Storm in a Teacup: The Physics of Everyday Life(30)

** 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.

*** One nanometer really is tiny—there are 25.4 million of them in one inch.

??? But there’s a limit. To increase the tension in the water to pull it up farther, the stomata must get smaller. And smaller stomata let in less carbon dioxide, so there is less raw material for photosynthesis. Theory suggests that the tallest a tree could possibly grow is 400–425 feet, because beyond that it wouldn’t be able to take in enough carbon dioxide to do any actual growing.

??? There’s also some evidence to suggest that the fog might actually go the other way too—entering the stomata to keep them full of water, not just preventing evaporation.

§§§ These devices go by the catchy name of “microfluidic paper-based electrochemical devices” or uPADs for short. A non-profit organization called Diagnostics for All has been set up to move this idea into the real world.





CHAPTER 4




A Moment in Time


ON A LAZY Sunday at lunchtime, an English pub is the place to be. The innards of these establishments often give the impression of having been grown rather than designed—a cluster of oddly-shaped spaces hidden in an ancient oak skeleton. You park yourself at a table positioned between polished brass bedpans and pictures of prize pigs, and order a proper pub lunch. It always arrives with a bowl of fries and a glass bottle of ketchup, but this combination comes at a price. For decades, these oak beams have witnessed an enduring ritual. The ketchup must be extracted from the bottle, and it won’t happen without a fight.

It starts when one optimist picks up the ketchup and just holds it upside down over the bowl of fries. Nothing ever happens, but almost no one skips this step. Ketchup is thick, viscous stuff, and the feeble pull of gravity isn’t enough to shift it from the bottle. It’s made this way for two reasons. The first is that the viscosity stops the spices sinking downward if the bottle is abandoned for a while, so you don’t ever need to shake it to make sure it’s well mixed. But more importantly, people prefer a nice thick coating on each French fry and you can’t get that if the ketchup is runny. However, it’s not on the fry yet. It’s still in the bottle.

After a few seconds, having established that this bottle of ketchup is just as immune to gravity as every other one they’ve ever encountered, the hopeful fry-eater starts to shake the bottle. The shaking gets gradually more violent, until it’s time to try thumping the bottom of the bottle with the free hand. Just as the others at the table are starting to lean back to avoid the fracas, a quarter of the contents of the bottle whooshes out all at once. What’s weird is that the ketchup clearly can flow very easily and quickly—the thick blanket of ketchup now covering the bowl (and probably half the table too) provides ample evidence of that. It just doesn’t, until it does, and then it flows with considerable enthusiasm. What’s going on?

The thing about ketchup is that if you try pushing on it very slowly, it behaves almost like a solid. But once you force it to move quickly, it behaves much more like a liquid and flows very easily. When it’s sitting in the bottle or perched on a fry, it’s only being pulled weakly by gravity, so it behaves like a solid and stays put. But if you shake it hard enough and start it moving, it behaves like a liquid and moves very quickly. It’s all about time. Doing the same thing quickly and slowly gives you completely different results.

Ketchup is mostly sieved tomatoes, jazzed up by vinegar and spices. Left to itself, it’s thin and watery, and doesn’t do anything interesting. But lurking in the bottle is 0.5 per cent of something else, long molecules made up of a chain of linked sugars. This is xanthan gum: Originally grown by bacteria, it’s now a very common food additive. When the bottle is standing on the table, these long molecules have surrounded themselves with water and are slightly tangled up with other similar chains. They hold the ketchup in place. As our ketchup enthusiast shakes the bottle harder, these long molecules get slightly untangled, but they re-tangle pretty quickly. As the thumping on the bottom of the bottle shunts the ketchup around more quickly, the tangles keep breaking, and at some point they’re pushed out of place more quickly than they’re re-tangling. Once this critical point is passed, the solid-like behavior vanishes and the ketchup is on its way out of the bottle.*