Physics Around the Clock E-Book

Michael Banks

• Herausgeber: The History Press
• Kategorie: Wissenschaft und neue Technologien
• Sprache: Englisch

Seitenzahl: 431

Veröffentlichungsjahr: 2025

Beschreibung

'This is one of those brilliant books where there is an amazing fact on every page showing just how much science underpins our everyday world.' – Mark Miodownik, author of It's a Gas: The Magnificent and Elusive Elements that Expand our World Have you ever found yourself wondering how to brew the perfect cup of coffee, finally rid your garden of snails or how to pull off a seamless Mexican wave? Well, wonder no more. Grounded in cutting-edge scientific research, this entertaining book answers these questions and many more, and explains what physics can reveal about so many seemingly ordinary aspects of your daily routine, from cooking breakfast and walking the dog to playing games and watching a film. So, whether you're navigating rush-hour traffic, choosing the best seat on a plane, or just trying to win at rock, paper, scissors, Physics Around the Clock shows that physics isn't just about lab experiments and equations – it's relevant to everything you do and is happening all around you, every second of every day.

Leseprobe

Praise for Physics Around the Clock

‘Physics Around the Clock explains, in an easy and engaging way, how from morning till night, we’re surrounded by fascinating physics that hides in plain sight.’

James Kakalios, physics professor at the University of Minnesota and the author ofThe Physics of Everyday Things

‘Physics is all around us – even as we go about our seemingly mundane daily lives, as Michael Banks ably demonstrates in Physics Around the Clock. Whether it’s your morning coffee, daily commute, walking the dog, cooking dinner, playing Monopoly or Texas Hold ’Em, or debating whether it’s better for gunslingers to draw first while watching classic spaghetti Westerns, a physicist somewhere has studied it. And Banks is here to explain it all to you in a truly compelling read.’

Jennifer Ouellette, author of The Calculus Diaries

‘This is one of those brilliant books where there is an amazing fact on every page showing just how much science underpins our everyday world.’

Mark Miodownik, author of It’s a Gas:The Magnificent and Elusive Elementsthat Expand our World

For Claire, Henry and Elliott

First published 2025

The History Press

97 St George’s Place, Cheltenham,

Gloucestershire, GL50 3QB

www.thehistorypress.co.uk

© Michael Banks, 2025

The right of Michael Banks to be identified as the Author of this work has been asserted in accordance with the Copyright, Designs and Patents Act 1988.

All rights reserved. No part of this book may be reprinted or reproduced or utilised in any form or by any electronic, mechanical or other means, now known or hereafter invented, including photocopying and recording, or in any information storage or retrieval system, without the permission in writing from the Publishers.

British Library Cataloguing in Publication Data.

A catalogue record for this book is available from the British Library.

ISBN 978 1 80399 582 3

Typesetting and origination by The History Press

Printed and bound in Great Britain by TJ Books, Padstow, Cornwall

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CONTENTS

Introduction

PART 1 – MORNING

1 Wake Up and Smell the Physics

2 Breakfast with Einstein

3 Looking after Yourself

4 Creature Comforts

PART 2 – DAYTIME

5 The Great Outdoors

6 Planes, Trains and Automobiles

7 Life’s a Sport

8 Under the Weather

9 Friends and Family

PART 3 – EVENING

10 Thrills and Spills

11 Fun and Games

12 The Key Takeaway

13 Physics on the Screen

Epilogue

Acknowledgements

Notes

INTRODUCTION

IF I ASKED YOU to outline a typical twenty-four-hour day, what would you say? Perhaps you would describe sleeping for roughly eight hours each night, with seven hours spent working or studying and then whatever is left squandered on commuting, chores or perhaps taken up with looking after children. If you are lucky then you may mention having a few extra hours for yourself before bed to watch TV, play a board game or read a book, such as this one. While you might think that your day is unique, it turns out that a large swathe of the world’s population spends a similar amount of time doing the same sort of tasks. At least, that is according to researchers at McGill University in Canada, who in 2023 came up with a typical ‘global human day’.1

To do so, they gathered data taken between 2000 and 2019 by national statistics agencies and international organisations, representing fifty-eight countries, or about 60 per cent of the world’s population. Once they crunched through all the numbers, they discovered, somewhat unsurprisingly, that sleep, or bedrest, makes up the largest single chunk of ‘activity’, occupying – and those with young children look away now – about nine hours each day.* The remainder, or some fifteen hours, was then split between three categories: the largest, with a similar amount of time as sleeping, was spent on ‘human outcomes’, such as making yourself look presentable, caring for children, reading, watching TV, playing sport, going for walks, and just generally chilling out; another three hours were consumed by activities on ‘external outcomes’ like food preparation, as well as cleaning and tidying the house; the remaining two or so hours were taken up with socialising or commuting.†

Many of the activities that we carry out day in, day out, we do so at the same time and for the same duration (especially sleeping) each day.‡ You probably have breakfast within the same ten-minute window every morning, have a shower just before 8 a.m., and leave for work, take the kids to school, or the dog out for a walk thirty minutes later. We are creatures of habit, after all. While some of these deeds may be considered rather ordinary and even downright boring, taking the time to stop for a moment and examine what is happening underneath your very nose (or eyes) can reveal a whole host of surprising and extraordinary phenomena. Taking the dog out for a walk in the rain, for example, might be a pain, especially when the pooch comes back inside the house and carries out a ‘wet dog shake’. Yet the ability to make water droplets fly off in every direction can be a matter of life and death for your mutt, helping to stave off hypothermia. Modern high-speed photography has revealed that the movement is so effective it can eject some 70 per cent of water from a dog’s fur in just a few seconds, going some way to explain why you, and the furniture, get so wet in the process (for more, see Chapter 4).

That is what this book is about: delving a little deeper into the seemingly ordinary aspects of everyday life to reveal the fascinating science and physics that lay beneath. Doing so reveals how the beauty and versatility of physical laws can manifest themselves everywhere, whether it is the way that Cheerios§ mysteriously clump together when floating in a bowl of milk at breakfast to how opening a bottle of champagne (presumably not in the morning) results in a supersonic plume of gas – the same physics that occurs in the exhaust of jet fighters. Indeed, searching for interesting or new phenomena does not always require building billion-pound experiments, such as underground particle colliders or launching huge astronomical observatories into the depths of deep space. While no one can deny that particle smashers, such as the Large Hadron Collider at the CERN particle-physics laboratory near Geneva or NASA’s James Webb Space Telescope, have done, and continue to do, amazing science, such endeavours often attract the most media attention, resulting in what most people take physics to be. Instead, in the pages ahead we will examine the more familiar world around us, whether in the simple sound of dripping water into a water-filled container to how a dandelion pappus drifts effortlessly for metres in the wind to turn a lawn into a summer meadow.

We begin by exploring the morning routine and the intriguing physics that can be seen as we have breakfast. As the typical ‘global human day’ shows, commuting is a significant part. We will also delve into the interesting dynamics involved with getting around town, as well as investigate the science involved in other common daily pursuits, such as enjoying the garden. Finally, we end with the physics behind popular evening pursuits, such as cooking dinner, playing your favourite board game or the network theory behind your favourite movie. Each chapter is self-contained, and doesn’t necessarily follow the other, so if you have no interest in ball sports, or the spread of pathogens makes you feel a bit icky, then simply move on.

Along the way we will meet the scientists who find fascination and intrigue in examining the how and why in everyday phenomena. Their discoveries highlight how following your curiosity can reveal countless marvels that exist in the mundane. Even during the rush of our daily lives.

_____________

* Admittedly this number is high because it also includes children, who generally sleep longer than the eight hours a day that an adult may sleep.

† You might wonder why ‘work’ or ‘study’ is missing, but the authors already included work activities into the other time categories (such as food production). On top of that, as it is an average global human day, the number of hours spent ‘working’, averaged across all humans, is only 2.6 hours per day.

‡ While the ‘global human day’ is remarkably consistent across people of different countries and backgrounds, there are a few cases where it isn’t. Those from low-income countries, for example, spend more time on farming or food preparation than those in higher-income nations.

§ Other ringed-shaped breakfast cereals are available.

1

WAKE UP AND SMELL THE PHYSICS

WHAT BETTER WAY TO start the day than with a nice, warm cup of coffee – especially if it is brought to you as you lay in bed. If you are lucky enough to receive that level of service, then you will already know what is coming, thanks to the aroma of the cup of java as it nears. There are thought to be more than a thousand chemical compounds that are extracted from ground coffee, which give the drink its unique flavour and smell. Yet taste isn’t everything on offer: it’s the C8H10N4O2 that really counts.* Coffee is the beverage brewed from the roasted and ground cherries, or beans, of the coffee plant. Although there are more than a hundred coffee plant species that have been catalogued to date, only two varieties are used commercially: C. arabica, or Arabica coffee, and C. canephora, which is commonly known as Robusta. Arabica, which accounts for roughly two thirds of global coffee production,† is thought to have a sweeter and smoother taste than Robusta, but comes with the downside that it can only be grown in a few places in the world where climate conditions are favourable.‡ Robusta, as the name suggests, is more resistant to disease and adverse weather conditions than its Arabica cousin.

A cup of coffee is the result of growing, harvesting, roasting and grinding coffee beans before passing water through the grounds to extract all that goodness. The first flavour that emerges when roasting beans is acidity but thanks to a chemical process called the Maillard reaction,§ chocolatey, nutty or caramel flavours soon emerge. What you will likely be in control of is how to brew the coffee itself (if you happen to be a coffee connoisseur then you may also grind – or even roast – your own coffee beans. Fancy!). Coffee can be made several ways, with each way having a slightly different technique and differing results. Some enjoy the simplicity and ease of instant coffee for a quick caffeine fix.¶ Instant coffee granules can be manufactured in a few ways, but one method involves brewing coffee beans and then evaporating the resulting liquid at a high temperature and low pressure. The remaining condensed extract is freeze-dried at -40°C and broken up into small crystals. These are the ones that you scoop into your cup before adding hot water, and perhaps a splash of milk and some sugar. While a cup of instant coffee may offer a quick and easy caffeine fix, it only contains about 70mg** of caffeine per cup compared to some 80–100mg for an espresso shot.

Over the past twenty years consumers have not only become more conscious about where their coffee comes from but also more precious (perhaps snobbish?) about how it is made. For decades, I was easily content with the ease of instant coffee, perhaps branching out to filter or a trip to the local coffee shop at the weekend for an espresso. But I never looked back when I bought a ‘bean-to-cup’ espresso coffee machine just before the COVID-19 pandemic, which resulted in coffee shops temporarily closing during the lockdowns. Since then, I have enjoyed the ease each morning (and afternoon) of pressing a button, hearing the beans being ground, followed by a nice caffeine hit. What I never quite appreciated, however, was the physics that goes into making the perfect cup of coffee, requiring the precise combination of water temperature, flow rate and sometimes pressure to get the balance of flavours just right. So, tossing aside instant coffee, how can physics help to make that perfect cup?

If I said that volcanic eruptions and grinding coffee beans have something in common, the first thing that might come to mind is being too eager to drink from your takeaway coffee cup and scolding your mouth and tongue. In fact, both processes involve the production of a lot of static electricity. Static occurs due to an imbalance between negative and positive charges on an object as it is rubbed together with another object. These charges build up on the surface of an object until they find a way to be released or discharged – the classic experiment being to rub a balloon on someone’s hair. The hair will become positively charged by losing electrons, while the balloon will be negatively charged, gaining electrons. Bringing the two together results in the hair rising towards the balloon as they attract.

Grinding coffee involves fracturing the beans via rotating metal plates in a burr grinder.†† This grinds the beans to a fine ground but, as it does, it produces static, with individual coffee grains acquiring an electrical charge. The static produced can be high enough to cause the small coffee grains to clump together as well as stick to the grinder. To get around this, some baristas add a drop of water to the beans before grinding to reduce the static friction – a method known in the industry as the Ross droplet technique. Not much was known about how this process impacts the resulting brew until 2023, when Christopher Hendon at the University of Oregon in the US teamed up with volcanologists – yep, people who study volcanoes – to measure the static electricity produced when grinding roasted coffee beans.1

Hendon, who goes by the moniker ‘Dr Coffee’, has spent his career studying coffee and first became interested in the tipple when doing a PhD at the University of Bath in the UK. He was surprised to find that while there are certain rules when it comes to coffee-making (more on that later), not much scientific research had been done on what goes into making the perfect cup. ‘What sets coffee apart from many other drinks is that you have to do something before the point of consumption,’ Hendon told me. ‘You can open a bottle of beer or wine and drink it how it is intended to be, but for coffee you must make it. From that perspective, it’s inherently an experimental beverage because it entices the consumer to play around with the variables until they arrive at something they think tastes good.’

Back in the US, Hendon and colleagues began running public coffee-making demonstrations at Oregon University, when a couple of volcanologists came by to sample what was on offer. They became intrigued about the static that was produced when the coffee beans were being ground. After discussing this with Hendon, they became, for want of a better word, attracted to the idea of investigating the charge that can build up during bean grinding. The experiments the team carried out involved modifying an instrument known as a ‘Faraday cup’, which is used to measure the electric charge of volcanic ash. This is a material that can be so electrically charged as the particles collide and fragment that when it gets spewed into the atmosphere it can even cause volcanic lightning storms. The result of their efforts was a small metal vessel – about the size of an espresso cup – that the team placed under a burr grinder and used it to measure the static charge of individual grains as they were ground. They found, as predicted, that the charge on the grounds was larger the more finely the coffee was ground. What wasn’t so expected was that lightly roasted beans had a positive charge while darker roasts had a negative charge. Unroasted beans contain about 50 per cent water, but the roasting process removes liquid so that dark roasts only contain about 10 per cent water. The higher the internal moisture content of the beans, the lower the charge on the grains, suggesting that water reduces the static by acting as a lubricant during grinding.

When Hendon and colleagues then compared espresso made with identical coffee beans that were either ground with or without a drop of water, they discovered that grinding with water resulted in a more consistent and stronger brew. They attribute this to water reducing the friction during grinding and thus stopping the grounds from clumping. This allows the water to move more consistently, and more slowly, through the uniformly compacted grounds during brewing to help extract more flavour. Hendon and colleagues discovered that adding a splash of water resulted in 10 to 15 per cent more ‘yield’ – the fraction of the ground coffee that dissolves and ends up in the final drink – compared to not adding a drop of water.

So, if you like to grind your own beans at home, perhaps think about adding just a drop or two of water before grinding to stop the grains from clumping, which will hopefully result in a tastier coffee.

Brewing coffee from ground coffee beans is all about hot water moving through a bed of coffee grains so that it can absorb the flavours and oils of the coffee as it passes through. The basic physics of what happens when water percolates through a bed of granular‡‡ material such as ground coffee was first worked out in the nineteenth century by the French hydrologist Henry Darcy. In the 1840s, Darcy was contracted to provide a clean supply of water to the French town of Dijon, which a few years later would become known for its mustard. Between 1855 and 1856, Darcy undertook a series of experiments in which he tested how water was filtered as it flowed through a cylinder of sand. He had to calculate how many cylinders would be required to filter the necessary amount of water, and from the experiments he empirically§§ obtained what is now known as Darcy’s law, which describes the speed of a liquid as it moves through a bed of granular material. Darcy’s law has had countless applications in everyday life, such as describing how water, oil or gas flow from petroleum reservoirs – and, of course, in coffee extraction.

Darcy’s law states that the flow rate of a liquid depends on several factors, one of which is the viscosity of the liquid. This is the resistance that a fluid has to a change in shape or movement. A fluid with a large viscosity, such as honey, resists motion because its molecular make-up produces a lot of internal friction. A fluid with low viscosity, like water, flows easily because its molecular make-up results in very little friction when it is in motion. If you have a cup of water and a cup of honey and tip both, the water, with its lower viscosity, will flow faster. Other factors that affect the liquid’s speed through a granular bed include the pressure difference between where the liquid enters and where it exits. Yet when it comes to applying Darcy’s law to brewing coffee, the key parameter is the permeability of the granular bed of coffee beans. A material’s permeability is a measure of how easily it lets liquid through. When it comes to coffee making, it can be changed by compressing the coffee grains into a firm bed, which is why you will often see baristas ‘tamp’ their coffee into a compressed puck before putting it in the machine. This helps to pack the grains together so that water can’t find an easy escape route through the puck, which would result in a weak brew. Another way to alter the permeability is to change the size and shape of the grains. Finer grinds maximise the surface area of coffee that is in contact with the water, which means it can take longer to pass through. But if the bed is compressed too much, then it can take too long and this can result in extracting higher levels of organic acids, leaving a bitter taste. ‘Making coffee is about extracting the molecules that taste good as fast as possible,’ adds Hendon.

So how does this work in practice? The simplest coffee-making contraption from a physics perspective, and one that is found in countless domestic kitchens, is the French press. Here, hot water is poured on to coarse or large irregular-shaped coffee grains that are held in a cylindrical container. In this ‘immersion’ scenario, the water is always in contact with the grains, leeching the coffee compounds into the liquid. Over time, the grounds rise to the top of the liquid to reside under the base of the plunger, which is then used to push the grounds back through the water. If you have ever unknowingly scooped coffee grounds into a French press without any feel for how much is the right amount, then the recommended method is 54g of coffee grounds for 1 litre of boiling water. If the above recipe is always used then the only parameter that can be changed is the time the water is in contact with the grains, which is usually suggested to be five minutes before plunging. And if you have ever wondered how much force is needed to push the plunger through the water, in 2021 physicists carried out a combination of kitchen- and laboratory-based experiments to determine that 32 Newtons¶¶ is the magic number. This force took the plunger in a 1 litre French press from top to bottom in about fifty seconds.2 Now you know.

Things get a little more complicated for filter coffee, which remains a popular way to brew despite the rise of espresso machines. This method involves putting the coffee grounds into a conical paper or metal filter and then pouring near boiling water from the top. The water slowly percolates through the coffee, thanks to gravity, over several minutes and as it does so the water washes over the coffee grains, extracting some of the flavour before passing through the filter and dripping into a glass container placed on a heated plate. The key parameter here is the time it takes the water to wash over the coarse coffee grains. A finer grind makes the water seep through more slowly, so it can increase the extraction, but it also results in more unwanted bitter compounds dissolving into the water. Filter coffee is Hendon’s preferred method in the morning, which he makes thanks to a Moccamaster automatic that helps to automate the process of adding water. ‘It saves me the five minutes it would take to pour water through the beans,’ he adds, ‘and it makes good coffee, well, good enough for me.’

Our physics-based coffee-brewing venture gets a bit more interesting when some vapour pressure is thrown into the mix. This happens in the Italian ‘moka’, which was invented in 1933 by the Italian engineer Alfonso Bialetti and was widely commercialised in 1946 by his son Renato via the trademark Moka Express. In a moka, which is still prevalent in Italy today, a bottom container is filled with water and on top sits a basket that holds finely ground coffee beans (the grounds should be packed loosely and not compressed). The bottom compartment plus basket is then screwed on to an empty upper chamber, which is often in the shape of a teapot (see figure opposite). The moka pot is then put on the stove and is heated from the bottom compartment. The heat gradually turns the water into steam, the pressure rises and in turn the steam ‘pushes’ the water up the middle funnel and through the coffee grains in the basket and into the top teapot compartment above.***

In a moka pot, the pressure from the steam drives the hot water up through a bed of coffee grains and into the top compartment.

By measuring the flow rate through a bed of ground coffee, in 2007 scientists applied Darcy’s law to the moka, finding that the coffee bed’s permeability is akin to sandy silt or clean sand (showing that Darcy’s experiments with sand columns does have something in common with brewing coffee).3 Yet two years later another moka pot study found that things are slightly more complicated, and the permeability of the coffee bed was not constant over time but reduced as the pressure of the water and its flow rate changed during the brewing process.4 Despite being a seemingly simple device – what some might call a ‘poor person’s espresso’ – the physics of what is going on inside a moka pot is far from straightforward. So, how can you improve the output? In 2008 (the physics of the moka pot was obviously a hot topic in the late 2000s), physicists carried out controlled experiments in the lab. They first sealed a moka pot with water at room temperature and heated it by about 10–20°C per minute – what typically happens in a domestic kitchen.††† They found that half of the resulting coffee was extracted when the temperature of the water was only about 65°C. This not only contradicted the idea that the water temperature had to reach boiling point before coffee emerged, but also hinted that the temperature was too low to extract as many of the desired flavours as possible. To get around this issue, the researchers first heated the water to about 70°C before putting it into the moka pot. This resulted in near boiling water being transferred during the whole brewing process, helping to produce as much of that great flavour as possible. If you rely on a moka pot for your morning coffee, next time perhaps try starting off with hot water and see if it tastes any better.5

Perhaps the ultimate way to prepare coffee – especially for busy coffee shops – is the espresso‡‡‡ machine. In simple terms, water is boiled to produce steam, and this is collected in a ‘group head’ at the top of the machine that pressurises the steam and uses it to drive water through the bed of grounds that are held in a portafilter, which then drips into a cup held below it. Espresso normally comes in a fine grind and the Specialty Coffee Association – a non-profit, membership-based organisation that represents thousands of coffee professionals around the world – defines an espresso as a 25–35ml beverage prepared from about 7–9g of ground coffee.§§§ The association states that water should be heated to 92–95°C before being forced through the granular bed under 9–10 bar of static water pressure for between twenty to thirty seconds.6 If the water is hotter, it risks burning the coffee grounds, resulting in a bitter brew, whereas if the temperature is much less than 90°C then – as we have seen what can happen with the moka pot – the flavours will not be extracted efficiently, resulting in weaker coffee.

In a filter or other ‘pour over’ coffee technique, about 1.5 per cent of the coffee material is dissolved in the beverage, but for espresso it is about 10 per cent – so almost ten times more concentrated. While adding water to an espresso shot results in the same concentration of coffee mass as a filter or coffee made with a French press, it will taste different, being perhaps more intense. This is because many molecules in coffee are volatile – easily lost to the atmosphere – but adding pressure, as in the espresso technique, helps to lock those molecules inside the resulting drink. The addition of pressure driving the water through the grounds is also thought to help extract more of the rich oils into the resulting coffee. Indeed, if you do it right then you will see the result – the ‘crema’, a golden-brown layer of tiny bubbles of carbon dioxide that are coated in proteins and oils of the coffee.

The espresso is widely seen as the most complex way to make coffee – Hendon calls it ‘a high-performance version of filter coffee’. But with that complexity brings a certain susceptibility to inconsistency. While espresso machines control the temperature and pressure of the water, the rest is left to the barista.¶¶¶ Even for skilled baristas operating top-of-the-range coffee machines, creating cups of espresso that taste consistent from one to the next can be tricky. The coffee industry in the US is worth $343 billion, or 1.5 per cent of US gross domestic product, which means that any improvements to the process can have a big impact – as we have seen in the Ross droplet technique by simply adding a drop of water before grinding.

As espresso machines, grinders and beans are different, that means there is, unfortunately, no magic formula to tell you the right parameters to use at home. Coffee is a tricky drink to prepare because there are so many factors that can be changed from the temperature and amount of water to the coarseness of the beans and how compact they are. ‘My main tip when preparing coffee is that every single decision you make, either consciously or unconsciously, will impact extraction,’ notes Hendon. ‘The first thing you can do is try and measure everything you have done, whether it’s the brew time or temperature of the water to better understand why you’re getting that particular output.’

This dependence on experimenting means that coffee is perhaps the perfect physics-based drink, and goes some way to explain why it is the beverage of choice for physicists. Yet a little knowledge of what is going on when making that morning tipple can also help you to deliver the best possible coffee.

You might think that once you have finished your cup then that is where the physics ends. Well, there is one last thing. It is always hard to keep any liquid from trickling down the side of a mug as you take your mouth away after a sip. If you leave your cup in one place for a while you may see a ringed stain left behind. After all, it’s why we have coasters. Likewise, if you leave a couple of drops in your finished coffee and let them dry, you may see that the droplets looks like a ring – lighter in the middle with a dark ring around the outside. This effect can be seen for other types of drinks, such as wine, and it is likely to have been observed hundreds of years ago too, but it is only in the past couple of decades that it has been made clear what was going on. It is now known – rather unimaginatively – as the coffee-ring effect.7

If you take a droplet that contains tens of little dark particles inside and watch it dry under a microscope, you will see that the particles all seem to move to the drop’s edge, where they collect, resulting in a dark ring as the drop dries. The question is, why are the particles attracted to the outer edge rather than just being spread evenly over the drop’s surface? The explanation is that as a drop evaporates, the edge of the drop stays pinned. While the drop stays the same diameter, the height of the droplet decreases as it evaporates, so it ends up a bit like a pancake, or a spherical cap. The evaporation does not happen evenly but is quicker at the edge of the drop. As this happens, liquid at the edge is replenished by the interior, and this creates a current towards the edge. The particles inside the drop then flow outwards to the edge, where they pile up, one at a time, into a tightly jammed packing, which produces a dark ring as the drop finally dries.

You might think that the coffee-ring effect is just an annoying aspect that must be cleaned, but knowledge of it can also have lifesaving applications. Scientists are using the effect to design simple and cheap ways to diagnose diseases, such as malaria, which kills more than a million people a year.8 In such a test, a patient’s blood is mixed with a liquid containing gold nanoparticles and then left to dry. If malaria is present in the blood, then a unique protein**** that is secreted by the malaria parasite will be present. This protein then binds with the nanoparticles to form clusters and when the drop dries these clusters move to the edge, leaving a visible ring. If no malaria is present, then the clusters don’t form and there is no ring.

A simple test for malaria all thanks to the same physics that happens with your finished cup of coffee.

_____________

* This being the chemical formula for caffeine. Of course, some people prefer or can only drink decaffeinated coffee. This can be achieved in several ways, but one method is to steam the green beans before soaking them in a solvent, such as methylene chloride, which removes about 97 per cent of the caffeine. The beans are then dried and roasted.

† Arabica is thought to have been created naturally from two parent species – Coffea canephora and Coffea eugeniodes – around 600,000 years ago in the forests of Ethiopia. This is some 300,000 years before modern humans, refuting the claim that humans bred the species.The amalgamation of the two parent plants is thought to have resulted in Arabica’s flavour and its large and complex genome.

‡ Some estimates predict that, by 2050, suitable areas for growing Arabica will fall by about 80 per cent due to climate change, see Imbach, P., Fung, E., Hannah, L., et al., ‘Coupling of Pollination Services and Coffee Suitability Under Climate Change’, Proceedings of the National Academy of Sciences, vol. 114, no. 39 (2017): 10438–10442.

§ Named after the French chemist Louis-Camille Maillard, who first described the process in 1912. The reaction between sugars, heat and amino acids is also responsible for giving certain foods their distinct flavour – think toasted marshmallows or seared steaks.

¶ The amount of caffeine in your bloodstream peaks about fifteen to forty-five minutes following consumption.

** A milligram being 0.001g.

†† Burr grinders involve two revolving abrasive surfaces that grind the beans, while blade grinders use a propeller-like blade akin to a blender. Burr grinders produce a more consistent grind than blade grinders.

‡‡ Granular as in resembling or consisting of small grains or particles.

§§ This means that the law was used to describe the results of experiments rather than being based on theory or logic.

¶¶ Newton is the unit of force. About 32 Newtons is the same force a 2-year-old child can exert when pushing with their thumb – althoughI don’t recommend that you try this using a small child.

*** In 2012, physicists used a beam of subatomic particles called neutrons, which were produced at the Paul Scherrer Institute in Switzerland, to take real-time images of what was happening inside the moka pot as it was heated. For a video, see youtu.be/VESMU7JfVHU.

††† Another tip is to make sure the safety valve on the bottom of the device is functioning. If the coffee bed is strongly compressed, meaning the water cannot escape and the steam safety value doesn’t open, the pressure inside can turn the moka pot into a bomb.

‡‡‡ Espresso meaning ‘quick’ in this context.

§§§ Coffee shops in many countries use a higher coffee mass of 15–22g, resulting in larger-volume beverages of 300ml, much to the dismay of Italians.

¶¶¶ A study in 2021 found that coffee is perceived to taste sweeter if it features latte art; see Hus, L. and Chen, Y-L., ‘Does Coffee Taste Better with Latte Art? A Neuroscientific Perspective’, British Food Journal, vol. 123 (2021): 1931–1946.

**** Called histidine-rich protein II, or HRPII.

2

BREAKFAST WITH EINSTEIN

WHILE THE UK HAS a growing love for coffee, it might not be the first drink you consume in the morning. A cup of tea is my first hot beverage of the day (well, I am English) and, thankfully for this book, there is also a lot of interesting physics at play here, too. All tea comes from the tropical plant known as Camellia sinensis, which grows best in a warm climate with long days, cool nights and an abundance of rainfall. The varying types of tea such as green, black and oolong are due to how the leaves are processed after harvest – black tea being fermented while green tea isn’t, for example. According to legend, tea has been brewed for centuries, beginning in China in around 2,700 BC, but it took thousands of years before it became a popular drink in the country. To make tea, ‘kettles’, typically made from bronze or iron, were used to boil water. Then in the nineteenth century, copper became more prevalent, given that it conducts heat more efficiently. Later that century the copper teapot, similar to how it looks today, made its way into homes.

In this speed read of tea-making history, the first electric kettle was also being developed around the end of the nineteenth century. In 1891, the US-based Carpenter Electrical Company released an electric teapot with a heating element separate from the water compartment, meaning it took some ten minutes to boil water. Improvements to electric kettles came in subsequent decades, which included the commonsense idea of placing the immersion heating element into the actual water container. Yet traditional steam kettles continued to be popular, especially with the advent of gas-cooker tops in the early twentieth century. While electric kettles are now commonplace in Europe, in the US the steam kettle is still widely used. This is due to cultural preference as well as the fact that the mains voltage in the US is 110–120 volts (compared to 230–240 volts in the UK), meaning it can take just as long to boil water in an electric kettle as it can in a traditional steam one. Once the water is boiled, an electric kettle automatically switches off thanks to a channel within the kettle, typically inside the handle, that carries steam to a thermostat near the base, which when heated to near 100°C will trip the power off. Those using a steam kettle, on the other hand, will be made aware that the water has boiled due to the characteristic noise of a kettle whistle. This is a cylindrical duct placed at the end of the spout, which includes two circular plates that are closely spaced apart inside (see figure on page 30). Both plates have a hole in the middle that allows the steam to pass through.

Despite this whistling noise having been heard for well over a century, nobody fully understood the physical mechanism behind it until 2013 when acoustic engineers Anurag Agarwal and Ross Henrywood, from Cambridge University in the UK, tackled the problem. Agarwal first became interested in the whistling kettle when doing a PhD in acoustics in the US. He discovered that the phenomenon was first tackled by the nineteenth-century British physicist and mathematician John William Strutt (known more widely as Lord Rayleigh).* In Rayleigh’s 1877 book The Theory of Sound, he compared the mechanism to how birds produce birdsong. But even the great physicist admitted that ‘much remains obscure’ when it came to how the sound is produced. ‘Lord Rayleigh didn’t have microphones and similar equipment back then, which would have made it difficult to study, so we thought we would make the measurements and validate his theory,’ Agarwal told me.

They found that Rayleigh’s theory didn’t quite apply to whistling kettles. To investigate further, Agarwal and Henrywood tested a series of whistles of different lengths by forcing air through them at various speeds.1 The pair found that once the water is near boiling, the steam going through the kettle’s spout produces sound at a single, fixed frequency.† When they investigated this surprising result, they discovered that the noise is generated in the same way as when you gently blow over the open neck of a wine bottle. This creates something called a Helmholtz resonator, which in the case of a wine bottle causes sound to radiate from the neck of the bottle at a fixed frequency. In a similar way, the air inside the whistle reverberates like the air in the neck of a wine bottle, producing a characteristic hum at a constant, single frequency.

A kettle whistle features two plates that each have a small hole. The fast-moving steam entering the whistle’s first hole forms a jet, while the second hole acts to produce mini vortices that are responsible for the characteristic whistling noise.

Once the water in the kettle is on a rolling boil, however, steam is pumping out and travelling much faster. This is when another sound – the whistling we are all accustomed to – kicks in. As the steam in the spout enters the first hole of the whistle, it contracts into a fast-flowing stream of steam. This jet of steam is unstable and starts to break up as it makes its way through the whistle’s cavity to the next plate, producing sound waves in the duct between the plates. By the time it gets to the second plate the steam jet hits the hole and produces vortices outside the spout. These mini whirlwinds just happen to produce sound at the same frequency as the sound waves in the duct; the note produced being determined by the size and shape of the hole openings and the length of the spout – a bit like a flute. Agarwal found that it is exactly these vortices – a phenomenon called vortex shedding – that causes the sound. The frequency of the sound also increases with the flow rate of the steam, which is why you may hear the sound change the more the water boils. This vortex shedding is the same effect that happens when wind blows over telephone wires, or when the air travels over roof bars on top of your car. Both produce a whistling noise that is not so dissimilar to the physics of the whistling kettle.

Once the kettle is boiled, if you are not in a frantic race to get the kids off to school or head off to work, then you may instead prefer to sit down with a pot of tea. How civilised. The issue for teapot aficionados is pouring the tea while avoiding liquid trickling down the underside of the spout and on to the table. This is known as the ‘teapot effect’, a term first coined in 1956 by the Israeli physicist Markus Reiner.2 The phenomenon occurs when the liquid coming out of the spout ‘sticks’ to the tip and does not flow out cleanly, resulting in some of it trickling down the underside of the spout. It was thought that surface tension and adhesion of the liquid to the surface was behind the effect. Surface tension is the effect you can see on the surface of water that results in a ‘film’, which allows some insects to walk on water. Deep in a liquid, water molecules are surrounded by other water molecules on all sides, which results in the interactions between the molecules balancing out – giving no net tension. In other words, the molecule pulling above is balanced out by the molecule pulling below, and so on. However, molecules at the very top of the surface do not have neighbours above them – only at the sides or below. This results in the molecules bonding more strongly with their neighbouring molecules along the surface, creating a sticky ‘surface film’, much like the stretchy elastic sheet that pond skater insects use to literally walk on water.

In 1956, however, Reiner discovered that surface tension and adhesion alone were not enough to describe what was going on. Instead, he proposed that when a fluid flows against a surface, it shears. In other words, the part of the liquid that is away from the surface travels faster than the part that is at or near the surface, which is affected more by friction, and that makes it stick to the spout. Pouring quickly, however, helps to avoid this, as the liquid ‘detaches’ from the surface and flows freely out.

Thirty years later, in 1986, the physicist Joseph Keller, at Stanford University, proposed that the main culprit behind the teapot effect is actually pressure. As the liquid begins to turn out of the teapot, the pressure in the liquid at the pouring lip is lower than atmospheric pressure, since a pressure drop is required to balance the centrifugal forces.‡ This means that the air ‘pushes’ the tea against the lip and the outside of the spout, which causes the drip to pour down the underside of the spout. No need for surface tension. In 1989, Keller improved his model to include gravity to explain the point at which it starts to trickle down the underside of the teapot and on to the tablecloth.§

Case closed? Not quite. But the end of the dribbling teapot trauma finally came closer than ever in 2021 when researchers led by Bernhard Scheichl from the Vienna University of Technology in Austria declared to the world that they had formulated a ‘complete – even though quite technical – theory’ of the teapot effect. ‘I hadn’t actually heard of the teapot effect before,’ Scheichl told me. ‘But when we looked into it, we realised that no one had really fully explained what was going on.’ By filming teapots as they poured water, the researchers discovered that a small liquid drop formed just under the sharp edge of the spout so that the area always remained wet. The size of this drop, however, depends on the speed that the liquid is flowing: if too slow then the drop acts to direct the entire flowing liquid down the underside of the spout, but when poured at a faster rate the water detaches from the drop and pours out.3 It is this drop, they found, that is behind the teapot effect.¶ The team also carried out a theoretical analysis showing that the effect is an interplay between inertia** of the moving fluid, agreeing with the earlier findings by Keller. Other factors include the properties of the fluid itself and the flow of the fluid in a narrow space due to cohesion and adhesion, known as the capillary effect.

That’s all very well, but how do you stop it? One possible approach to dribble-free pouring is to coat the inside of the spout with butter. It is hydrophobic, so repels water and gives the liquid a little extra ‘push’ out of the teapot. Scheichl says, however, there are other ways that thankfully don’t involve adding a taste of butter to your tea. One is to use a ceramic teapot, as it is more hydrophobic than, say, glass. The second it to buy a teapot that has a thin, sharp-edged lip on the spout, which allows the water to flow more freely and not get stuck. If it is too curved, then this helps the drop to form underneath the spout. The third has to do with the angle between the spout opening and the neck of the teapot. If it is almost 90° or alternatively very small, then this will also promote the drip. The key is to have an angle somewhere in between, looking somewhat like a ‘less-than’ sign, i.e. ‘<’. And a final trick is not to fill the teapot to the brim so that you can pour the liquid out smoothly.

Scheichl and colleagues also concluded from their work that gravity is not a factor in the teapot effect. This only acts to direct the jet of tea and plays no role in its dynamics near the spout. So, if we ever colonise another planet, prospective tea-drinking explorers would still have to put up with stained tablecloths.

If you close your eyes (not now, unless you have an audiobook) and imagine the sound of hot water being poured into a cup, you will probably be able to do so quite effectively. This rather mundane sound has a particular impact on the brain; after all, if you listen to it long enough you might need to go to the toilet. If you ever visit a Moroccan tea house you will see tea being poured from a great height – well over 30cm from the cup – without seemingly splashing the table, or worse, the customer. This pouring technique is done to trap air bubbles in the liquid to produce a layer of foam on top of the drink, which not only adds to the aesthetic appeal of the cup of tea but also to the tasting experience, enhancing its aromas (more on how bubbles can impact a drink’s taste in Chapter 10). If you pour water out of a typical plastic bottle, the jet of water that flows happens to be uniform and smooth, known as laminar flow. As you pour, the water hits the surface of the liquid already in the cup and makes little sound. But if you get a teapot and pour water from the same height, you may see that the jet of water breaks up and is louder when it hits the water’s surface. This is thanks to the Plateau–Rayleigh instability, in which Lord Rayleigh (him again) showed in the late nineteenth century that a vertically falling column of liquid breaks up into drops if its length exceeds its circumference, or π times its diameter.††

In 2023, researchers in South Korea set out to investigate the reasons behind this intriguing discrepancy between noise and liquid break-up.4 They placed a nozzle with a diameter of a few millimetres about 10cm above a water-filled cylinder. They then sent a jet of water through the nozzle and on to the cylinder and recorded the sounds that were made with an underwater microphone. Using a camera, they also imaged the bubbles that were produced as the jet hit the surface of the water. They did this for several different nozzle heights, discovering that the key to making noise wasn’t so much the height that the liquid is dropped from, but rather due to the jet stream breaking into droplets. When the jet of water broke up, a louder noise was produced due to more air bubbles being trapped under the water’s surface – as happens when pouring tea from a great height (to find out what happens when a single drop falls, see the next chapter). As a smaller-diameter jet breaks up more easily than larger-diameter ones, this explains why pouring from a bottle with a larger opening can be quieter than pouring water from a small-nozzle teapot. Yet the flip side is that when larger-diameter jets do break up, they contain larger drops of liquid that then causes more bubbles and more noise. In any case, they found that if you want to pour your tea as quietly as possible you should do so from a height that is no more than a few centimetres from the surface of the water. Or you could just do it from a great height instead and make your tea pouring a bit more theatrical.