Walking on Water (Now in Slow Motion)
December 4, 2012 1 Comment
Alright, I lied, this post isn’t strictly speaking about walking on water. It’s about walking on water and cornstarch–a combination you’re probably familiar with if you’ve ever been a kid or raised a kid. When I was in elementary school we called it “Oobleck,” a reference to this book. I’m sure it goes by a lot of different names, but that’s the one I’ll use here.
Oobleck, famously, flows like a liquid unless it is exposed to sudden force, at which point it “seizes up” and acts much more like a solid. As a result, you can walk on it, if you do it quickly enough, because the sudden forces of your footfalls make it go momentarily stiff. But you can’t stand on it, because while you’re still exerting a force on the Oobleck with your weight, that force is no longer sudden.
It’s all summarized nicely by the antics of these nice gentlemen on some sort of Barcelona-Based, Bill-Nye style science show. You don’t have to speak Spanish to appreciate it, but if you do, please tell me, is Google correctly translating the name of the show (“Hormiguero Cientifico”) as “Scientific Ant Hill”?
This is in fact a rather “old” clip by internet standards. What caught my attention this week is this newer video showing the same trick, but in super slow motion:
So, what causes this phenomenon? There’s actually some debate. Roughly speaking, however, there are so many tiny cornstarch particles floating in the mixture that they can’t move past each other very well. When they have enough time, they can eventually get out of each other’s way. But when they’re forced to move quickly, they simply jam up. It’s the same reason that in a fire drill, you’re told to stay calm and walk in an orderly manner. If everyone suddenly rushes for the exits, you can form a logjam that no longer flows like a fluid.
Fluids like Oobleck are called “Non-Newtonian,” because they don’t behave like the “simple,” more typical fluids studied by Sir Isaac Newton. More specifically, this one is called a “dilatant”– a fluid which gets harder to move through when subjected to sudden forces. Contrast this with squeeze-bottle salad dressings, which often use a substance called xanthan gum* to turn them into “pseudoplastics”: fluids which flow easily when experiencing sudden forces (like the shaking of a bottle) but which otherwise remain appetizingly thick.
Another, more advanced Oobleck trick here. By the way, if you’re interested in Oobleck but have never bothered to make your own (a dilatant dilettante, you might say**) then please take the plunge at some point. It’s very easy, and no amount written explanation can replace the experience of handling Oobleck directly. I suggest a 2:1 ratio of cornstarch to water. Put the starch in a bowl, and add the water a little at a time, mixing with your fingers. The starch will sort of “resist” spreading out into the water, so you have to help it along.
*Don’t Worry! It’s a lot less sinister and artificial than it sounds
** No, I didn’t write this entire post just as a setup for this pun. Might have if I’d thought of it first, though.