metamaterials

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

Focussing the sound at Spike and Earl

soya latte ginger beer

Soya latte and a ginger beer at Spike and Earl.

A few months ago, news came that the coffee roasting company Old Spike had opened a new café, Spike and Earl, down in Camberwell. Operating on similar principles to Old Spike, Spike and Earl aims to serve excellent coffee (and food and cocktails) with a social conscience. By employing those who have previously been homeless, Spike and Earl offers an employment (and training) route for people who may not easily otherwise have the opportunity. So although Camberwell is a bit of a trek, I was looking forward to trying this new place. As it was a late afternoon in November and the menu suggested that the dairy alternatives were only soya or oat, I decided to try a soya latte. (For any reader with a nut allergy, the current fashion of using almond milk means that you should always ask first if your cappuccino contains nuts). The baristas were friendly and confident in assuring me that they do not use almond milk (no danger of nut-cross contamination) but that their brownies did contain nuts (so I sadly had to pass on the brownie opportunity). My partner in these café reviews opted for a ginger beer.

There were a series of high tables with stools on the left hand side of the café. Presumably many people can therefore be accommodated when it gets crowded. However, at the time of our visit, it was fairly empty and we made our way to the rear of the café. Behind us, and behind closed glass doors, was a coffee roaster that we later discovered was part of the Old Spike roasting expansion. It’s always a nice touch to see coffee roasting happening as you drink but perhaps we needed to arrive earlier for that.

Bricks with holes Spike and Earl

Holes in bricks at Spike and Earl. Just a foot-hold or a suggestion for a great piece of engineering?

Drinks arrived together with complementary water and the soya latte was very smooth. Almost caramel like in the sweetness and very drinkable. It makes a pleasant change to have a latte once in a while. Light was playing tricks around the room as the sun was setting and the inside lights were becoming more prominent. But the striking thing about Spike and Earl was that the bricks used to support the tables and line the walls all had holes in them. On the wall running along the side of the café, (windows were on the other side), pot plants were placed in the holes giving the impression of the beginnings of a green wall. The holes in the bricks supporting the table meanwhile made an excellent footstool and were complemented by holes in the stools. A latte of course is largely made up of holes, or at least bubbles. The foam structure consisting mostly of air. How is it that some structures can be made better owing to what they don’t contain rather than what they do?

For example, if you imagine the difference between a latte and a cappuccino (but made out of metal rather than milk) that can be the difference between a successful tooth implant and a failure. We know from our coffees that bubble size can have a significant structural effect. But how about more fundamental properties, can the holes in bricks change things such as the way sound propagates?

Interior wall at Spike and Earl

More bricks with holes at Spike and Earl, this time with some plants escaping from them. The start of a green wall?

You may have heard about how different structures can be engineered to make materials “invisible” to certain frequencies of light. Imaginatively named “invisibility cloaks” are made by designing materials with patterns on them that change the path of an incident light beam. Because the effect on the light beam is due to the structure in the material rather than purely from the material itself, these materials have become known as ‘meta-materials’. When you remember that microwaves are a form of light, it is perhaps easy to see some of the applications of this research and one reason that it has attracted a lot of funding.

However there is an acoustic type of metamaterial that is far more similar to the bricks in Spike and Earl and that may find applications in medical imaging (ultrasound). Earlier in 2017, a team from the universities of Sussex and Bristol published a study about acoustic metamaterial ‘bricks’. Each brick had a differently shaped hole through the centre of it which delayed the incident sound wave by a specific phase interval (you can say it ‘slowed’ the wave). In order to work efficiently, the brick had to be of a height equal to the wavelength that the researchers were interested in and a width equal to half that wavelength. As they were investigating ultrasound, the bricks were therefore about 4.3mm square and 8.66 mm high.

By assembling the bricks together, the researchers found that they could steer a focussed beam of sound or even change the shape of the sound beam. This would have applications as diverse as targeting cancer cells with ultrasound to levitating a polystyrene bead. You can read more about their research here (or, if you have access to Nature Communications, their paper can be downloaded here).

soya latte Spike and Earl

Layering at the end of my soya latte. What would you think about?

Just for fun, assuming that the bricks supporting the table at Spike and Earl could be similarly turned into acoustic metamaterials, we could calculate the musical note that they would best work with. Estimating the brick at about 15cm square and remembering that is approximately  half the wavelength (λ/2) and using the speed of sound in air to be 330 m/s, we can calculate the frequency to be:

f = c/λ

f = 330/0.3 = 1100 Hz

Which is the musical note C#6 (with an explanation of nomenclature here).

As I finished my soya latte, strata of milk lined the cup. Reminiscent of the Earth’s layers or perhaps, metaphorically, our strata of understanding, there is certainly plenty more to ponder at this interesting new(ish) addition to the London café scene. So next time you are in Spike and Earl, do let me know what you end up thinking about, you never know where these thought trains may take you.

Spike and Earl is at 31 Peckham Road, SE1 8UB

 

Can you see me? At 123 Gasing, KL

Coffee at 123 Gasing

Latte, Long black and chocolate muffin at 123 Gasing, PJ, KL

There are times when you can sit and observe things for quite a while before noticing the physics that becomes a cafe-physics review. There are other occasions when the subject of the review is staring you in the face indeed, it is practically there written for you, on a noticeboard in black and white. Such was the case at 123 Gasing, a cosy and quirkily decorated cafe located, strangely enough at 123 Jalan Gasing (ie. Gasing Road), in PJ, Kuala Lumpur. We enjoyed a lovely breakfast of scrambled egg, long black and a latte (along with a very rich chocolate muffin). The coffee is from Degayo (according to Malaysian Flavours) which means that it is practically a local food product (originating as it does from neighbouring Indonesia). Coffee with minimal ‘food miles’. The only point of regret about our time at 123 Gasing was that we didn’t manage to spend longer there.

decoration at 123 Gasing

Birds on the wall at 123 Gasing.

It is the decoration that strikes you as you look around this cafe. A couple of painted birds sit on top of an electrical wire, prompting the question “why do birds not get electrocuted when they sit on a wire?”. Another question painted to a notice board on the wall asks “what is it that we need that we cannot see or feel?” (answer at the end of this post). Yet it was another thought on another noticeboard that prompted this cafe physics review. That thought suggested invisibility (see picture below).

The idea of invisibility has fascinated story tellers and philosophers for millennia. Trying to render objects invisible is, understandably, very desirable for the military and the defence industry. Although we have always had access to camouflage and deception, it is only relatively recently that it has become feasible to talk about invisibility cloaks as a real possibility.

A sign at 123 Gasing

Am I invisible?

What has moved “invisibility cloaks” into the realm of reality has been the advent of a field called “metamaterials”. As the name suggests, metamaterials are not materials that occur naturally but materials that we manufacture. Combinations of different materials or repeating patterns of a specific material that interact with light in a way that the material itself would not do. The classic example is a so-called split-ring resonator (SRR). These are rings (that were first made with copper) which have a slice cut out of them. Many such rings are arranged in a repeating, lattice pattern. Due to the engineered pattern of the copper, these lattices interact with light in a way that ordinary copper does not (for details click here). Specifically metamaterials can be engineered to bend light around objects so that it appears that the object is not there.

In order to work, the artificial structures (e.g. the copper rings) must be smaller than the wavelength of light that is to be ‘bent’. This means that microwaves (which have a wavelength ranging from a few cm to a few m) can be manipulated far more easily than visible light (with a maximum wavelength of 700 nm, or about 1/100th of the size of a grain of espresso grind). At first sight this may seem disappointing until we remember that even devices that only work with microwaves would have a clear application for the defence industry (radar).

already disturbed

Hopefully not a comment on current scientific funding

There are many ethical and philosophical questions that follow from the fact that it is now within our reach to render some objects invisible. It is not a scientific question as to whether we should do it, the scientific question is whether we can. Where science and ethics collide though is in the funding issue. A subject such as this with obvious applications receives far more funding than fields that advance our understanding but do not enhance our weaponry. Indeed, one of the researchers involved in this field describes how he was “offered large sums of money (almost on the spot)” when he spoke of the potentials of the “Harry Potter project”¹. Something that is alien to those of us who work in less fashionable subject areas where funding is a constant struggle. Government funded science quickly becomes dominated by a quest for application and technology. In effect we bypass the ethical questions of whether we should do this because it is this that will get funded. Science that is not driven by obvious applications will not get funded.

Is this what we want? Should the humanities and philosophy play a role in helping to determine what research is beneficial for society and so which research receives funding? Should ethical considerations play a part in funding considerations, or should scientific research all be about the devices that we can use? It is certainly something to ponder while sipping on our long blacks.

Answer to the question “what is it that we need that we cannot see or feel? Answer in 1990 – Air, answer in 2000 – Wi-fi (though personally I think maybe this should be the answer in 2015, the given answer of “2000” was still a bit early for widespread wifi).

Further reading and [1]: “The Physics of Invisibility” Martin Beech, New York, Springer, 2012