It’s not traditional to take up gardening during your final year as an undergraduate, but the house I was sharing with three friends had a garden, and the temptation was too much. In the odd spare hours between work and sport that year, I enthusiastically chopped down the huge forest of nettles that had taken over, and discovered buried treasure in the form of rhubarb plants and rose bushes. My dad laughed at me for planting potatoes (“typical Pole,” he said), but they were only part of my new vegetable patch. Most exciting of all, there was a grimy greenhouse, filled with rubble and a grapevine. Seedlings (leeks and beetroot, I think) could grow in shelter before joining the vegetable patch in the spring. In late February I sowed seeds in trays, and waited for new plants to grow.
After a while, it was noticeable that there weren’t any seedlings, but there were a lot of snails. I’d arrive with my watering can to find a smug mollusk parked in the middle of each tray, surrounded by bare soil and the occasional green hint of a chewed-off shoot. Not to be defeated, I chucked the snails outside, resowed the seeds, and placed the trays on top of bricks to make it harder for the snails to crawl in. Two weeks later, the nascent seedlings were gone, and there were more snails than ever. I tried a few different approaches, none successful, until I had only one idea left. This time, I took pairs of empty flower pots and balanced upside-down tea trays on top, so they were like giant mushrooms with two stalks. I greased the edges of each one, and put the seedling trays on top of the tea-tray mushrooms. After replacing the compost, I sowed the last of the seeds, crossed my fingers, and went back to studying condensed matter physics.
The seedlings grew undisturbed for about three weeks. And then the inevitable day came when I found one fat, happy snail where the seedlings should have been. I remember standing in the greenhouse, analyzing in forensic detail the possible routes that this creature could have taken. There were only two. Option one: It could have crawled up the inside walls of the greenhouse and out across the underside of the roof, and then somehow have dropped off at exactly the right location to land on a seed tray. This seemed unlikely. Option two: It had crawled along the bench and up the flower-pot sides, slimed its way upside down to the outer edge of the tea tray, crawled around the edge without falling off, and then trekked along the top of the tea tray to the seedlings. In either case, I had to admit that it had probably earned the bounty.? How could a snail do that? Both cases involved it crawling upside down, while glued to the surface only by its own mucus. If you watch a snail move, it’s different from a caterpillar—it doesn’t lift itself off the surface as it goes. It’s just stuck to its slime, and yet somehow it manages to shift itself along. But that slime is the snail’s secret weapon, because it behaves just like ketchup.
If you watch a snail moving, you won’t see very much because the outer rim of its foot is just moving at a constant slow speed. Everything on the edges is happening slowly, so the mucus is like the stationary ketchup: thick and gloopy and hard to move. But underneath, in the middle, muscular waves are traveling from the back of the snail to the front. Each wave is pushing forward on the mucus really hard, and it’s forcing the mucus to shift very quickly. And just like the ketchup, the mucus is shear-thinning, so if you’re shunting it very fast it suddenly flows very easily. The snail is sailing over the top of this liquid mucus on those muscular waves, taking advantage of the lower resistance. It needs the thick slime as well, so that it has something to push against. The only reason why snails (and slugs) can move is that the same mucus can behave either like a solid or a liquid, depending on how fast they force it to move. The huge advantage of this method is that they don’t fall off the underside of things because they never lift themselves away from the surface.
How does the slime manage this trick? It’s a gel of very long molecules called glycoproteins, all mixed up together. When it’s just sitting still, chemical links form between the chains, so it behaves like a solid. But when you push hard enough, the links suddenly break, and all the long molecules can slither over each other like strands of spaghetti. Let it sit still again, and the links will re-form; and after only a second, you’ll have a gel again.
If I had known all this, could I have protected the seedlings? Not by choosing a surface that the snails couldn’t stick to or climb over, it turns out. The mucus can stick to pretty much anything you’d find around the house—including non-stick pan liners. Experiments have shown that snails can even stick to super-hydrophobic surfaces, the ones that water can hardly touch. It’s a pretty amazing achievement, but probably one best appreciated by those who don’t have precious seedlings to protect.
The same mechanism also explains non-drip paint. When this paint sits still, it’s thick and gooey. But when you push on it with a brush, it becomes much less viscous, and it’s easy to spread a thin, even layer of it over a wall. As soon as you take the brush away, the paint goes back to being very viscous and so it doesn’t run off down the wall before it’s dried.
KETCHUP AND SNAILS are small, but this same bit of physics can have serious consequences on a much larger scale. Christchurch in New Zealand was a charming and peaceful city when I visited it in 2002. The land there is made up of sediment, layer upon layer of tiny particles deposited by the Avon River over successive millennia. It’s a beautiful location, but the city was sitting on a time bomb. At 12:51 p.m. on February 22, 2011, a magnitude 6.3 earthquake struck just 6 miles away from the city center. The earthquake itself was bad enough, throwing people into the air and tearing buildings apart. But the sediment that the town was built on was only strong and solid if it was stationary. Just like the ketchup, powerful shaking turned it into a liquid. The small-scale details are a tiny bit different—instead of the bonds between long molecular chains breaking, water sneaks in between sand grains and pushes them apart, allowing them to flow. But the overall physics is the same: When it’s agitated quickly, the solid ground starts to flow like a liquid.