Month: February 2016

Categories
Coffee review General Observations

Setting standards at Brill, Exmouth Market

Brill, Exmouth Market, neon, architectural history
The neon lit “Brill” from the back of the cafe. You can also see evidence of an old arch in the brickwork, an old doorway?

Brill on Exmouth Market has quite a history. Originally a record store, it has evolved into a music shop/cafe more recently. On my recent visit, I ordered a very good Americano (beans from Officina Coffee Roasters) and although cakes were on sale, it was a small bar of Green & Blacks chocolate that appealed to me a bit more that day. It is a small cafe and so the few seats that are upstairs were occupied. This turned out to be a good thing though because I noticed a sign indicating that there were more seats downstairs, which actually meant that there was seating in a lovely little courtyard/garden at the back of Brill. Although it was originally locked (it was February and fairly dismal when I visited, who in their right mind would want to sit in the garden?), the friendly staff unlocked it and quickly cleaned one of the tables so that I could enjoy my coffee and chocolate in peace in central London. Indeed, the occasional (inevitable?) sound of sirens in the distance only served to emphasise the tranquility of the courtyard. The courtyard has four tables and a glitter-ball in the corner hanging from a tree. There was a lot to appreciate outside, both in terms of the science and the history of the place: Leaves deposited by vortices in corners of the yard with brickwork that suggested a significant re-build has occurred to this cafe.

But from my vantage point, it was the word ‘Brill’, lit up in neon lighting inside the cafe, that caught my attention. Neon lights are always interesting to me because their colour is so suggestive of the atoms that make up the light. The colour of a neon light is determined by the energy levels of the atoms that make up the light, the gas ‘neon’ shines red, hence neon lights. But if you wanted blue ‘neon’ lights you could use mercury as the vapour in the tube instead of neon, it is all about the energy levels of the atoms in the gas in the tubes.

glitter ball, disco at Brill Exmouth Market
A glitter ball in the corner of the courtyard at Brill

Under certain conditions, cadmium also emits a red light which brings us to the subject of this cafe-physics review: The definition of length. How is it that we can all agree on what ‘one metre’ is, or even one ‘inch’? Perhaps you are wondering how the red light emitted by cadmium, (or neon), relates to the definition of the metre? It’s about standards and definitions. Up until about 1960, the standard unit of length (the metre) was measured with reference to an actual, physical, metal rod kept in Paris with two scratches carved into it, one metre apart. Any arguments about the precise length of a metre could be settled by referring to the metre, this metal bar in Paris. But of course there were problems, the first of which was that the metre was in Paris. Perhaps you would think it easy to make copies? Yet in the nineteenth century this was already becoming a problem, the measurements that were being made were becoming too precise. Anders Ångstrom’s pioneering work with spectroscopy (investigation of elements by the colours that they emit/absorb) revealed a small difference between the metre kept in Uppsala (where Ångstrom was based) and that kept in Paris. Although the difference was tiny, when it was compared with what people had started to measure, it became significant. Then there was the question of the scratches: Would you measure the metre between the furthest two points of the scratch? Or the closest? Then an even worse problem was discovered: The rod was shrinking! If you’re tempted to abandon metric units and hark back to Imperial units, bear in mind that the UK Imperial Yard was shrinking even faster. No, something had to be done and that something involved changing the definition of the metre fundamentally.

neon sign, light emission
Neon signs have characteristic colours due to the electron transitions in the ionised gases

It is here that cadmium comes in to the story. Rather than use a physical length that we could all measure, the people whose job it is to define our base units decided that the definition of the metre would be with reference to the wavelength of the red light of Cadmium. I do not know why they did not want to use the red of neon lights but even with cadmium it quickly became apparent that there was a problem. The problem was that cadmium exists as several isotopes, all having a very slightly different ‘colour’ of red light that they emit. So, rather than cadmium, in 1960 they settled on the orange line of Krypton as the definition of the metre. One metre was then defined as 1650763.73 vacuum wavelengths of Krypton. That was the definition for over twenty years before the definition of the metre was updated again in 1983. It is now defined as “the length travelled by light in a vacuum during a time interval of 1/299792458 of a second”.

Perhaps it is not a definition that you or I could use, we’d probably still refer to our metre rule! Nonetheless this definition does allow people to perform experiments that need very precise and very accurate measurements of lengths. These standards are important for extremely sensitive measurements such as that needed to detect gravitational waves with the LIGO experiment, reported a few weeks ago. The neon lights at ‘Brill’ do indeed suggest a story that goes way back in time, both for the cafe and for the science.

Brill is at 27 Exmouth Market, EC1R 4QL

Spectroscopy information from “Spectrophysics”, by AP Thorne, Chapman and Hall Ltd, 1974

Categories
General Observations slow Tea

Tea Gazing

Milky Way, stars, astrophotography
The Milky Way as viewed from Nebraska. Image © Howard Edin (http://www.howardedin.com)

A recent opinion piece about last week’s announcement of the detection of gravitational waves at LIGO drew my attention to a quote from Einstein:

“The most beautiful experience we can have is the mysterious. It is the fundamental emotion that stands at the cradle of true art and true science. Whoever does not know it and can no longer wonder, no longer marvel, is as good as dead, and his eyes are dimmed.“

Einstein was not the only scientist to have expressed such sentiments. Many scientists have considered a sense of wonder to be integral to their practice of science. For many this has involved gazing at the heavens on a clear night and contemplating the vastness, and the beauty, of the universe. Contemplating the twinkling stars suggests the universe outside our Solar System. Watching as the stars twinkle gives us clues as to our own planet’s atmosphere. Of course, it is not just scientists who have expressed such thoughts. Immanuel Kant wrote:

“Two things fill the mind with ever-increasing wonder and awe, the more often and the more intensely the mind of thought is drawn to them: the starry heavens above me and the moral law within me.“*

light patterns on the bottom of a tea cup
Dancing threads of light at the bottom of the tea cup.

The other evening I prepared a lovely, delicate, loose leaf jasmine tea in a teapot. I then, perhaps carelessly, perhaps fortuitously, poured the hot tea into a cold tea cup. Immediately threads of light danced across the bottom of the cup. The kitchen lights above the tea cup were refracted through hot and not-quite-so-hot regions of the tea before being reflected from the bottom of the cup. The refractive index of water changes as a function of the water’s temperature and so the light gets bent by varying amounts depending on the temperature of the tea that it travels through. Effectively the hotter and cooler regions of the tea act as a collection of many different lenses to the light travelling through the tea. These lenses produce the dancing threads of light at the bottom of the cup. The contact between the hot tea and the cold cup amplified the convection currents in the tea cup and so made these threads of light particularly visible, and particularly active, that evening. It is a very similar effect that causes the twinkling of the stars. Rather than hot tea, the light from the distant stars is refracted by the turbulent atmosphere, travelling through moving pockets of relatively warm air and relative cool air. The star light dances just a little, with the turbulence of the atmosphere, this way and that on its way to our eyes.

Marcus Aurelius wrote:

“Dwell on the beauty of life. Watch the stars, and see yourself running with them.“†

Marcus Aurelius of course didn’t have tea. Watch the dancing lights in the tea cup and see yourself sitting with it, resting a while and then watching while dwelling on the beauty in your cup.

*Immanuel Kant, Critique of Practical Reason

†Marcus Aurelius, Meditations

Categories
Coffee review General Observations Science history Sustainability/environmental

Keeping it local at Lumberjack, Camberwell

Lumberjack coffee Camberwell
Lumberjack Camberwell with the (not quite) inukshuk in the window

I came across Lumberjack last week while spending an afternoon in Camberwell looking for interesting cafes to “cafe-physics” review. I was actually on my way to a cafe further along the road when a couple of wooden structures in the window attracted my attention. Thinking that they were “Inukshuk” we decided to go in and try this new cafe. It turned out to be a good choice because, even though the structures were not in fact inuksuik, they had brought us into this lovely little cafe. We arrived shortly before closing but I still had time to enjoy a very good long black (with beans from Old Spike Roastery). Complementary water was brought over to the table. It would have been great if we had arrived just that bit earlier so that we could have had more time to properly appreciate this friendly cafe. The interior is bright and smartly decorated with wooden tables and shelving as well as plenty of seats at the back. The wooden furniture is explained by the fact that the cafe is the trading arm for London Reclaimed, a charity that provides employment and carpentry training to 16-25 year olds from SE London while making bespoke furniture from reclaimed timber. The cafe too aims to provide training and support to encourage 16-25 year olds into work and a future career. In terms of the ‘physics’ bit of this review, the interior of the cafe certainly has plenty to observe, from the pendulum like light fittings to the detail of the wood. But as this cafe is metaphorically, and in some ways literally, built on/with wood and as Lumberjack boasts on its website that “almost everything you’ll find in store, from the coffee to the furniture, are sourced as locally and homemade as possible” it is only appropriate that this cafe-physics review should focus on wood, trees and a tree very specific and local to London; the London Plane tree.

Long Black coffee in a red cup
A Long Black at Lumberjack with the grain of the wood showing underneath

With their characteristic mottled bark, London Plane trees are a recognisable sight along many a London street. The bark absorbs pollutants from the street before bits of bark fall off, taking the pollution with them and leaving the tree with its mottled appearance. Their root structure and resistance to pruning or pollarding helps to ensure that (mostly) they can survive happily in the crowded confines of London pavements. They are indeed very much a tree that seems almost specially adapted to London. Yet the connection between the London Plane and London goes deeper than that. The first ever record of a London Plane tree was in the seventeenth century, just up the road from Lumberjack, in the Vauxhall Gardens of John Tradescant the Younger. The London Plane is in fact a hybrid tree, thought to be a cross between the American sycamore (first recorded in London in 1548) and the Oriental Plane (first recorded in London in the C17th). Both these trees were found in Tradescant’s gardens and it is possible that the hybrid tree, the now ubiquitous London Plane, was actually first grown in Vauxhall.

Even though London is full of Plane Trees, it is not very common to find plane wood furniture. Rather than the grain visible in the tables at Lumberjack, Plane wood shows a “lacy” structure that gives furniture made with plane a distinctive pattern. Although unsuitable for outdoor furniture, plane-wood can be used to make indoor furniture and indeed some London based cabinet makers have even documented obtaining usable timber from recently felled London Planes.

Tomb of the Tradescants
The Tradescant Tomb at St Mary’s, Lambeth

And it is this that takes us to the physics part of the cafe-physics review. Perhaps it is the areas (and the parks) that I walk through, but it seems to me that there has been a fair amount of tree felling in London over the last six months or so. Part of the reason for this must be to ensure that the trees in our parks and that line our streets are safe and not going to fall down in high winds. Many trees that fall down in high winds do so because they get uprooted. However it is also possible, in very high winds for the whole tree to snap. Indeed, when researchers mapped the wind speeds through a forested area of Southern France during a storm in January 2009 they found that when the wind speed exceeded ~40 m/s (90 miles per hour), more than 50% of the trees broke in the wind, irrespective of whether these trees were softwood (pine) or hardwood (oak). A very recent paper by a Paris-based group (published last week in the journal Physical Review E) confirmed that irrespective of the species of tree or the tree height, the trunks of trees were liable to snap at a critical wind speed. The team combined experiment and theory to establish that the critical wind speed scaled with the tree’s diameter and height. However, because trees generally treble their diameter as they double their height, the effect of the diameter change was (almost) cancelled by the height difference between trees. Surprisingly, this critical wind speed did not depend on the elasticity of the tree, so there is no difference between a softwood such as pine and a hardwood like oak or plane. The researchers calculated the critical wind speed needed to break a tree to be 56 m/s, very close to the 40 m/s observed in that January storm.

Lumberjack can be found at 70 Camberwell Church St, SE5 8QZ

If you have a cafe that you think needs a cafe physics review, please let me know. Comments always welcome, please click the box below.

 

 

 

Categories
Coffee cup science General Home experiments Observations slow

Coffee ring bacteria

coffee ring, ink jet printing, organic electronics
Why does it form a ring?

We have all seen them: Dried patches of coffee where you have spilled some of your precious brew. The edge of the dried drop is characteristically darker than the middle. It is as if the coffee in the drop has migrated to the edge and deposited into a ‘ring’. It turns out though that these coffee rings are not just an indication that you really ought to be cleaning up a bit more often. Coffee rings have huge consequences for the world we live in, particularly for consumer electronics. Various medical and diagnostic tests too need to account for coffee ring effects in order to be accurate. Indeed, coffee rings turn up everywhere and not just in coffee. Moreover, the physics behind coffee rings provides a surprising connection between coffee and the mathematics of bacteria growth. To find out why, we need to quickly recap how coffee rings form the way they do.

When you spill some coffee on a table it forms into droplets. Small bits of dust or dirt or even microscopic cracks on the table surface then hold the drop in the position. We’d say that the drop is pinned in position.

artemisdraws, evaporating droplet
As the water molecules leave the droplet, they are more likely to escape if they are at the edge than if they are at the top. Illustration by artemisdraws.com

As the drop dries, the water evaporates from the droplet. The shape of the drop means that the water evaporates faster from the edges of the drop than from the top (for the reasons for this click here). But the drop is stuck (pinned) in position and so cannot shrink but instead has to get flatter as it dries. As the drop gets squashed, water flows from the centre of the drop to the edges. The water flow takes the coffee particles with it and so carries them to the edge of the drop where they deposit and form into a ring; the coffee ring. You can see more of how coffee rings form in the sequence of cartoons below and also here.

However in this quick explanation, we implicitly assumed that the coffee particles are more or less spherical, which turns out to be a good assumption for coffee. The link with the bacteria comes with a slightly different type of ‘coffee’ ring. What would happen if we replaced the spherical drops of coffee particles with elliptical or egg shaped particles? Would this make any difference to the shape of the coffee rings?

Artemisdraws
As water evaporates from A, the drop gets flatter. Consequently, the coffee flows from A to B forming a ring. Illustration by artemisdraws.com

In fact the difference is crucial. If the “coffee” particles were not spherical but were more elliptical, the coffee ring does not form. Instead, the elliptical particles produce a fairly uniform stain (you can see a video of drying drops here, yes someone really did video it). The reason this happens is in part due to a pretty cool trick of surface tension. Have you ever noticed how something floating on your coffee deforms the water surface around it? The elliptical particles do the same thing to the droplet as they flow towards the edge. (Indeed, the effect is related to what is known as the Cheerios effect). This deformation means that, rather than form a ring, the elliptical particles get stuck before reaching the edge and so produce a far more uniform ‘coffee’ stain when the water dries.

E Coli on a petri dish
A growing E. Coli culture. Image courtesy of @laurencebu

By videoing many drying droplets (containing either spherical or elliptical particles), a team in the US found that they could describe drying drops containing elliptical particles with a mathematical equation called the Kardar-Parisi-Zhang equation (or KPZ for short). The KPZ equation is used to describe growth process such as how a cigarette paper burns or a liquid crystal grows. It also describes the growth of bacterial colonies. Varying the shape of the elliptical particles in the drying drop allows scientists to test the KPZ equation in a controllable way. Until the team in the US started to ask questions about how the coffee ring formed, it was very difficult to test the KPZ equation by varying parameters in it controllably. Changing the shape of the particles in a drying drop gives us a guide to understanding the mathematics that helps to describe how bacterial colonies grow. And that is a connection between coffee and bacteria that I do not mind.

As ever, please leave any comments in the comments section below. If you have an idea for a connection between coffee and an area of science that you think should be included on the Daily Grind, or if you have a cafe that you think deserves a cafe-physics review, please let me know here.