Hooke hadn’t just shown the way to the world of the small; he’d thrown open the doors and invited everyone in for a party. Micrographia inspired some of the most famous microscopists of the following centuries, and also whetted the scientific appetite of fashionable London. And the fascination came from the fact that this fabulous bounty had been there all along. The annoying black speck buzzing around rotting meat was now revealed as a minute monster with hairy legs, bulbous eyes, bristles, and shiny armor. It was a shocking discovery. By then, great voyages had crossed the world, new lands and new people had been discovered, and there was great excitement about what was to be found in faraway places. It hadn’t really occurred to anyone that navel-gazing might have been severely underrated, and that even belly-button fluff might have much to say about the world. And once you’d got over the shock of the flea’s hairy legs, you could see how they worked. The world down there was mechanical, it was comprehensible, and the microscope made sense of things that humans had noticed for years but hadn’t been able to explain.
But even that was just the start of the voyage into the world of the small. Over two centuries more would pass before the existence of atoms was confirmed, each one so tiny that you’d need 100,000 of them to make a line as long as a single one of the cork cells. As the famous physicist Richard Feynman was to point out many years later, there’s plenty of room at the bottom. We humans are just lumbering about in the middle of the size scales, oblivious to the minuscule structures that our world is built of and built on. But 350 years after the publication of Hooke’s Micrographia, things are changing. We can do more than just peer into that world like a child peering into a protective glass museum case, forbidden to touch. Now we’re learning to manipulate atoms and molecules on that scale; the glass is off the case, and we can join in. “Nano” is in fashion.
A major part of what makes the world of the tiny both fascinating and extremely useful is that things work differently at that level. Something that’s impossible for a human might be an essential life skill for a flea. All the same laws of physics apply; the flea exists in the same physical universe as you and me. But different forces take priority.? Up here in our world, there are two dominant influences. The first is gravity, pulling us all downward. The second is inertia; because we’re so big, it takes a lot of force to get us moving or to slow us down. But as you get smaller, gravitational pull and inertia also get smaller. And then they find themselves in competition with the other weaker forces that were there all along, but insignificant. There’s surface tension, the force shifting the coffee granules about as the coffee puddle dries. And then there’s viscosity. Viscosity in the world of the small is why we don’t get a nice layer of cream on top of the milk anymore.
It was always the gold-and silver-topped milk bottles they went for. If you were early enough, and careful when you opened the front door, you’d catch them at it. Bright-eyed perky little birds, perched on the top of the bottle, snatching hasty sips of cream from the hole they’d pecked in the thin aluminium bottle top while keeping a beady eye on the world around them. As soon as they knew they were caught, they were off, probably to try their luck at the neighbor’s doorstep. For about fifty years in this country, blue tits were masters at stealing cream. They learned from each other that right below the flimsy lid there was a rich fatty treasure, and that knowledge spread throughout the UK blue-tit population. Other bird species didn’t seem to cotton on to this trick, but the blue tits were waiting for the milkman every morning. And then the game came to an end quite suddenly, not just because of plastic milk bottles but because of something more fundamental. For as long as humans have been milking cows, the cream has risen to the top. But these days, it doesn’t.
The bottle that the hungry blue tit was hopping about on contained a mixture of all sorts of goodies. Most of milk (nearly 90 percent) is water, but floating around in that are sugars (that’s the lactose that some people can’t tolerate), protein molecules assembled into minuscule round cages, and bigger globules of fat. All of this is jumbled up together, but if you leave it to sit for a while, a pattern emerges. The fat globules in milk are tiny—between one and ten microns in size, which means you could fit somewhere between 100 and 1,000 of them in a line between the millimeter markers on a ruler. And those tiny blobs are less dense than the water around them. There’s less “stuff” in the same volume of space. So as they’re being jostled about with everything else, there’s a tiny difference in where they go. Gravity is pulling the water around them downward a tiny bit harder than it’s pulling the fat globules, and the fat is very gently squeezed upward. That means that the fat is ever so slightly buoyant, and will very slowly rise up through the milk.
The question is: How fast will it rise? And here’s where the viscosity of the water starts to matter. Viscosity is just a measure of how hard it is for one layer of a fluid to slide over another layer. Imagine stirring a cup of tea. As the spoon goes around, the liquid around the spoon has to move, flowing past other liquid next to it. Water isn’t very viscous, so it’s very easy for those layers to move past each other. But then think about stirring a cup of syrup. Each sugar molecule is holding on to the ones around it very firmly. To move these molecules past each other, you’ve got to break those bonds before the molecules can move on. So it’s hard work to shunt the fluid about, and we say that the syrup is viscous.
In the milk, the fat globules are pushed upward because they’re buoyant. But if they want actually to move upward, they have to shove the liquid around them out of the way. As part of that pushing process, the nearby liquid has to slide over itself, so its viscosity matters. The more viscous it is, the more resistance there is to the fat globules rising.
Right under the blue tit’s feet, this battle is going on. Each fat globule is being pushed upward by its buoyancy, but it experiences a drag force because of the liquid around it having to move to let it pass. And the same forces acting on the same sort of fat globule come to a different compromise for different globule sizes. The drag has a much greater effect when you’re small, because you have a large surface area relative to your mass. You’ve only got a small buoyancy to use to shove quite a lot of the surrounding stuff out of the way. So even though the smaller fat globule is in exactly the same liquid, it rises more slowly than a bigger one. In the world of the small, viscosity generally trumps gravity. Things move slowly. And your exact size matters a lot.
In the milk, the larger fat globules rise faster, bump into some smaller, slower ones, and stick to them, forming clusters. These clusters experience less drag for their buoyancy because they’re even bigger than individual globules, so they rise even faster. The blue tit just has to sit and wait at the top, and breakfast will arrive at its feet.