Brighton

A pebble puzzle at The Lanes Coffee House, Brighton

The Lanes Coffee House Brighton, Coffee Brighton

The Lanes Coffee House, Brighton

Hidden somewhere deep in “The Lanes” in Brighton is a great little café that goes by the (somewhat appropriate) name “The Lanes Coffee House“. The Lanes is a set of old, narrow streets that form quite a maze, a great place to explore if you are a visitor to Brighton. There are two entrances to The Lanes Coffee House, which is fortunate as we may not have found the ‘front’ entrance while wandering aimlessly around a few weeks ago. With plenty of seats inside, and a few at the back, The Lanes Coffee House has a lot to notice if you like to sit in a café, enjoying your coffee, without the distractions of your phone or laptop. Pictures by a local artist decorate the walls, with each picture featuring a Brighton scene and The Lanes Coffee House in a “Where’s Wally” type format. A window opens up to the narrow lane outside and there is also plenty to notice inside the café where we enjoyed an Americano and Soya hot chocolate. Sadly on the day that we tried The Lanes there weren’t any nut-free cakes at the counter and so, as compensation, we had to have a bag of fudge.

We sat at a table on which there was a tiny metal bucket that was used for holding sugar (see picture). With Brighton being a sea-side town, this immediately conjured up images of sandcastles and picnic sandwiches. The only problem with this image of course is the fact that Brighton has a pebble beach rather than a sandy one. So rather than think about sandcastles I got thinking about something slightly different: If you were to partially fill a bucket with some sand, put a stone from the beach in the bucket and then fill the rest of the bucket with sand, how could you easily retrieve that stone without fishing around in the sand for it?

missing a spade but it is a bucket

Bucket with sugar at The Lanes

This question came up, in a slightly different way, in a paper published nearly 30 years ago called “Why the Brazil nuts are on top“. That paper dealt with the question of why, in a package of mixed nuts, the Brazil nuts (i.e. the largest nuts) were frequently found to be on top when the package was opened. Termed, the “Brazil nut effect”, it turns out that this question is not just an odd bit of physics but has relevance to the packaging industry and earthquake dynamics. If you were to shake the bucket of sand containing that one pebble, the stone would rise up through the sand, eventually coming to the surface. The question is, what is driving this ‘anti-gravity’ type behaviour? Why do the heavy objects rise to the top?

Obviously, being a coffee website, to do the experiment with sand and a stone is a bit, well, dull. So, using dried, used, grounds (well, what do you do with your used coffee grounds?) and a green coffee bean*, I repeated the experiment on a smaller scale, in a shot glass. I put the green bean at the bottom of the shot glass and then covered it with coffee grounds (up to half way up the shot glass). To prevent too much mess, I covered the shot-glass with a piece of clingfilm. In order to avoid too much vertical shaking, the glass was hit repeatedly from the side in order to ‘shake’ it. It took 40 seconds of vigorous shaking but the green bean eventually started to poke above the surface. The heavy large bean had moved up through the glass upon being shaken, just as the Brazil nuts end up on top in a bag of mixed nuts. Just how had that happened?

Brazil nut effect

Now you see it, after shaking the green bean rises to the surface

It turns out that there are several things going on. Firstly, as the 1987 paper discussed, the small grains of sand (or ground coffee) can fall into voids under the large Brazil nut/stone/coffee bean. This has the effect of pushing the nut/stone/coffee bean upwards until it reaches the surface. The coffee bean could move down through the grounds but this would require the collective movement of a large number of coffee grounds under the bean to form a void large enough for the bean to fall into, something that is fairly unlikely. So the coffee bean will move up, the coffee grounds will move down. Then there is an effect that is more familiar to coffee drinkers, convection. The idea here is that although all the particles in the glass (or bucket) can move upwards as the container is shaken, only the small particles can move down again through the small voids created at the sides of the container. Rather like the movement of milk in a hot coffee, this again has the effect that the small particles push the larger particle up.

After this though things start to get more complicated. Along with what has been termed “the inverse Brazil nut effect”, it seems that air pressure may play a role in the Brazil nut effect. Who knew that Brazil nuts could be so complicated! Perhaps though, to finish our train of thought on The Lanes we could turn this effect into a non-physics, almost allegorical, meditation? If we, as a society, are collectively shaken enough, can small, individual actions do what at first seems impossible? I wouldn’t want to push the analogy too far, but it does seem an interesting problem on which to dwell for a further five minutes while you are sipping on your long black (or, as this is officially the first day of summer, your cold brew).

(If you’d rather think about the physics of sandcastles, you can find out more about them here).

*The green coffee bean came from a jute coffee sack that had been given to me by Amoret coffee for my composting worms. Thanks Amoret!

The Lanes Coffee House is at 59D Ship Street Brighton, BN1 1AE

 

Now you see it now you don’t at Bond St Coffee, Brighton

Outside Bond St Coffee Brighton

Bond St on Bond St, Brighton

A couple of weeks back, I tried the lovely Bond St. Coffee in Brighton on the recommendation of @paullovestea from Twitter. It was a Saturday with good weather and it turns out that this particular café is (understandably) very popular and so, sadly, to begin with we could only sit outside. That said, it was a lovely spring day (sunny but a bit chilly) and so it was quite pleasant to watch the world go by (or at least Bond St) while savouring a well made pour-over coffee. All around the café, the street decoration hinted at times past. Across the road what was obviously a pub in times gone by has turned into an oddities store. Air vents to a space underneath the window seating area in Bond Street café itself suggested an old storage space. A seat in the window appeared to have been re-cycled from an old bus seat.

But it was the pour-overs at Bond St. Coffee that had been particularly recommended and they certainly lived up to expectations. I had a Kenyan coffee roasted by the Horsham Roasters. The V60 arrived at our bench seat/table in a metal jug together with a drinking glass. The angle of the Sun caught the oils on the surface of the coffee, reminding me of Agnes Pockels and her pioneering experiments on surface tension. Pouring the coffee into the glass I thought about the different thermal conductivities of glass as compared to metal and how I had put both down on the wooden bench. How was heat being transferred through these three materials? And then, as I placed the metal jug back on the bench I noticed the reflections from the side of the jug and thought, just why is it that you can see through the colourless glass but the metal is grey and opaque?

Metal jug and transparent glass

Metal jug, glass cup. V60 presentation at Bond St Coffee

On one level, this question has a simple answer. Light is an electromagnetic wave and a material is opaque if something in the material absorbs or scatters the incoming light. In a metal, the electrons (that carry the electric currents associated with the metal’s high electrical conductivity) can absorb the light and re-emit it leading to highly reflective surfaces. In glass there are no “free” electrons and few absorbing centres ready to absorb the light and so it is transmitted through the glass.

Only this is not a complete answer. For a start we haven’t said what we mean by ‘glass’. The glass in the photo is indeed transparent but some glasses can be more opaque. More fundamentally though, there is a problem with the word ‘opaque’. For us humans, ‘visible’ light is limited to light having wavelengths from about 400nm (blue) to about 780nm (red). ‘Light’ though can have wavelengths well below 400 nm (deep into the UV and through the X-ray) and well above 780 nm (through infra-red and to microwaves and beyond). We can see the spread of wavelengths of light visible to us each time we see a rainbow or sun dog. Other animals see different ranges of ‘visible’ light, for example, bumble-bees can see into the ultra-violet. So, our statement that glass is transparent while metal is opaque is partly a consequence of the fact that we ‘see’ in the part of the spectrum of light for which this is true.

Sun-dog, Sun dog

Sun dogs reveal the spectrum of visible light through refraction of the light through ice crystals.

For example if, like the bumble-bee, we could see in the UV, some glass may appear quite different from the way it does to us now. Even though the glass in the photo lacks the free electrons that are in the metallic jug, there are electrons in the atoms that make up the glass that can absorb the incident light if that light has the right energy. There are also different types of bonds between the atoms in the glass that can also absorb light at particular energies. The energy of light is related to its frequency (effectively its colour*). Consequently, if the energy (frequency/ wavelength) of the light happens to be at the absorption energy of an atom or an electron in the glass, the glass will absorb the light and it will start to appear more opaque to light of that colour. Many silicate glasses absorb light in the UV and infra-red regions of the electromagnetic spectrum while remaining highly transparent in the visible region. High purity silica glass starts to absorb light in the UV at wavelengths less than approx 160nm†. Ordinary window glass starts to absorb light in the nearer UV†. In fact, window glass can start to absorb light below wavelengths of up to ~ 300 nm, the edge of what is visible to a bumble bee: The world must appear very different to the bumble bee. At the other end of the scale, chalcogenide based glasses absorb light in (our) visible range but are transparent in the infra-red.

Looking at how materials absorb light, that is, the ‘absorption spectrum’, enables us to investigate what is in a material. It is in many ways similar to a ‘fingerprint’ for the material. From drugs discovery to archaeology, environmental analysis to quality control, measuring how a material absorbs light (over a wider range of frequencies than we can see) can tell us a great deal about what is in that material.

Perhaps you could conclude that whether something is opaque or crystal clear depends partly on how you look at it.

 

Bond St Cafe is on Bond St, Brighton, BN1 1RD

*I could add a pedantic note here about how the colour that we see is not necessarily directly related to the frequency of the light. However, it would be fair to say that a given frequency of light has a given ‘colour’ so blue light has a certain frequency, red light a different frequency. Whether something that appears red does so because it is reflecting light at the frequency of red light is a different question.

†”Optical properties of Glass”, I Fanderlik, was published by Elsevier in 1983.