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I promised you guys a polymer lecture, and I had a good boring one about polymer stabilized systems all mapped out in my brain. Lucky for you, I spent the day engaged in some very cutting-edge research on polymers, or at least that’s how we explained it to our boss when he came into the lab and found us examining the flow properties of silly putty. We’d had two samples of silly putty running down a filing cabinet for almost a month, and were analyzing the data. So let’s talk about silly putty instead.

Silly putty is a great polymer with a bunch of science packed into it. Silly putty is one of those accidental inventions, the by-product of war-era research on synthetic rubbers. While the story goes that the inventor sent samples of his bizarre polymer to scientists all over the world and none of them could think of a damn thing to do with it, I suspect they weren’t even trying. Most scientists when given an object will immediately spew out six different applications they’d like to apply it to, of which one might work and if we’re lucky won’t be prohibitively expensive. Either way, the entrepreneurs persevered by realizing that just because something is impractical doesn’t mean people don’t want it and today silly putty is not only a popular toy, but has found plenty of applications.

Silly putty’s weirdness stems from it’s odd flow properties: it bounces like a rubber ball, but can be smushed easily if pressed slowly. Some materials react differently to stress depending on how quickly the stress is applied. This is called a ‘viscoelastic’ response, where visco = flowy and elastic = bouncy. Most things give you a choice, either or but not both, but silly putty is in a special catagory of materials that can do both, depending on how you poke them.

“Quickly” is a relative term, and part of learning to be a scientist is learning how to combine a bunch of material parameters and spit out a decent time scale. Silly putty happens to have a very human-friendly natural relaxation timescale where shocks that are quick to us (bouncing, hammering) are also quick to the putty and slower but people-friendly stresses like kneading are slow enough to cause the flow response. Some materials that flow, like glass (which is not in the same family of materials as silly putty at all), flow so slowly that you wouldn’t notice a difference if you had a dozen lifetimes to sit there and watch,so we can pretty much always treat them as a solid. I’ve heard that stained glass windows in old-school European cathedrals are supposed to be almost imperceptibly thicker towards the bottom, and that this has been attributed to glass flowing downward under the influence of gravity, but at least one of my professors believes that this famous and fun example is probably a load of crap. I’ve never sat down and done the math myself, so I have no idea if hundreds of years is enough time to notice a difference. Point is, how long it takes a material to respond to different forces can be very important when it comes to considering how and if you can use it in a particular application, and silly putty might not be as fun if it had a different relaxation time.

There is a very good PDF on this topic over at the Cambridge Polymer Group’s website, including pictures of a sheet of silly putty with a bullet going through it. Also of note is this experiment where 50 pounds of silly putty were dropped off a parking garage.

You can make silly putty very easily at home with glue and a few other things, but some recipes involve ingredients that you may not have laying around and I’ve never tried any of the elmer’s glue ones myself so I don’t know how well it would work. A viscoelastic fluid that is easy to make at home and actually a little cooler (but a lot messier) than silly putty is cornstarch and water. Pretend you’re making a thickening agent for stew, but reverse the relative amounts of cornstarch and water so that it is mostly cornstarch. You’ll end up with a material that you can pour from one hand to the other but feels like a brick wall if you slap it. Also it will cover everything it touches in a fine dusting of starch, so tell your kids to keep it outside.