Man, those Brits have some awesome TV programming. Not only does Jen-Luc Piquant adore the BBC's modern Sherlock reboot, she's also a diehard fan of all those Inspector Morse mysteries ("Oh, for god's sake, Lewis!") and the weirdly oddball Blackadder series ("I have a cunning plan!"). The British also do a mighty fine job on the science end of things, whether it be serious, Attenborough-narrated fare like Frozen Planet, or something more playfully irreverent and silly, like Brainiac: Science Abuse, which ran for nearly five years (2003-2008).
I was reminded of the Brainiac series this morning, when one of their YouTube clips popped up in my feeds: host Richard Hammond (who now co-hosts the hugely popular Top Gear) coaxes his co-host Jon Tickle into demonstrating an experiment. It's impossible to walk on water -- unless one happens to be Jesus, who "cheated" by being of divine origin --but it's entirely possible to walk across custard. Check it out:
So what gives? Well, as the Brainiacs explain in the video, it's an example of a non-Newtonian fluid -- or, as I like to think of it, "oobleck." That's a nod to Dr. Seuss, specifically, a tale called Bartholomew and the Oobleck.
Bartholomew is a royal page in the Kingdom of Didd. King Derwin is a bit of a dunderhead who decides he's bored with plain old water-based rain and show, and orders the casting of a magic spell that causes a green sticky substance to rain down from the sky. The stuff was called "oobleck," and as often happens with magic spells, it turned out to be more troublesome than entertaining, gumming up the entire kingdom until the creatively pragmatic Bartholomew figures out how to save the day.
Oobleck is a favorite substance of mine, because it just can't decide whether it wants to be a solid or a liquid -- it swings both ways! It's worth pointing out that the Brainiacs didn't use custard made from scratch with eggs and milk and sugar -- they used custard powder, which is mostly comprised of cornstarch. And one of the most common at-home science experiments you can do involves water and cornstarch mixed in proper proportions, to give you an instant (almost) non-Newtonian fluid.
Isaac Newton first delineated the properties of what he deemed an "ideal liquid," of which water is the best example. One of those properties is viscosity, loosely defined as how much friction/resistance there is to flow in a given substance. The friction arises because a flowing liquid is essentially a series of layers sliding past one another. The faster one layer slides over another, the more resistance there is, and the slower one layer slides over another, the less resistance there is. Anyone who's ever stuck their arm out of the window of a moving car can attest that there is more air resistance the faster the car is moving (air is technically a fluid).
That's the basic principle. But the world is not an ideal place, and not all liquids behave like Newton's ideal liquid. In Newton's ideal fluid, the viscosity is largely dependent on temperature and pressure: water will continue to flow -- i.e., act like water -- regardless of other forces acting upon it, such as being stirred or mixed. In a non-Newtonian fluid, the viscosity changes in response to an applied strain or shearing force, thereby straddling the boundary between liquid and solid behavior.
Physicists like to call this a "shearing force": stirring a cup of water produces a shearing force, and the water shears to move out of the way. The viscosity remains unchanged. But non-Newtonian fluids like oobleck? Their viscosity changes when a shearing force is applied.
Ketchup, for instance, is a non-Newtonian fluid, which is one reason smacking the bottom of the bottle doesn't make the ketchup come out any faster; in fact, it slows it down, because the application of force increases the viscosity. Blood, yogurt, gravy, mud, pudding, and thickened pie fillings are other examples. And so is oobleck. They aren't all exactly alike in terms of their behavior, but none of them adhere to Newton's definition of an ideal liquid.
The substance becomes thicker, or more viscous, in response to agitation, compression or other similar applied forces: punch the oobleck, and it hardens into a solid, softening into a fluid again once the energy dissipates. Compress it into a ball and toss it in the air, and it will quickly lose its shape and flatten before it lands.
And if you have an old stereo speaker lying around, you can pour a bunch of oobleck onto it and then watch the stuff respond to the musical beat.
(Side note: non-drop paint exhibits the opposite effect, brushing on easily but become more viscous once it's on the wall. And under rare circumstances, liquid hydrogen and liquid helium can become superfluids, exhibiting zero viscosity at extremely low temperatures.)
Similar shear-thickening fluids are already being used on prototype bullet-proof vests and sporting equipment, because their sensitivity to impact means they can better absorb the energy of a high-velocity projectile or hard impact, while still being flexible enough for wear comfortably. For instance, in 2003, scientists at the University of Delaware treated the fabric of Kevlar vests with a shear-thickening fluid (silica particles suspended in polyethylene glycol).
Under normal conditions, the molecules of the treated material are weakly bonded and can move around with ease; that's why the material is so flexible. But the shock of any impact -- a hard fall, or incoming bullet -- will cause those chemical bonds to strengthen so the molecules lock into place and the fabric becomes instantly rigid. Once the force from the impact dissipates, the bonds weaken again and the vest becomes flexible again. The US and Canadian skiers in the 2006 Winter Olympics wore a similar form of "smart armor" manufactured by a British company called d3o Labs.
The Brainiac clip ends with hilarious footage of Tickle trying to get out of the custardy oobleck, which proves much more difficult than one might expect -- again, because of the unusual non-Newtonian properties. The more you struggle, the more the stuff resists.
It's very similar to how quicksand behaves. If you want to escape quicksand, it's best not to struggle too frenetically, but slowly and patiently work your way to firmer ground. That's because quicksand is also a non-Newtonian fluid, despite being made up of fine grains of sand or silt; when mixed with clay and salt water, it becomes a colloid hydrogel. So quicksand appears solid when it is undisturbed, but even a tiny (like, 1%) change in the stress on it will cause its viscosity to decrease quite suddenly, and the person walking across it will sink into the sand, after which the sand and water mixture will separate to form something akin to a solid. (Apparently it's the salt that's the blame for that trapping power.)
What could be more fun than a big tub of non-Newtonian fluid? Oobleck: make a batch today!
Adapted from an August 2007 post from the archived blog.
Lee, Young S., Wetzel, E.D., and Wagner, N.J. (2003) "The ballistic impact characteristics of Kevlar soven fabrics impregnated with a colloidal shear-thickening fluid," Journal of Materials Science 38, 2825-2833