The tiny world where surface tension, capillary forces, and viscosity dominate over gravity and inertia has always been a part of our everyday life. The mechanisms may be invisible, but the consequences are not. And these days we’re not just spectators admiring the elegance and exoticism of what happens down there. We’re beginning to be engineers, working within it. There’s a word for the rapidly developing field of Lilliputian plumbing, the manipulation and control of fluids flowing through narrow channels: “microfluidics.” It’s not a familiar word to most of us now, but it’s going to have a huge impact on our lives in the future, especially when it comes to medicine.
Today, people with diabetes can monitor their blood sugar using a simple electronic device and a test strip. A tiny drop of blood touched to the test strip will immediately whoosh into the absorbent material due to capillary action. Tucked away in the tiny pores of the strip is an enzyme, glucose oxidase, and when this reacts with blood sugar it produces an electrical signal. The hand-held device measures that signal, and voilà!—an accurate measure of blood sugar appears on the screen. It’s easy to see this as a description of the obvious—paper soaks up a fluid so that it can be measured. So what? But this is just a crude demonstration of the principle. It gets a lot more sophisticated than that.
If you can move a fluid through tiny tubes and filters, gather it in reservoirs, mix it with other chemicals along the way, and see the results, you have all the components of a chemistry lab. No need for glass test tubes, hand-held pipettes, and microscopes. This is the premise of the growing “lab-on-a-chip” industry, the development of tiny devices to carry out medical tests. Nobody likes having a whole vial of blood extracted from them, but a single drop isn’t too hard to part with. Smaller diagnostic devices are often cheaper to make and easier to distribute. And you don’t even need to make them from fancy modern materials like polymers or semiconductors. Paper might do just fine.
A group of researchers at Harvard, led by Professor George Whitesides, is on the case. They have engineered diagnostic test kits about the size of a postage stamp, made of paper, but containing a maze of water-loving paper channels with waxed water-hating walls. When you touch a drop of blood or urine to the correct part of the paper, capillary action drags it through the main channel, splits it up, and reroutes it to lots of different test zones. Each one contains the ingredients to do a different biological test, and each reservoir will change color depending on the test results.§§§ The researchers suggest that someone a long way from a doctor could do the test locally, take a picture of the result with their phone, and e-mail it to a distant expert who could interpret it. As ideas go, it’s brilliant. Paper is cheap, the device doesn’t require power, it’s lightweight, and a flame is all you need to dispose of it safely. As with all these devices, it’s got a lot of checks and balances to face before we know whether the simple-sounding idea can deal with the real world. But it’s hard not to be convinced that one way or another, devices like this will be a big part of medicine in the future.
The genius of all this is that when we look at a problem, we may be able to choose to do the engineering on the size scale that makes the problem easiest to solve. It’s like being able to choose which laws of physics you want working for you. Small really is beautiful.
* Sorry. Really. If it helps, what I’m about to say is just as true for instant coffee, so you don’t ever need to waste shots of fancy coffee on science.
? We can go a long way into the world of the small without having to deal with the strangeness of quantum mechanics. That really kicks in when you explore what’s happening to individual atoms and molecules, and there’s an awful lot that’s bigger than that and still smaller than what we can see. That middle bit is interesting because we can understand it intuitively (something which is impossible by definition when it comes to the rules of the quantum world), even though we can’t see it clearly.
? As someone who loves the variety and spice of life, I’m always a bit sad when I see this word. Making everything the same definitely has its uses, but sometimes it does just sound as though it’s taking the fun out of life. Especially if you’re a blue tit.
§ Their rise is slowed even more by the extra protein coat that surrounds each of the new smaller globules; this weighs them down a bit, so they’re even less buoyant than they were before. This has been measured in quite a lot of detail. You’d be surprised at how much science has gone into a pint of milk.
? If you’re interested in reading more on this, the biologist J. B. S. Haldane wrote a very famous short essay in the 1920s called “On Being the Right Size.” It’s here: http://irl.cs.ucla.edu/papers/right-size.html. The most memorable quote from it is very painfully true: “To the mouse and any smaller animal it [gravity] presents practically no dangers. You can drop a mouse down a thousand-yard mine shaft; and, on arriving at the bottom, it gets a slight shock and walks away, provided that the ground is fairly soft. A rat is killed, a man is broken, a horse splashes.” As far as I’m aware, no one has actually done this specific experiment. Please don’t be the first. And certainly don’t blame me if you do.
# If you keep stirring your milk, the cream won’t rise to the top because it keeps getting mixed back in. The same principle applies here—particles don’t sink down very far because they keep getting mixed back in by air currents that move faster than they’re falling.
** Especially mine. I am a bubble physicist, after all.
?? You can see this effect for yourself if you put a droplet of water on something that is fairly hydrophobic—a tomato does the job. The droplet will sit up, mostly off the surface. Then just touch it with a cocktail stick with a tiny bit of detergent on the end, and the drop will immediately spread out sideways. I recommend washing the detergent off the tomato before eating it.
?? This balance—how much water is attracted to a solid surface compared with how much it’s attracted to itself—helps with all sorts of problems. The most important one for any Brit is the question of why some teapots dribble from the spout as you finish pouring, sending tea down the side of the pot and onto the table rather than into the cup. The answer is that the teapot is just too attractive to water. As the flow slows, the forces sticking the water to the spout dominate over the momentum carrying the water forward. You can solve this by having a hydrophobic teapot, one that doesn’t attract the tea at all. Sadly, at the time of writing, no one seems to be selling them.
§§ It was actually an apology. On a trip to Krakow, I had promised a fabulous dinner in the Jewish quarter of the city, but this was before the days of smartphones, so I got lost. I led twelve hungry people on a merry dance around many dark and empty streets, failing to find any restaurants at all, never mind the excellent ones I was aiming for. We ended up eating in a McDonald’s instead. I felt that apple pie was the least I could do to make up for this.
?? Of course, the fat and protein and sugar in the milk don’t evaporate, so they’re left behind and the towel still needs a wash.
## The clock tower of Westminster, the one that houses Big Ben, is 315 feet tall. These trees really are giants.