Month: September 2018

Categories
Allergy friendly cafe with good nut knowledge Coffee review General Observations Science history slow

Feeling the Earth move at Pritchard and Ure, Camden

Egg no pales, coffee, garden centre
Fried egg on cactus leaves. Cactus festival at Pritchard and Ure, Camden

Good coffee in a garden centre, in (nearly) central London, with some physics thrown in? Today’s cafe-physics review seems unlikely on several levels. And then it becomes even more unlikely as you realise that this garden centre and café are also a social enterprise where people “disadvantaged in the labour market” are helped back to employment through working here. All in all, Pritchard and Ure represent a great café to have come by.

Pritchard and Ure can be found in the gallery space of a warehouse type shop that houses the Camden Garden Centre. They serve Workshop coffee together with an extensive selection of alternative drinks and food. As it was lunchtime we enjoyed a spot to eat which gave me an opportunity to try cactus (it was cactus festival at the garden centre). Cactus leaves with re-fried beans and a cactus-water mocktail which came together with a reusable metal straw. The straws were being sold (together with brush straw cleaners) at the counter. After lunch there was a very well made long black (interestingly I was given the choice to have it either as a 6oz or an 8oz, ie. more or less water depending on whether I wanted more or less coffee taste) and resisted (somehow) one of the tempting cakes before having a wander in the garden centre.

equations art work coffee Camden
But are they real?
The equations are the writing on the wall at this cafe.

There are of course many things that you can notice and connect with/to in a garden centre. Plants, biosphere, windows and greenhouse effect, the carbon cycle, the nature of colour, the list could go on. In addition to all of these, to the left of the counter was an art piece on the wall with a list of various equations and comments. Were all these equations real? One thing in particular though in this café/garden centre was particularly mesmeric: the disco ball suspended as a pendulum from a beam across the ceiling. Initially we watched as the ball just glinted reflected light as it slowly swayed to and fro in its oscillation. It took 22 seconds to cover 5 oscillations while I estimated it was 7m in length. Knowing that there is a formula for calculating the period of oscillation I wondered, was my estimation any good?*

During the hour it took us to enjoy lunch, the position of the Sun moved in the sky so that the disco ball started to reflect an array of polka dots of light onto the walls surrounding us (you can see these in the photo). Owing to the combined rotation and oscillation of the ball it wasn’t too easy to measure the time period from these oscillations but about 4 seconds per swing (as I had obtained by merely watching the ball) seemed comfortingly correct. The sun slowly moved round and these dots danced until at some point the sun had moved far enough that the glitter ball was no longer in direct light. But had the Sun moved or the Earth rotated underneath it? We all know the answer (or at least we think we do), but we could use the pendulum to prove it (and to calculate our latitude).

discoball cafe
Disco ball pendulum together with polka dot reflected sunlight. The view from the gallery at Pritchard and Ure.

In various science museums around the world, different Foucault pendulums swing to and fro all day above circular patterns on the floor. The pendulums appear to rotate clockwise in the northern hemisphere and anti-clockwise in the southern hemisphere thereby illustrating the rotating earth underneath the pendulum. The idea is of course that the pendulum continues to swing in the same plane as it was when it was started off but as it is swinging the earth is rotating underneath it giving an apparent rotation of the pendulum swing over the course of a day. If we were at the north (or south) pole, the period of one complete rotation of the pendulum through a circle on the floor would take 24h. As most of us are not at the pole (and Pritchard and Ure certainly is not), the period of complete rotation is lengthened by a corrective factor proportional to the sine of the latitude. Consequently, it is perfectly feasible for us to calculate our latitude by observing a pendulum swinging for long enough in the absence of any breeze.

It is a great piece of evidence for the rotation of the earth (and by implication the fact that the earth is not flat and that the sun is not going round the earth each day). It’s also a very simple (hiding some complicated maths) demonstration that anyone could set up if they wished to carefully do so. So next time you see a disco ball suspended as a pendulum in a café, you would have another reason to start singing “I feel the Earth, move, under my feet…”

Pritchard and Ure is in the gallery of Camden Garden Centre at 2 Barker Drive, St Pancras Way, NW1 0JW

*7 m is an over estimate of the length of the pendulum based on the period of the oscillation. A length of 7m would give a time period of 5.3 seconds, whereas 22 seconds for 5 oscillations is about 4.4 seconds for one giving a calculated length of just under 5m. More details about how to calculate this are here.

Categories
General Home experiments Observations Uncategorized

Exploring the sound of coffee

coffee at Watch House
We’re used to thinking about the aroma of coffee and how it looks, tastes, even how it feels, but what about how it sounds?

How much attention do you pay to your brewing coffee? You know the aroma, how the coffee blooms, you anticipate the taste and feel the warmth of the steam rising off the brew. But what about the sound? Admittedly this depends on your brew method, but what about the sounds as you filled the kettle or prepared a pour over brew? It turns out that the sound of dripping water was the subject of a recent paper in Scientific Reports.

Perhaps take time to watch a tap dripping into a bowl of water. Or maybe use this as an excuse to make another coffee by drip brewing. Each drop falling onto the water (or coffee) below first deforms the water’s surface then, as far as we can see, rebounds up with a splash of a returning drop or droplets. The phenomenon of what causes the characteristic sound of the drip has been investigated for over 100 years but in 1959 it was established using high speed photography that there were four key phases to any drip sound. First, the drop fell on the liquid, then a cavity formed just under the water surface and an air bubble formed just under that. Finally the water surface recoiled leading to a jet of droplets returning from the surface. It has been thought that the sound, that ‘plink’ of the dripping tap, was caused by that trapped air bubble expanding and contracting as it moved through the water under the water’s surface¹. But this has now been confirmed, along with some other interesting, coffee related, observations using ultrafast video recording (30 000 fps for most of the work, 75 000 fps for some of the extra details).

lilies on water, rain on a pond, droplets
Like the sound of falling rain? What causes the dripping sound of a tap?

The authors of this recent paper describe what must have been a fun experiment to do, dripping water into a tank below. You can see some of the videos of the droplet entering the water by scrolling down to the “supplementary information” in the paper. Two microphones (one above, one below the water surface) recorded the sound waves coming from the dripping ‘tap’ simultaneously with the video recording so as to match the timing of the sound with what was happening in the video. The microphone above the water surface largely recorded the same sound waveform as the microphone under the water with one crucial exception. When the authors lined the tank with MDF wood, the underwater sound was ‘damped’ quite quickly, in comparison the bare tank amplified the sound and so the sound wave took much longer to decay. Above the surface however, it didn’t matter whether the tank was lined or not, the sound signal remained the same. This may sound somewhat insignificant, but it means that it cannot be the sound created by the wobbly bubble itself merely propagating through the surface of the water. If this were the case, the microphone above the water surface should show the same signal as the microphone under the water’s surface. Instead the authors suggest that the oscillating bubble causes the surface of the water immediately above it to vibrate (in the bit that is depressed owing to the droplet having fallen into it) and it is this that we hear above the water surface.

science in a V60
Droplets on the surface of a brewing V60 may also form owing to a temperature difference between the dripping drops of coffee and the coffee ‘bath’ underneath.

It is a beautiful set of experiments but how can it link to coffee (apart from with the dripping)? It is in the way that it gives us the chance to experience our coffee with experiments involving more of our senses than just smell, touch and taste. Firstly, the study emphasises the connection between the drop’s diameter and speed to the sound of the drip (the best sounds are for drops between 1mm and 5mm diameter). This suggests that by changing the brewing parameters (whether you prepare your V60 in a jug or a mug or change the filter paper to a metal kone for example), you may hear a change in the sound of the drips. Do you? Secondly, it has been suggested that the sound that is formed is dependent on the temperature difference between the dripping drop and the water bath underneath. A temperature difference between drop and bath would also explain an odd phenomenon I noticed in the V60 a while back. Do you notice a difference in the sound of the brewing coffee when you prepare cold brew pour over as opposed to a standard breakfast brew? Lastly, the authors of this study found that they could suppress the sound of the plink by reducing the surface tension of the water bath that they were dripping water into. In their case they added washing up detergent to the bath. This seems an awful waste of coffee but is it possible that something intrinsic to our coffee brew could do the same thing? Oil will also change the effective surface tension of the water and different coffees (and different roast strengths) change the oil content of the brewed coffee. Have you noticed any change in the sound of the drips of the coffee depending on how dark a roast coffee you use?

It may not make ground-breaking science but it does offer us an opportunity to pay even more attention to our coffee. Does the sound of your coffee reveal the beauty of the physics at work just under its surface?

¹ Some history of the investigation into the dripping sound as well as the experiments can be found in: Phillips et al., “The sound produced by a dripping tap is driven by resonant oscillation of an entrapped air bubble”, Scientific Reports, 8, 9515 (2018)

Categories
Coffee review Observations Science history

Metrology and the Press Room, Twickenham

Press Room coffee Twickenham
The arrival of the pour over at the Press Room, Twickenham.

It is not often that I have an errand to run in Twickenham, but when one popped up just two weeks after reading Brian’s Coffee Spot review of The Press Room, it was obvious where we were going to have a coffee. The Press Room serves pour over coffees (along with a good selection of other drinks). It is always great to find somewhere that serves pour overs well and so I had no hesitation in ordering a Nicaraguan “Los Altos” prepared by V60. Hot chocolate was available as white, milk or dark chocolate and there were a number of alternative non-dairy milks on offer as well as a large variety of tea. A lovely feature of The Press Room is that they offer suspended coffees, the idea being that you buy a coffee now for someone later who may not otherwise be able to afford one. The total number of coffees (given/claimed) is recorded on a blackboard behind the counter. It was nice to see that at the time of our visit 800+ coffees had been paid forward (and just less than 800 claimed), suggesting that the Press Room is having a positive effect on its local community.

clock wall Twickenham coffee
The large clock on the wall at The Press Room in Twickenham.

A great thing about ordering a pour over is watching as the barista expertly prepares your coffee, taking the time to do this properly. To be fair, this is part of the reason that finding a café serving pour-overs is becoming more difficult. After a while, the coffee was brought over to our table together with a bowl ready for me to place the filter cone on it when I was ready to enjoy the coffee. After taking the obligatory photograph, and pondering when would be the best time to remove the filter from the top of the mug and place it onto the empty bowl, the clock next to us took our attention. It is a large time piece that dominates this corner of the room. It is revealing to consider how the accuracy and availability of clocks have changed the way we live as a society.

Considering measurement (of time and other things), I used to be in this area more frequently a few years ago when I worked on a project in collaboration with the National Physical Laboratory (which is down the road, on the same bus route that Brian’s Coffee Spot notes takes you to a few good cafés). Partly, NPL’s work is to ensure that we know how to measure things properly. Take the pour over I enjoyed at The Press Room. A known amount (perhaps 12 g) of coffee was weighed out before 200 g of water was poured slowly over the coffee. But how do you know that the 12 g measured at Press Coffee is the same 12 g as you measure at home? And while perhaps it may not be critical for the coffee culture (even the most extreme home-brewer does not need to know the amount of coffee they are using to the nearest 0.000 002%), knowing accurately how heavy something is can be extremely important. Hence the need for a standard kilogram (and a standard metre, second, Candela etc) so that we have a way of knowing that what you call a kg is the same as what I call a kg.

coffee bowl pour over
The coffee that escaped! But was it a measure of my patience or hesitation?

Oddly, the kilogram is the last fundamental unit still defined with reference to a physical object (the other fundamental units are seconds, metres, Kelvin, Amperes, Moles, Candelas). The kilogram reference block is a PtIr alloy kept at the International Bureau of Weights and Measures near Paris. However all this may change next year depending on a decision due in November 2018. If all goes to plan, from May 2019 all units will be defined with respect to natural constants such as the speed of light etc. For the kilogram, this has meant measuring mass relative to a magnetic force generated by a coil of wire in a device known as a Kibble balance. In this way, the kg can be defined with respect to Planck’s constant and an era in which we measured substances relative to known objects will end.

On a day to day level though, how much do these things matter to us? Sometimes the way we measure things affects how we view them (and therefore what questions we ask next). Take for example temperature. We are used to measuring degrees of ‘hot’, so on the centigrade scale 0ºC is the freezing point of water and 100ºC is the boiling point. But it wasn’t always this way. Celsius devised his original scale to measure degrees of cold so 0º was the boiling point of water and 100º was the freezing point (you can read more about that story here). It is arguable that changing to measuring degrees of ‘hot’ enabled us to more easily conceptualise the idea of heat as energy and the field of thermodynamics. Certainly for a while, considering the idea of ‘degrees of cold’ meant that some looked for a substance of ‘cold’ called “frigorific“¹. There’s a similarity here with the coffee at The Press Room, was the amount of coffee in the bowl used to hold the filter after I removed it from the mug a measure of my impatience before trying the coffee or my hesitation at testing the coffee? How we ask that question affects how we view the coffee and the café (for reference, I would take the positive interpretation: the amount of coffee in the bowl measures my impatience; I was eager to try the coffee).

droplets on the side of a mug
Condensation on the side of the mug. These droplets can reveal many aspects of physics, which do you think about?

Partly this suggests some of the ways in which language, and philosophy, underpin all science. It certainly suggests one further connection with this bright and comfortable café. Erich Fromm in “To have or to be”² considered an interesting linguistic usage that reveals our way of being. Do we “have an idea”, or do we “think”? Are we consumers or people with experiences? Do we wish to have, to acquire, to consume or do we wish to exist, to be. Our language affects how we perceive the world which in turn changes the language we use about it. Linguistically, depending on how we interpret the cafe’s name “The Press Room”, we either have a café that offers a space to read the latest news or one that is reflective of the coffee brewing process (specifically espresso); a space to get up to date or one in which to contemplate? The symbol of the café visible in the frontage of the shop and on the mugs suggests the latter, but maybe it is something we need to experience to truly know?

¹Inventing Temperature, Hasok Chang, Oxford University Press, 2008

²To have or to be, Erich Fromm, Jonathan Cape, 1978

The Press Room is at 29 London Road, TW1 3SW