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Science
by Pat Murphy & Paul Doherty

QUICKSAND AND KETCHUP

 
WHAT DO ketchup, Silly Putty, and the soil of the planet in Vonda McIntyre's short story "Screwtop" have in common?

Need a hint? To make it easier, we'll add a few more items to the list: corn starch mixed with water, quicksand, and mayonnaise. Does that help?

We'll get to the answer in a bit, but first we're going to talk about the strangeness of ketchup.
 

YOU WANT FRIES WITH THAT?

In our roles as scientists and science fiction writers, we look for anomalies—stuff that's weird in one way or another. Anyone who has ever tried to douse a French fry with the gooey red sauce that's a favorite of most nine-year-olds will acknowledge that ketchup is weird.

You grab a bottle from the refrigerator door and tip it, but the ketchup doesn't flow. So maybe you shake the bottle or maybe you thump on the bottom or maybe you take the advice of the folks at Heinz and tap with two fingers on the bottle neck, right about where the 57 is on the Heinz bottle. Subjected to your ministrations, ketchup flows—maybe nicely or maybe in a big glob. One way or the other, it lands on your fries—and then it sits there, not flowing away, but staying put.

That brings us to Isaac Newton.

Newton probably never actually encountered ketchup. Ketchup, like so many other things, was invented in China and it didn't become a staple in the British diet until 1740, after Newton's death. But even so, Newton, that giant of scientific inquiry, contributed to the understanding of ketchup. Back in 1687, when Newton was taking a rest from inventing calculus and discovering gravity, he mathematically described the behavior of fluids.

At this point, we must caution you against the rookie mistake of thinking that fluids and liquids are the same thing. Liquids are one type of fluid, but not all fluids are liquids. A fluid is a substance that flows—and the air flowing around an airplane wing is just as much a fluid as the water flowing from a tap or the ketchup flowing (however reluctantly) from the bottle.

In a solid, molecules are stuck together and pretty much stay in the same place, but in a fluid they're free to move past each other. How easily the molecules move past one another determines the fluid's viscosity or resistance to flow. Water is a low-viscosity liquid—it's easy to move a spoon through water. Honey is a high-viscosity liquid—you can move a spoon through it, but you have to push harder.

In the ideal fluid defined by Newton, viscosity doesn't vary when you stir the fluid or thump on it or otherwise mess with it. When you stir a Newtonian fluid with a spoon, the speed of the spoon depends on how hard you are pushing. If you push harder, it moves faster, with the speed increasing at the same rate as your force. Apply a given force to the spoon and it will reach a constant velocity through the water and a slower constant velocity through the honey. In each case the terminal velocity of the spoon will be proportional to the applied force on the spoon. It's independent of the velocity itself.

What's going on in the fluid where you are moving that spoon around? The fluid right near the spoon flows a lot. Farther from the spoon, the fluid moves less. The relative motion between these adjacent layers of moving liquid is called shear.

That (finally) brings us back to ketchup, which is (drum roll, please) a non-Newtonian fluid. The viscosity of ketchup does not remain constant when you stir it. In fact, the more you stir ketchup, the easier it is to stir. Ketchup is not only non-Newtonian, it is also thixotropic, a word that comes from the Greek for touch (thixis) and turning or changing (tropos.) A rheologist (that is, a scientist who studies fluid flow) would categorize ketchup as shear-thinning, a fluid that gets runnier as you stir it. When part of the fluid is forced to slide or shear past the rest, the fluid thins or gets runnier. The faster you stir, the runnier it gets.

It's not only stirring that causes a shear-thinning fluid to flow. Vibration will do it too. With an electric motor and an eccentric weight, Paul says he can make a device that will vibrate. Then just touch it to the neck of a ketchup bottle to stimulate flow…or maybe you're better off just tapping with your fingers.

Though ketchup is known for its unwillingness to start flowing without forceful encouragement, it isn't the only fluid that acts like this. Whipped cream, yogurt, sour cream, cake batter, and mayonnaise are all shear-thinning fluids. So was the soil on the planet Screwtop in Vonda McIntyre's story of the same name, which liquefied under pressure.
 

OOBLECK

As scientists, we look for anomalies (aka weird stuff) and commonalities. How are two weird things the same? How are they different? Considering the non-Newtonian behavior of ketchup brings us to the distinctive but related peculiarities of something else from the kitchen: cornstarch.

If you haven't ever played with cornstarch and water, here is your chance to experience strangeness in your own kitchen. Mix a quarter of a cup of cornstarch with about a quarter of a cup of water. Stir slowly. Keep adding water until the mixture acts like a liquid when you stir it.

When you reach that point, stop stirring and tap on the surface of the liquid with your finger. It won't splash. In fact, the mixture will feel more like a solid. Grab a handful of the stuff and squeeze it into a ball. While you're squeezing, it'll be solid. Let up the pressure and it will ooze into a puddle. Science teachers call this mixture oobleck, named after the sticky green goo that fell from the sky in Dr. Seuss's 1949 book, Bartholomew and the Oobleck. For years, teachers and parents with bored kids stuck at home have been using this mysterious mixture to amuse and intrigue their charges.

As you have no doubt realized, oobleck is another non-Newtonian fluid. In a way, this stuff is the anti-ketchup. Ketchup is a shear-thinning fluid, and oobleck is a shear-thickening fluid. If you try to stir it, the speed of the spoon does not increase at the same rate as the force you apply. When the spoon moves more rapidly, the resistance to its movement increases faster than linearly. Moving the spoon through oobleck is extremely hard at high speeds. The difficulty depends on the speed itself.
 

THE QUICKSAND OF EXTRAPOLATION

Of course, like any good science fiction reader, you are probably wondering what more could be done with this strange mixture. You are thinking about possible extrapolations: if a mixing bowl filled with oobleck is entertaining, what fun could you have with a wading pool full of it?

Funny you should ask. At the Exploratorium, Paul filled a wading pool with oobleck and demonstrated, with a group of science teachers, that you can run across a pool of oobleck. Under the impact of each footfall, the oobleck becomes solid. But you have to keep running. Stand still and you'll sink as the oobleck oozes around your feet. Mythbusters did the same experiment in an episode about ninjas walking on water.

Paul has considered that oobleck walking could provide an incremental approach to learning to walk on water. Start with cornstarch and water and thin the mixture a little each day until you can run on water. Alas, he has not yet managed to make this work.

Playing with a pool full of oobleck is not just fun—it helps you prepare for unexpected encounters with quicksand, another shear-thickening fluid. Here's something to remember the next time you fall into a pit of quicksand: the faster you try to move, the harder moving will be. If you try to move quickly, you'll just tire yourself out.

Paul suggests you relax in the knowledge that the quicksand is denser than you are. You will not completely submerge in quicksand. In fact, the deepest you will sink is about to your armpits. As you relax, comfortable in your knowledge of physics, lean back. A slow-motion lean will allow your feet to float slowly to the surface where you can roll to the edge of the quicksand bog.

That's Paul's advice, and he should know. While he was hiking through a canyon in Zion National Park one afternoon, his foot plunged into a pool of quicksand. The hot sun had dried the surface sand, but under that dry topping there was wet sand and a flowing spring. Paul's initial effort to quickly pull his foot out to keep the water out of his boot was useless. He resigned himself to a wet boot and slowly pulled his foot out. Of course he then took his boots off and explored the physics of this amazing pool of quicksand.

Paul also says it's fun to find shear-thinning mud. He recommends standing in it and wiggling your feet. You will slowly sink as long as you keep vibrating.
 

SO WHAT?

Science fiction writers—particularly those associated with TV and movies—have long been fond of force fields and other instant shields that can protect you from harm—from the Starship Enterprise's shields to the force field generated by Iron Man's suit. All these defenses relate to the idea of body armor, which is even older than the idea of ketchup. A body armor made of linked metal rings, often referred to as chain mail, dates back to 300 B.C., and scale armor made of overlapping metal scales sewn to a backing in overlapping rows predates that.

Modern creators of protective equipment are trying to solve the same problems those ancient armor makers faced: How can you stop a weapon from penetrating the wearer's defenses and yet keep the armor light and flexible? Today inventors are looking to shear-thickening fluids for a possible answer to that question.

If you're into high-performance ski equipment or ballet shoes (and after all, who isn't?), you know that this science fiction idea has finally reached the marketplace in the form of orange-colored gel made by U.K.-based D3O lab. This high-tech plastic, called D3O®, is made of long-chain molecules suspended in a liquid. Move the gel at slow speeds and it flexes—the molecules slither about and change position. Hit the gel hard and fast and it solidifies as the molecules tangle and lock in place.

In the 2006 Winter Olympics, U.S. and Canadian ski teams used protective equipment incorporating D3O. D3O is currently used in ballet toe shoes, in shoulder, knee and elbow protectors, in tennis rackets, and in protective cases for smart phones. The British military is working with D3O to improve helmets and body armor for soldiers.

In comparing D3O with conventional body armor, the gel's inventor, Richard Palmer, chief executive of D3O Lab, says that it's like comparing RoboCop and Spider-Man. RoboCop is the bulky, heavy past; Spider-Man is nimble and flexible, moving into the future.

The U.S. army is also investigating the use of shear-thickening fluids to improve body armor. Experiments have shown that body armor woven from Kevlar can be improved by saturating the Kevlar fibers with a shear-thickening fluid. When a force is applied to the armor, the shear-thickening fluid stiffens and resists the penetration.
 

HIGH-TECH NON-NEWTONIAN FLUID

Other experimenters seek to improve on D3O using even more innovative materials. More than a decade ago, artists and science teachers at the Exploratorium started experimenting with ferrofluid, which is basically an oil with lots of tiny iron particles suspended in it. Ferrofluid is a non-Newtonian fluid of a variety that Newton never imagined. Stick this stuff in a magnetic field and the iron particles become magnetized and stick together, making the fluid freeze into interesting shapes determined by the magnetic field.

Paul makes his own ferrofluid using iron filings (the smaller the better) or magnetite sand grains he gathers by dragging a magnet through California coastal beaches. He mixes the iron particles with corn oil and places the ferrofluid into a clear plastic jar that used to hold peanut butter. Bring a refrigerator magnet near a jar of homemade ferrofluid and the liquid dances and moves with the movement of the magnet.

Today, magnetorheological fluid, a slightly more refined relative of ferrofluid, is being considered as a possible component of high-tech body armor that bears a passing resemblance to the Holzman shield in Frank Herbert's Dune books. Theoretically, body armor filled with MF could be activated by electromagnets when needed, freezing instantly to protect the wearer.

It's a lovely notion, but there are problems that don't arise with D30. Like how do you activate the body armor at exactly the right moment and in exactly the right part of the body? Pat imagines a future battlefield littered with warriors who have been suddenly immobilized by their suits—protected but frozen.
 

WEIRDNESS IS EVERYWHERE

As folks who specialize in noticing things, we have noticed that once we start studying something, we see similar stuff all over the place. In other words, start examining something that seems really strange and pretty soon you realize there's a lot more of it than you thought.

Non-Newtonian fluids are all over the place. Some occur naturally—like blood and lava. Some have been created for a particular purpose: like toothpaste that is formulated to come out of the tube and then stay on your toothbrush or paint that flows smoothly when you paint it on but doesn't drip once it's on the wall. (Imagine trying to paint a wall with honey (which would be difficult to brush on) or water (which would drip right off) and you'll appreciate the merits of non-Newtonian fluids.)

And of course, there is our favorite non-Newtonian fluid: Silly Putty. Initially created by research aimed at making synthetic rubber, it flows like a liquid, stretches like rubber, breaks apart like a brittle solid, and bounces like nobody's business. It has always seemed somewhat science fictional in its strangeness.

Writing this column (and doing the inevitable online searches that the topic involved) has led us to consider experimenting with a new mixture: Silly Putty infused with iron particles. Not only does it bounce and stretch and flow, the iron-infused putty crawls toward magnets and devours them. Could world domination be far behind?
 

__________________________________

The Exploratorium is San Francisco's museum of science, art, and human perception—where science and science fiction meet. Paul Doherty works there. Pat Murphy used to work there, but now she works at Klutz Press (www.klutz.com), a publisher of how-to books for kids. Pat's latest novel is The Wild Girls; her latest nonfiction title is Paper Flying Dragons, which comes with five dragons to fold and fly, plus a robodragon that you can construct from parts. To learn more about Pat Murphy's writing, visit her website at www.brazenhussies.net/murphy. For more on Paul Doherty's work and his latest adventures, visit www.exo.net/~pauld.

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