when fluids get weird

FrazzledJen-Luc2 You may have noticed a few changes to the blog this week, most notably the departure of my esteemed co-bloggers who came on board in 2008. Basically, co-blogger Diandra reluctantly informed me this week that she can no longer blog for the cocktail party because she needs to focus more on her personal and professional obligations (among other things, she recently relocated to head up West Virginia University's WVNano, an interdisciplinary research center that targets materials, devices, and biomolecular systems for use in public security, health, energy, and environmental applications). Lee and I had already been chatting about her lack of time to blog, and Allyson and Calla made infrequent (though always welcome!) appearances. It just seemed like a good time to revert back to the individual format. I am sad to lose such awesome contributors, and I know the readers will miss them too — but I told them they're welcome to pop into the cocktail party any time with a guest post should they feel the urge.

The other changes are to the design, namely, a switch to a three-column format with a few other minor tweaks. We've had the same look for five years now, and over the next few months I'll be revamping the design further with a new banner, color scheme, updated blogroll and so forth. What won't change: the content — although the goal is to mix up with long monster posts with shorter ones, thereby reining in somewhat my habitual long-windedness. And Jen-Luc Piquant will preside as always over the festivities.

With that out of the way, we now now return to our regularly scheduled science! Via the Improbable Research blog, I came across this nifty video for New Scientist, in which a guy named Chris Chiswell jogs on the non-Newtonian fluid known as "oobleck" (basically cornstarch-mixed-with-water), until he stops jogging and sinks into it.

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! 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. (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.)

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!

UPDATE: A Facebook pal pointed me to this awesome appearance by Dr. Who's Matt Smith on British TV explaining about Non-Newtonian fluids (a.k.a. batter). Why don't we see this on The View or Good Morning America?


5 thoughts on “when fluids get weird”

  1. Ketchup does flow better when you smack the bottle, because it is a shear thinning non-newtonian liquid. Increased stress decreases its viscosity. Shake any bottle of ketchup and it’ll pour out very easily.

  2. Loved the “oobleck” vid! Especially since it references one of the cooler Dr. Seuss stories. Another fun non-Newtonian fluid can be made with a mixture of water, white glue, and borax. By changing the proportions you can achieve an impressive range of behaviors. ’tis a great way to entertain bored children on a rainy afternoon. (Note that the bored children are optional.)

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