Viscosity matters when something small is moving through a single fluid—fat globules rising through milk or a tiny virus falling through the air. Surface tension, its partner in the world of the small, matters at the place where two different fluids touch. For us, that’s usually where air touches water, and everyone’s favorite example of air mixing with water is a bubble.** So let’s start with a bubble bath.
The sound of a bathtub filling up is distinctive and jolly. It announces the imminent reward after a hard day, a soak to recover from a particularly tough tennis match, or just a bit of pampering. But the moment you pour in a bit of bubble bath, the sound changes. The deep rumble gets softer and quieter as the foam builds up, and the place where the water stops and the air starts gets harder to identify. Pockets of air are trapped inside watery cages, and all it took was a tiny amount of stuff from a bottle.
It was a group of European scientists in the late nineteenth century who picked apart the puzzle of surface tension. The Victorians loved bubbles. Soap production expanded dramatically between 1800 and 1900, and the white suds washed the workers of the Industrial Revolution. Bubbles provided the Victorians with good fodder for moralizing; they were the perfect symbol of pure cleanliness and innocence. And they were also a nice example of classical physics at work, just a few years before Special Relativity and quantum mechanics came along and poked a sharp pin in the ballooning idea of a neat, tidy and well-behaved universe. But even so, the serious men with top hats and beards didn’t work out the secrets of bubble science all by themselves. Bubbles were so universal that anyone could have a go. Enter Agnes Pockels, often described as a mere “German housewife,” but really a sharp-minded critical thinker, who used the limited materials available to her and a decent dose of ingenuity to examine surface tension for herself.
Born in 1862 in Venice, Agnes was of a generation very firmly convinced that the woman’s place was in the home. So that’s where she stayed while her brother went off to university to study. But she learned advanced physics from the material he sent to her, carried out her own experiments at home, and generally kept up with what was going on in the academic world. When she heard that the famous British physicist Lord Rayleigh was starting to take an interest in surface tension, something she had done many experiments on, she wrote to him. He was so impressed with the letter describing her results that he sent it to be published in the journal Nature so that it could be seen by all the greatest scientific thinkers of the day.
Agnes had done something very simple and very clever. She had suspended a small metal disk (something about the size of a button) on the end of a string and let it sit on the surface of the water. Then she had measured how much force it took to pull it away from the surface. The mystery was that the water held on to the disk; you had to pull harder to get it off the water surface than you would have had to pull to pick it up off the table. That pull from the water is what we call surface tension; so in measuring the pull Agnes was measuring surface tension. She could then investigate the surface of the water, even though the thin layer of molecules responsible for the pull was far too small for her to see directly. We’ll see how in a minute; but first, back to the bath.
A bath full of pure water is a jostling swarm of water molecules playing a very crowded game of bumper cars. But one of the things that makes water such a special liquid is that all those molecules are very strongly attracted to all the other water molecules around them. Each one has a larger oxygen atom and two small hydrogen atoms (that’s the two Hs and an O in H2O). The oxygen sits in the middle with the two hydrogens stuck to it on either side, making a shallow V-shape. But although the oxygen is very strongly attracted to and bonded to its own two hydrogen atoms, it’s also flirting with any others that happen to go by. So it’s constantly tugging on the hydrogen from other water molecules. This is what holds water together. It’s called hydrogen bonding, and it’s very strong. In the bath, water molecules are constantly pulling on the other water molecules around them, tugging the whole mass of water together.
The water molecules on the surface are a bit left out. They are being pulled by the water molecules underneath them, but there’s nothing above them to pull back. So they’re being pulled down and sideways but not up: and the effect of this is to make the surface behave like an elastic sheet, pulled tight over all the water molecules below the top layer, and pulling itself inward so that it is as small as possible. This is surface tension.
As you run the tap, air gets carried downward into the bath, making bubbles. But when these bubbles float up to the surface, they can’t last. The round dome of the bubble is stretching the surface and the surface tension isn’t strong enough to haul it back. So the bubbles burst.
One of the things that Agnes did was to set up her button so that it was being pulled upward, but not quite hard enough to pull it off the surface. And then she touched the surface of the water nearby with a drop of something like detergent. After a second or so, the button would pop off the surface. The detergent had spread across the water, and it had reduced the surface tension. All it takes to reduce the surface tension is to provide a thin top layer so that the water molecules don’t have to be the ones right at the surface.
When it’s finally time to add the bubble bath, it’s time to say goodbye to a clean, flat, minimal surface. That dollop of scented gloop gets carried down into the water and immediately does its best to hide at the edges. Each molecule has one end that loves water and one end that hates water. If the end that hates water can find some air, it will stay with it, but the water-loving end isn’t giving in either. So anyplace where water touches air, a thin layer of bubble bath sits right at that surface. It’s just one molecule thick, and each molecule is the same way around so that the water-loving ends are all still submerged in water, and the water-hating ends are all still in air. With this thin coating, a large surface isn’t a problem. The bubble bath doesn’t provide the strong pull that water does, so the elastic sheet effect becomes really weak. It’s time for a surface party, and that’s what the foam is. By reducing the surface tension, bubble bath makes it easier for bubbles to last because their large surface is much more stable.