Helen Czerski - Bubbles: A Ladybird Expert Book

Helen Czerski

---

BUBBLES

with illustrations by
Chris Moore

Every effort has been made to ensure images are correctly attributed, however if any omission or error has been made please notify the Publisher for correction in future editions.

UK | USA | Canada | Ireland | Australia

India | New Zealand | South Africa

Michael Joseph is part of the Penguin Random House group of companies whose addresses can be found at global.penguinrandomhouse.com

First published 2018

Text copyright Helen Czerski, 2018

All images copyright Ladybird Books Ltd, 2018

The moral right of the author has been asserted

Cover illustration by Chris Moore

ISBN: 978-1-405-93470-1

Bubbles are beautiful, ephemeral, fun, fragile, jolly and slightly unpredictable. Were all familiar with them, but we dont often ask what they actually are.

A bubble is a pocket of gas enclosed in liquid. It sounds simple, and exists because states of matter have boundaries. A gas and a liquid cant merge without becoming purely a gas or purely a liquid. Packaging a blob of gas within the liquid is an efficient way to arrange a mixture.

Humans have loved bubbles for centuries. The ancient Sumerians may have had a word for bubble: gakkul, used mostly in the context of beer (and often celebrated for generating happiness).

Bubbles took off in public life with the Victorians and their fascination with soap and cleanliness. Soap consumption in the UK increased from 25,000 tons in 1801 to 100,000 tons in 1861, and as the germs were scrubbed away bubbles floated into everyones lives, ephemeral symbols of the new world being built by the Industrial Revolution.

Many great scientists in the Western world Lord Rayleigh, Isaac Newton, Robert Hooke, Agnes Pockles (see opposite, clockwise from top right) and more studied bubbles seriously. They recognized that bubbles could tell us a lot about the nature of the physical world, and they poked, prodded and listened to find out what it was. In the years since, weve learned that this bulbous arrangement of liquid and gas does things that neither could do by itself.

This book has a message: never underestimate a bubble.

We all encounter our first bubble very early in life, and its almost always a soap bubble. Theyre easy to make and fun to play with, but theyve got such a delicate structure that its amazing they can exist at all. A soap bubble is a pocket of air trapped inside a very thin film of water. Soap molecules coat the inner and outer surfaces of the water so that it doesnt touch the air at any point. This double coating keeps the water layer stable even though its only about 0.001 mm thick.

Soap bubbles are usually spherical because this shape can contain a fixed amount of gas with the minimum surface area. But the real source of their beauty is their iridescence. The thickness of the soap film is so close to the wavelength of light that when light bounces between the walls some colours are reinforced and others cancel themselves out. The colour you see depends on the angle you look from and the thickness of the soap film at that point.

The thin water layer gradually drains downwards because of gravity (some evaporates from the surface). Glycerol slows down the drainage, which is why its commonly added to bubble solution. Putting dyes in the soap solution doesnt change the colour of bubbles because theres too little water within the soap film to absorb light. But Japanese astronaut Naoko Yamazaki blew coloured bubbles on the International Space Station. In zero gravity, the water layer is thicker and doesnt drain downwards, so bubbles really can have a single colour.

Soap bubbles, while pretty, are only the beginning. The real world of bubbles is underwater, where a bubble is more robust, and much more interesting.

Underwater bubbles are harder to watch than soap bubbles, but theres much more to see. The most obvious difference is their shape.

A perfectly spherical underwater bubble is a rare beast. Bubble shape is determined by a hierarchy of invisible forces in close competition the bubble surface is constantly resculpted as these forces jostle for position.

The simplest bubble is a sphere, because its possible to have a nice tidy balance when there are only two forces in the battle: the gas pressure pushing outwards and the surface tension (the tendency of a water surface to act like an elastic sheet), which acts to reduce the surface area, squeezing the surface inwards. As the volume of the squeezed gas decreases, its pressure increases until a truce is reached a perfect sphere.

But the only place youll see this is in zero gravity. Down here on Earth, gravity pulls the water downwards, forcing bubbles to rise. And that brings in a third player the surrounding liquid pushing on the bubble surface.

Large rising bubbles are flattened into a spherical cap shape, and the bigger the bubble, the more umbrella-like its shape becomes. Turbulent water is full of swirls and mini-currents which can stretch and squish bubbles into lots of weird shapes, and even break them in two. Those forces from the liquid are constantly competing with the surface tension and the bubbles inner pressure.

Almost all bubble shapes can be made by distorting a normal spherical bubble. But there are exceptions, and nature has an expert at producing them: the dolphin.

If youre a dolphin, a bubble is a great toy. And many species of dolphin and whale are so good at playing with bubbles that theyve learned how to make a very unusual shape: a ring bubble or toroid. These bubbles are doughnut-shaped a tube stretched around in a circle and theyre always travelling. Theyre the aquatic equivalent of a smoke ring.

The reason that these toroidal bubbles can hold their strange shape is that theyre really just the core of a spinning doughnut of water. The easiest way to make them is to blow a normal bubble but to add a pulse of pressure at the end, so that a jet of water pierces the middle of the bubble at the last moment. This jet of water will then flow around the outer surface of the bubble ring and back up through the centre, swirling round and round to form a vortex thats curved around in a circle. The ring travels forward, spinning all the time, and it becomes wider and thinner as it goes. Its tricky to get this right in the laboratory, but the dolphins are superstars at it. Both wild and captive animals have been seen making toroidal bubbles, and they often bat the rings to break them up or swim right through their new hoops.