident on it and doesn’t reflect or emit any visible light back. Until 2019, the world’s blackest material was “Ventablack”, but even this material only absorbed 99.96% of the light shining on it. Late in 2019, a new material was discovered that absorbs 99.995% of light shone onto it. And not just that, it absorbs 99.995% of light from the ultraviolet to the THz (between infrared and microwave). The material is truly black. But the inventors of this material were not trying to make a black material, they weren’t even really interested in optics. The invention came courtesy of a collaboration with an artist.

At the time, Diemet Strebe was the artist in residence at MIT. The scientists at MIT who would go on to discover this new black material, were interested in the electrical properties of carbon nano tubes (CNTs). CNTs are a layer of carbon atoms (arranged in the hexagonal structure familiar for layers of graphite) wrapped into a tube. Each tube may be just a few nanometers diameter but they can grow hundreds of micrometers long. Depending on how they are wound into a tube, CNTs can have a very low electrical resistance. But this electrical advantage is lost when you try to attach them to a metal like aluminium because the surface of aluminium always oxidises. Unlike aluminium, aluminium oxide is a brilliant electrical insulator. Which is great if you want to study effects in which the electrical current is blocked, but terrible if you want to utilise these fantastically conducting CNTs. This was the problem that the scientists at MIT wanted to solve. Their solution was to remove the oxide using salt water and then deposit the CNTs on top. (When phrased like this it sounds such a simple idea, “why did no one do this before”, but there are many experimental steps needed to be able to grow CNTs onto substrates such as aluminium and it has taken many years to get to this point). Once the CNTs were deposited, the authors found that not only did they have a good electrical conductor, the material was really black.

Coffee love. Some evidence of foam ripening at the surface of an oat milk latte.

What happened next is where the art comes in. Professor Brian Wardle who led the study was quoted as saying “Our group does not usually focus on optical properties of materials, but this work was going on at the same time as our art-science collaborations with Diemet. So art influenced science in this case.”

Thinking about how black the material was, the team decided to measure its optical absorption, which is when they discovered that they had broken the record previously held by Ventablack. And it was then that the art came back in. Strebe took a $2m natural yellow diamond and covered it with this ultra-black material. The result is striking (link). The composition, called “The Redemption of Vanity” could cause us to pause and ponder what we value as a society. What makes a sparkling diamond so valuable? How do we start to see objects by the fact that we can’t see them at all? If we extend this contemplation to our surroundings of Chelsea Green, we may wonder at this small little triangle of grass with its couple of benches. What does it reveal or hide? Does it help us to know that this is the last remnant of Chelsea Heath*, a bit of common land in which occupants of the surrounding manor houses down on (what is now) Cheyne Walk had the right to graze their livestock. Throughout this green, cows wandered up from the Thames as recently as the seventeenth century. A part of London that has disappeared, obscured by modern buildings yet held in memory by the street names and, the names of housing blocks.

As for why this material absorbs so much light, it is still an open question. It is known that the arrangement of the CNTs (including their alignment) can make a material coated in them very black. Even Ventablack is made from CNTs. It is a question that will probably continue to be discussed over many coffees. But is it a question that we would be asking again at all if it weren’t for the interaction in this case of art and science? Another point of contemplation that we can enjoy while looking into our coffee and just wondering ‘why’?

Amar Cafe is on Chelsea Green, 15 Cale Street, London, SW3 3QS and at several other telephone box locations.

*London Encyclopaedia, 3rd edition, Weinreb, Hibbert, Keay and Keay

Connectivity at Populus, Singapore

Inside Populus

The cool interior of Populus

A friend recommended that we try a café in her neighbourhood near Outram, in Singapore. So, with some time to spare we walked up the hill and into the welcoming air conditioning of The Populus Cafe in Neil Road. The coffee is roasted by 2º North and there is an extensive menu of both coffee based drinks and a seasonal filter selection. There was also a range of iced drinks on the menu which in the Singapore heat were tempting, but I opted for the 6oz Long Black. Given more time, or a second visit, I would certainly try the filter, but this time an appointment across the island was calling. There seemed too to be a very good lunch selection on the menu, but again, the lunch appointment elsewhere meant that it was just the 6oz long black that day.

It should be possible to take some time back from a busy schedule full of appointments and concerns and sit back and ponder the connections in any café. Having run from one set of concerns and soon to have to go off to another, would this be possible in 30ºC+ heat? The comfortable space of Populus provided a perfect place to test this question. Sitting back while sipping my coffee, the first thing that struck me was the wood, arranged in different geometrical patterns on the walls. The floors too were decorated with hexagonal stone tiles while the door was glass. There was a different pattern on the door, but what was it? Staring at it for a while, I thought about the schematics you sometimes see for a connected world, each of us a point connected to the others (how true is it that we are all 6 hand shakes away from everyone else?). Maybe this fitted with the name of the café? My companion in these reviews instead thought of crystals and the way that crystal structures are represented with lines between the atoms. While loading the photograph of the door onto the website, I saw a pattern of flowers. It seems that the pattern on the door formed and reformed with each new view.

door, Populus

Looking through the door of Populus. What patterns do you see?

Then there was the cup. A black coffee in a black cup, with umbral and penumbral shadows (as pointed out by @Bob_MatPhys on Twitter). A few years ago there was great excitement about a new material that had been made to be blacker than any known material. The substance used a coating of carbon nanowires (of just 20nm diameter which is about 1/1000 the size of a grain of espresso grind) to absorb light across the visible, ultra-violet and infrared spectrum. And just as nano-structuring a material helps it to appear the ‘blackest’ object ever, so changing the structure of a material can make it invisible to other parts of the electromagnetic spectrum such as microwaves. Quite why various defence companies and governments would invest so much into this research I will leave for your imagination (it is not to avoid heating soup). A more peaceful and beautiful side of the effect of nanostructure on optical properties is the way that the feathers of a peacock have a striking green-blue hue. It is another example of light interacting with a structure and so producing different optical effects; all is not as it appears.

coffee cup Populus

A black coffee in a black coffee cup. But what does black mean? And is something that is transparent always so?

And the fact that all is not as it appears gives another connection to the Populus cup. For although it seemed quite black to my eyes, it was clearly shining in the infra-red. The hot coffee inside was radiating through the cup and onto my hands. Which could prompt us to consider what ‘black’ really means? And for that matter, what about transparent? Just as ‘black’ only absorbs light over a certain set of frequencies, so transparent only lets light through over certain frequencies. The door that we can see through with our eyes may be opaque to a different frequency range that we cannot see. Just over two hundred years ago Carl Wilhelm Scheele deduced the presence of the infrared by contemplating how his stove heated him in the winter. Although he could not see them, the ‘heat rays’ seemed to come straight towards him and yet did not cause a candle flame to flicker, clearly the heat was ‘radiating’ like light rather than travelling like a breeze on the air.

The knowledge that structure, as well as pigment, provides the colour to our world, or that what is transparent at some frequencies may be opaque to others, these things give us plenty to think about, scientifically and perhaps more philosophically, while enjoying our black coffee. Which shows that even ten minutes spent sitting with your coffee can result in a series of thought connections that you may not have enjoyed had you rushed from appointment to appointment while checking your smartphone. That we could all enjoy a good ten minutes (or more) in the Populus Cafe!

The Populus Cafe is at 146 Neil Road, Singapore 088875