The Oozy Physics Of Oobleck

IRA FLATOW: You may be familiar with a common science demonstration done in classrooms. Here it is. If you mix cornstarch and water in the right proportions, you wind up with a material that seems to defy the rules of physics. It flows and settles like a liquid like you would expect it to, but when you try to pick it up quickly or stir it, it stiffens up.

The same thing happens with Silly Putty, quicksand, and paint. This type of material is called a non-Newtonian fluid. It also has a more fun name, oobleck. And for a long time, it’s been hard to prove why exactly this material acts like this. But now scientists have a better understanding of the underlying mechanism, and this understanding could help us create new smart materials.

Joining me to talk about this is my guest Dr. Heinrich Jaeger, professor of physics at the James Franck Institute at the University of Chicago. The study was done in collaboration with the James Franck Institute and the Pritzker School. Thank you for joining us.

HEINRICH JAEGER: Thank you, Ira. Thanks for having me.

IRA FLATOW: Nice to have you. You know I have done this many times. I love creating quicksand with cornstarch and water. And if you put your hand in it and if you lift it up slowly, you can easily remove your hand. But if you try to jerk it up quickly, the cornstarch water becomes like a solid. Please explain what exactly is going on here. What is a non-Newtonian fluid?

HEINRICH JAEGER: Yes. So a non-Newtonian fluid really is a fascinating material that can adapt and dramatically change its behavior all by itself, depending on how it is forced to flow. And what I want to do to explain that is contrast that with what we would call a Newtonian fluid. Typically, that’s a pure liquid like water. And how easily it flows depends on its resistance, its viscosity, which is simply a materials property. So it does not depend on how you handle it.

Imagine you move your hand through water. It puts up some resistance to flow. And that is this viscosity. But a non-Newtonian fluid now can adapt its viscosity. Smartly, it can flow more easily– for example, when we push it. Or it can dramatically resist flow when we push it. And these are two extreme cases of non-Newtonian fluids that we call either shear thinning or shear thickening.

IRA FLATOW: Well, can you explain that exactly? What’s going on in the particles inside that dish where my hand is?

HEINRICH JAEGER: Yes. So the prototypical non-Newtonian fluid is a dispersion or suspension of small particles in a liquid. And you might think that the liquid lubricates these particles as they flow past each other.

IRA FLATOW: Right, exactly.

HEINRICH JAEGER: And that can happen. In particular, this can happen for so-called shear thinning fluids. But there’s also the case, particularly when you add a lot of particles, where they get pushed together to a point that actually the liquid between them is expelled, and they get into direct contact. And now they interact by friction. And that can dramatically increase the resistance to flow. In fact, it can even solidify.

IRA FLATOW: Yeah, yeah. Now I get it. That was good. Do you find these kinds of non-Newtonians in nature, or are they all man-made?

HEINRICH JAEGER: No. In fact, they exist in nature. So I told you that typically pure liquids are Newtonian. So they don’t display that behavior. But when particulate matter is added, then this non-Newtonian behavior emerges. And an example is blood.

IRA FLATOW: Blood.

HEINRICH JAEGER: Blood, for example, is shear thinning. It flows more easily if it’s forced harder. Then there is paint. Paint also often is formulated by putting additives in there such that it will flow more easily when you brush it. That’s what you want, to get it off the brush onto the wall.

But then when it’s on the wall, you don’t want it to keep flowing. You want it to resist flow. And that’s exactly shear thinning behavior, so the opposite of oobleck.

IRA FLATOW: Yeah. This is Science Friday from WNYC Studios. Why has it taken so long to understand how these ooblecks work?

HEINRICH JAEGER: In part these shear-thickening, oobleck-type non-Newtonian fluids, they’re rarer. And they are so utterly counterintuitive. Just imagine any material. I force it. I push it. I would imagine it should get weaker.

Oobleck is now doing exactly the opposite. It becomes stronger. It potentially even solidifies, right?

IRA FLATOW: Right.

HEINRICH JAEGER: And it does that even reversibly. It goes back and forth. If I take the forcing off, if I don’t push it, it just reverses back to a liquid.

And this counterintuitive behavior is complex. It has been hard to understand. And maybe historically that had also to do with the fact that two very different communities were looking at that. Initially, it was the rheology community. And I think they started basically from the idea I take pure liquid, like water, and I add particles, a few, and see what might happen, right?

IRA FLATOW: Right.

HEINRICH JAEGER: Did you know that the person who was one of the first to calculate what would happen if you would put a few particles in a liquid and how its viscosity would change was no other than Albert Einstein?

IRA FLATOW: Really?

HEINRICH JAEGER: That was his– it’s a part of his PhD thesis, in fact.

IRA FLATOW: Wow. That’s something I’m sure a lot of us didn’t know.

[CHUCKLING]

But we like to know that now. You suggest that now that we better understand how non-Newtonian fluids work that they could be used to make new materials. Well, what kinds of materials are you thinking of?

HEINRICH JAEGER: Well, so we are very much interested in making materials, formulating materials that all by themselves, if you want autonomously, adapt to changes in conditions. You don’t need a computer to tell the material what to do, no feedback. So if you want an oobleck-like material, you want one that becomes more resistive to flow. And that could be useful, for example, for impact mitigation.

Think of wearable garments impregnated by fluids like that that would take up impact, that would help you maybe protecting against sport injuries, injuries at the workplace, prevent you from hurting yourself when you fall. There are other applications that have been proposed. One that I like particularly is speed bumps.

IRA FLATOW: Speed bumps. Oh, yeah. Now I see it. It’s like lying squishy flat when it’s not being touched, but then you hit it in the car, and now it’s suddenly a speed bump.

HEINRICH JAEGER: Exactly. So the idea with this non-Newtonian shear-thickening fluid is that the harder you force it, the more resistive it gets. So a car rolling over one of those at low speed would essentially just push the fluid aside and roll right over or through it. And if you go at faster speeds, the bump would get solid, and you would notice.

IRA FLATOW: That is cool. I like that. So you say it could be used in clothing. What other kinds of uses, let’s say, in clothing? Stab resistant because it would harden up. What other uses possibly of wearable stuff?

HEINRICH JAEGER: Well, I should explain maybe a little bit more. Of course, there are many ways of protecting yourself against impact, let’s say, during sports. But typically, this implies a garment or a part of a garment that is relatively rigid. And that could also then prevent you in terms of mobility.

And what would be really nice is a protective system that is very much not affecting your mobility when nothing happens, but then suddenly hardens up the moment there’s an actual impact. And that’s exactly what such a non-Newtonian fluid could do. So we would combine the fluid with another fabric, obviously, into a protective system.

IRA FLATOW: That’s all the time we have for today. I’d like to thank my guest Dr. Heinrich Jaeger, professor of physics at the James Franck Institute at the University of Chicago. This study was done in collaboration with the James Franck Institute and the Pritzker School.

HEINRICH JAEGER: Thank you, Ira. Thanks for having me.

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