A non-Newtonian fluid
Image credit: Rory MacLeod [CC BY 2.0], via Flickr

Like many people, I’ve tried hard to forget my days in junior high school. That was an unpleasant time in my life for all the usual reasons, and thankfully most of it is now a dim blur. But a few pleasant moments do stand out in my memory. One of those was a report I did for my ninth-grade science class. For reasons I no longer recall, the topic I chose was Pascal’s Law, and I must have prepared well for that 10-minute presentation, because I could probably stand up and give pretty much the same talk today, even though I never went on to study any more about it.

Pascal’s Law describes the behavior of fluids in a closed system, and says, to oversimplify somewhat, that the pressure the fluids exert is always the same throughout the system. This is the principle that enables hydraulic presses to work—a small amount of force applied to a piston pushing down on fluid can exert much more force on a larger connected piston, making it sort of like a liquid lever. The same effect has applications in everything from scuba diving to ventilation systems and dam construction.

Pascal, Meet Fisher

Although that little snippet of knowledge has stayed with me all this time, that marked the extent of what I knew about fluid mechanics until I wrote an article here about Space Pens. These pens, legendary for being able to write underwater, in zero gravity, in a vacuum, or in just about any other situation, use a special thixotropic ink, a substance that’s normally in a gel state but which turns into a liquid when it’s agitated—that is, when the ball rolls against it. It’s liquid just long enough to flow onto the paper, and then it turns semisolid again. (As spiffy as that is, some Space Pen ink turns out to have surprising problems, which you can read about in Space Pens vs. Purple Ink on my blog.)

Back in 2006, some very cool videos started making the rounds: people doing stunts with something called “non-Newtonian fluids.” If you’re not an engineer, that may not sound very intriguing, until you look at guys running across the surface of a vat filled with a solution of cornstarch and water, but sinking into it when they stand still. That’s a real eye-opener. (Mythbusters also did a segment on this phenomenon.) Or some other guys taking a handful of the same liquid goop and slapping it into solid balls, which then turn back into liquid as soon as the agitation stops. These are just a couple of the many wacky properties exhibited by non-Newtonian fluids—substances that change their viscosity in reaction to stress. (See YouTube for a long list of videos featuring non-Newtonian fluid experiments.) And sure enough, the ink from my Space Pen is in the same category.

Non-Newtonian fluids range from the exotic to the mundane. You’ve probably made a cornstarch-and-water mixture lots of times in your own kitchen, and if you made enough of it, you too could walk across its surface. Assuming the proportion of starch to water is right, the solution gets suddenly thick and firm when force is applied to it, as you may have noticed when trying to stir it when preparing a sauce. Stir more slowly, and it flows more easily.

Going with the Flow

But not all non-Newtonian fluids behave this way. Some of them get runnier when under stress, such as Space Pen ink and paints that adhere to a brush when at rest but glide on easily when the brush is applied to a surface. Also in the thixotropic category as well as in your kitchen: ketchup and honey.

There are still other varieties, too, which have different patterns of changing viscosity. Such varied substances as quicksand, Silly Putty, blood, dough, and gelatin fall under the broad non-Newtonian heading. By contrast, Newtonian fluids, or what most people think of as normal fluids, are those (like water) whose viscosity is determined only by temperature and pressure. Sure, water will get plenty firm if the temperature is reduced enough, but no amount of force can make liquid water behave like a solid.

The moral of the story? If you’re stuck in a non-Newtonian fluid—or junior high school—the trick is to remain calm. The more you struggle, the harder it gets.

Note: This is an updated version of an article that originally appeared on Interesting Thing of the Day on February 28, 2007.