COFFEE IS A fantastically valuable global commodity, and the precise black magic needed to extract perfection from this humble bean is a constant source of debate (and some snobbishness) for connoisseurs. But my particular interest in it doesn’t depend on how it was roasted or the pressure in your espresso machine. I’m fascinated by what happens when you spill it.* It’s one of those everyday oddities that no one ever questions. A coffee puddle on a hard surface is unremarkable, just a patch of liquid in a blobby shape. But if you leave it to dry, you’ll come back to find a brown outline, reminiscent of the line drawn around the body in a detective drama from the 1970s. It was definitely filled in to start with, but during the drying process all the coffee has moved to the outside. Scrutinizing a coffee puddle to see what’s going on is the caffeine waster’s equivalent of watching paint dry, but even if you tried, you wouldn’t see very much. The physics shunting the coffee around only operates on very small scales, mostly too small for us to see directly. But we can definitely see their consequences.
If you could zoom in on the puddle, you’d see a pool of water molecules playing bumper cars, and much bigger spherical brown particles of coffee drifting around in the middle of the game. The water molecules attract each other very strongly and so if a single molecule lifts up a bit from the surface, it immediately gets pulled back down to join the horde below. This means that the water surface behaves a bit like an elastic sheet, pulling inward on the water below it so that the surface is always smooth. This apparent elasticity of the surface is known as surface tension (of which much more a little later). At the edges of the puddle, the water surface curves downward smoothly to meet the table, holding the puddle in place. But the room is probably warm, and every so often, a water molecule escapes from the surface completely and floats off into the air as water vapor. This is evaporation, and it happens gradually, and only to the water molecules. The coffee can’t evaporate, so it’s effectively trapped in the puddle.
The clever bit happens as more and more water escapes, because the water edge is pinned to the table (we’ll see why a bit later). Water is so strongly stuck to the table that the edge has to stay where it is. But evaporation is happening at the edges more quickly than from the middle, because a higher proportion of the water molecules are exposed to the air there. The bit that you can’t see (as you try to persuade your coffee companion that watching paint dry really is the latest thing) is that the contents of the puddle are on the move. Liquid coffee from the middle must flow out to the edges to replace the lost water. The water molecules carry the coffee particles along as passengers, but when it’s their turn to escape into the air, the coffee can’t join them. So the coffee particles are gradually carried out to the edges, and once the water has completely gone, all that is left is a ring of abandoned coffee.
The reason I find this so fascinating is that it happens right in front of your nose, but all the interesting bits are just too small to watch properly. This world of the small is almost a whole different place. The rules that matter are different down there. As we’ll see, the forces we’re used to, like gravity, are still present. But other forces, the ones that arise because of the way molecules dance around each other, start to matter more. When you dive down into the world of the small, things can seem very weird. It turns out that the rules that operate on this small scale explain all sorts of things in our larger-scale world: why there’s no cream on the milk anymore, why mirrors fog up, and how trees drink. But we’re also learning to use those rules to engineer our world, and we’ll see how they’re going to help us save millions of lives through improved hospital design and new medical tests.
BEFORE YOU CAN worry about things that are too small to see, you have to know that they’re there. Humanity faced a catch-22 here—if you don’t know there’s anything there, why would you go looking for it? But all of that changed in 1665 with the publication of one book, the first scientific bestseller: Robert Hooke’s Micrographia.
Robert Hooke was the Curator of Experiments at the Royal Society, and so he was a generalist, free to roam among the scientific toys of the day. Micrographia was a showcase for the microscope, designed to impress the reader with the potential of this novel device. The timing was perfect. This was an era of great experimentation and rapid advances in scientific understanding. Lenses had been lurking around the edges of human civilization for a few centuries, mostly unappreciated and seen as novelties rather than serious tools for science. But with Micrographia, their moment had arrived.
The wonderful thing about this book is that although it wears the robes of respectability and authority, as befits a publication of the Royal Society, it’s unashamedly the product of a scientist at play. It’s full of detailed descriptions and beautiful illustrations, expensively produced and carefully presented. But underneath all that, Robert Hooke was basically doing what every child does when given a microscope for the first time. He just went around looking at everything. There are stunningly detailed pictures of razor blades and nettle stings, grains of sand and burnt vegetables, hair and sparks and fish and bookworms and silk. The level of detail revealed in this tiny world was shocking. Who knew that a fly’s eye was so beautiful? In spite of the careful observations, Hooke didn’t make any claims to in-depth study. In the section on “gravel in urine” (the crystals commonly observed on the insides of urinals), he speculates on a way of curing this painful affliction and then happily leaves the hard work of actually solving the problem to someone else:
It may therefore, perhaps, be worthy some Physicians enquiry, whether there may not be something mixt with the Urine in which the Gravel or Stone lies, which may again make it dissolve it, the first of which seems by it’s regular Figures to have been sometimes Crystalliz’d out of it. . . . But leaving these inquiries to Physicians or Chymists, to whom it does more properly belong, I shall proceed.
And proceed he does, dancing through mold and feathers and seaweed, the teeth of a snail and the sting of a bee. On the way, he coins the word “cell” as a description of the units that made up cork bark, marking the start of biology as a distinct discipline.