The Son of Man. The Lamb of God. The King of Kings. The Knave of Hearts. The Sultan of Swat. Jesus of Nazareth, also known as the Prince of Peace, and in America, the God of War, was said to perform a string of miracles at the beach town of Tiberias on the Sea of Galilee in Israel. One of them involved catching a great deal of fish with one net. Another, feeding several thousand people with very little food. And yet another involved walking on water to meet a boat full of his disciples, who were caught in a sudden storm.
Now, Clarke’s Third Law states that “any sufficiently advanced technology is indistinguishable from magic.” So nowadays, the miracle of the modern fishing industry, with its deep-sea trawlers, 150-mile longlines, and space-age tracking and echolocation technology, ensure that our nets can catch hundreds of thousands of fish at a time. (Though not for much longer.) And genetic engineering, bolstered by mechanized farming and artificial fertilizers, ensures we can feed the multitudes. (Though not for much longer.) But biologically and technologically speaking, how miraculous is it to walk on water?
Not very, if you’ve got the right tools, and the right size. The most classic example of animal locomotion on the water’s surface is the water striders, or water skaters, or water scooters, or any of the other collective names for these 500 species of insects that make up the Gerridae family. They are hunters that use surface tension to their advantage; where prey might swim, they float like a bubble. Their short front legs are for grabbing, their middle pair for “skating,” and the hind pair act as rudders. The secret to their unsinkability is the hydrophobic hairs on their legs. Each leg is covered in thousands of fine filaments called microsetae that spread the weight out on the water’s electric “skin” of surface tension, and the grooves in each filament trap tiny air bubbles which add to their buoyancy. So powerful is the effect that a water strider could carry fifteen times its own weight and still remain afloat, and a few species have even adapted to walk the waves of the open ocean.
But it’s not only insects that have the ability to walk on water. A few reptiles have also evolved to stay high and dry. And more advanced insects have discovered not just how to walk on water, but how to turn water into wine.
The most famous of these is the basilisk, also known as the Jesus Christ lizard. This small Central American lizard has the ability to run bipedally across the water’s surface for up to 15 feet. But the secret to the basilisk’s buoyancy was only recently revealed. The reason that it skeedaddles across the water where you would simply faceplant in a sandbar isn’t just a matter of size, but technique. Speed is a factor, but so is the stroke of its feet, which first slap the water vertically, then pushes back to “swim” with its toes before recovering the stroke. The downward slap makes the water highly resistant to the surface of the foot (if you’ve ever heard that hitting the water after jumping off the Golden Gate Bridge is like hitting concrete, it’s the same principle), but also traps a large bubble beneath the basilisk’s heel. So far, it seems that the two animals we’ve examined who walk on water aren’t walking on water as much as they’re walking on air.
What, then, to make of the pygmy gecko of Brazil? At just 2-4 cm long, this dollhouse reptile is so small that it can rest on the surface of puddles. But it only does this with the help of its hydrophobic, or water-repellent, skin. Because water cannot get a grip on the skin, and the gecko is less dense than the water, it simply cannot do anything but float — a handy adaptation when raindrops, to you, are the size of cannonballs, and every pond is an ocean.
(Side Note: I apologize for using two David Attenborough-narrated BBC nature videos in a row. I will stop using BBC video when anyone else produces nature documentaries that are anywhere near the quality.)
What exactly does that mean, to have hydrophobic skin? I mean, humans have hydrophobic skin, for the most part, but we still need floating deck chairs. What substance in the gecko’s skin makes it so hydrophobic that the animal performs a seeming miracle? Fat is a hydrophobic substance, but in this case it’s likely to be either wax or oil. A water strider’s feet are waxy, which keeps their microvellae from getting waterlogged. The terms “hydrophobic” and “water-repellent” are misleading, as it is actually the water, with its polarized electrical charge, that is repelling the non-polarized object, strongly preferring to bond only with itself. In other words, it’s not that a “hydrophobic” substance is afraid of water. Technically, it’s just that when it comes to waxy or greasy substances, water gets a little grossed out. Water can be a snob.
And what happens when two liquids with different surface tensions, like oil and water, meet? They slip. The transfer of mass along the interface between oil and water is called the Marangoni Effect, named for an Italian physicist. Of course, oil poured into water will float because of the difference in density, but that doesn’t explain how it moves. Surface tension is a contractile force, meaning that it pulls instead of pushing other objects. So a solution of two different liquids will have a gradient of surface tensions, with the areas of high surface tension pulling more liquid than the areas with low surface tension.
A very pleasant way to observe the Marangoni effect — or anything, really — is with a glass of wine. Wine is a solution of water and alcohol, alcohol having the lower surface tension. If you swirl wine around in the glass, you will notice a halo of liquid from which is dripping what the melancholic French call “tears,” the pious Germans call “church windows,” and the horndog Americans call “legs.” It used to be said that only a good wine had legs, but all it proves is that the wine is a) alcoholic and b) unhomogenously mixed. By swirling the wine, you make a halo of low-surface tension alcohol from which the “legs” or “tears” drip away in rivulets, pulled by the high-surface tension water. Bam. Marangoni.
You can observe it by putting a drop of dish detergent in a bowl of water. Unable to bond with the soap, the water pulls itself away. To illustrate how the Marangoni effect is used in biology, let’s return to the water striders. Their waxy feet make water pull itself away rather than pulling the feet under. But there’s an even cooler application: another family of waterbugs called the Microvelia. Not only can they walk on the surface of the water, they can propel themselves forward using the Marangoni effect. They dip the tips of their abdomens in the water and release a hydrophobic surfectant — an oil, basically — and by repelling the water, can scoot forward in the water at twice their walking speed like a propeller-less motor boat. So maybe it’s impossible to literally turn water into wine, but there’s at least one type of animal that can create tears of wine in the water. It’s powered by surface tension, and turns oil into velocity. And this summer, it will be puttering around in a pond near you. Maybe it’s short of performing a truly grand feat of magic, but it’s a pretty nifty trick. As they say, thank God for small miracles.