Storm in a Teacup: The Physics of Everyday Life(18)

Gravity pulling on solid objects is a familiar concept, mostly because we’re solid objects that are pulled on. But around the solid objects in our world, fluids are flowing—air and water are shifting around in response to the forces acting on them. I think it’s a great tragedy that we can’t usually see fluids shifting as clearly as we see leaves falling or bridges rising. Liquids experience the same forces, but they aren’t limited to holding the same shape, and so the world of fluid dynamics is beautiful: sweeping, whirling, meandering, surprising, and everywhere.

The lovely thing about bubbles is that they are everywhere. I think of them as the unsung heroes of the physical world, forming and popping in kettles and cakes, bioreactors and baths, doing all sorts of useful things while often only fleetingly in existence. They’re such a familiar part of our background that we often don’t really see them. A few years ago, I was asking groups of five-to eight-year-old children where you might find bubbles, and they all happily told me about fizzy drinks, baths, and aquariums. But the last group of the day were tired, and my cheerful encouragement was met with a grumpy silence and blank stares. After a long pause, and a lot of shuffling, one unimpressed six-year-old stuck his hand up. “So,” I said brightly, “where might you find bubbles?” The boy stared at me with a do-I-really-have-to glare, and announced loudly: “Cheese . . . and snot.” I couldn’t fault his logic, although I had never thought about either. It seemed likely that his experience of bubbly snot outranked mine, anyway. But for at least one animal, bubbly snot is the key to a whole lifestyle. Meet the purple marine snail, janthina janthina.

The snails that live in the sea generally scoot about on the ocean floor or on rocks. If you were to pry one off its rock, carry it a little way up into the water, and let go, it would sink. The ancient Greek polymath Archimedes (he of “Eureka” fame) was the first to work out the principle that determines when something is going to float and when it’s going to sink. He was probably much more interested in ships, but the same principle applies to snails and whales and anything else which is submerged or semi-submerged in any fluid. Archimedes worked out that there is effectively a competition between the submerged object (our snail) and the water that would be there if the snail wasn’t. Both the snail and the water around it are being pulled downward toward the center of the Earth. Because the water is a fluid, things can move around in it very easily. The gravitational pull on an object is directly proportional to its mass—double the mass of your snail and you double the pull on it. But the water around it is also being pulled downward, and if the water is pulled down more, the snail will have to float upward so there’s more room for water underneath. Archimedes’ Principle, stated for our hapless mollusk, is that there is an upward push on the snail equal to the downward gravitational pull of the water that should have been in that space. This is the buoyancy force, and every submerged object experiences it. In practical terms, it means that if the snail has a greater mass than the water that would fill a snail-shaped hole, it will win the gravitational battle and sink. If the snail has less mass (and is therefore less dense), the water will win the battle to be pulled downward and the snail will float. Most marine snails are more dense than seawater overall, and so they sink.

For most of their history, marine snails just sank and that’s the way it was. But at some point in the past, a “normal” marine snail had a bad day and got an air bubble trapped in its egg cases. The clever bit about buoyancy is that it’s only the average density of the overall object that counts. You don’t have to change the mass of the object. You can just change the space it takes up—and air bubbles take up lots of space. One day, a bigger air bubble was trapped, the balance tipped the wrong way, and the first marine snail took flight through the water and drifted up toward the sunlight. The door to the vast larder at the sea surface had been opened . . . but only for a snail that could puff itself up; and so evolution got to work.

Today, janthina janthina, the descendant of the first snails that got lost in space, is common in the warmer oceans of the world. Now bright purple, the snails secrete mucus, the same sort of slime you see on garden stones in the early morning, and use their muscular foot to fold the mucus and trap air from the atmosphere. They build themselves a large bubble raft, often bigger than themselves, to ensure that their total density is always less than the seawater they’re in. So they always float, upside down (bubble raft up, shell beneath), preying on passing jellyfish. If you see a purple snail shell on a beach, it’s probably from one of these.

Buoyancy can be a very useful and quick indicator of what’s inside a sealed object. For example, if you take identically sized cans of a fizzy drink, one diet version and one with a full load of sugar, you’ll see that the diet can floats in fresh water and the other one sinks. The cans have exactly the same volume, so the difference is all inside, and it’s all that dense sugar. A standard 12-ounce can of pop has 1.2–1.7 ounces of sugar inside it, and that extra mass counts, making the can overall more dense than water. That means it beats the water in the battle with gravity, and so it sinks. The mass of sweetener in diet pop is minuscule, so that can is basically just filled with water and air, and it floats. A slightly more useful example is a raw egg. Fresh eggs are more dense than water, so they sink and lie flat in cold water. But if they’ve been sitting in your fridge for a few days, they’ll have been gradually drying out, and as the water sneaks out of the shell, air molecules sneak into an air sac at the rounded end to fill the gap. An egg that’s about a week old will sink but stand up on the pointy end (so that the additional air is closer to the surface). And if the egg floats completely, it’s been around for a bit too long—have something else for breakfast!