NEIL LIN: Here's a simple experiment. We have a bag of corn starch and I'm going to mix this with water. And this is something you can get in your kitchen. So I'm going to pour in some corn starch in the beaker-- a lot of corn starch-- and then I'm going to add a little bit water to the same beaker.
Now, the next thing you should do is-- we're going to mix this to make a mixture. So cool thing about the mixture of corn starch and water is, if you gently play with-- if you stir does corn starch gently, it actually flows. It's like liquid.
But the thing is, if you start to do some severe motion or you try to stir it really fast, it actually becomes solid and it breaks. So you can really now take out a chunk of corn starch.
I want to show you a mystery here. We have a spinning wheel. It's driven by a motor, and this motor is actually connected to a battery so it's driven at a constant voltage. As you can see, this gear is rotating at a constant speed.
Now, I'm going to dip this a rotating flywheel into the fluid underneath the flywheel. So if the fluid is just a typical fluid like water, the flywheel will still rotate but slower. But because of the fluid we have here is corn starch and corn starch is a really unique thickening fluid-- it thickens when you stir it-- so when I do this, when I dip the flywheel into the corn starch, you see the motor stops. It's because the fluid becomes so viscous that stops the rotation of the motor.
But what I haven't told you is that the entire device is actually mounted on the speaker. And this speaker is able to vibrate the corn starch up and down vertically. And as I switch on the speaker, the strange thing is you can see the flywheel starts to rotate again. And if we switched off, it stops. And we can do this back and forth, repeat, and it seems that we have some control over the viscosity or the flowability of corn starch here by just switching on and off the speaker. And in this video we're going to explain how this experiment works.
ITAI COHEN: You can think of the corn starch in this container as comprised of fluid and these little tiny corn starch particles suspended in that fluid. And when you're sharing the corn starch, those particles end up making clusters, and those clusters rotate together and they disrupt the flow and that's what creates the thickening behavior.
Now, last time we showed that these clusters behave asymmetrically. And what do I mean by that? It turns out that when you compress the particles towards one another, because of contact interactions, they are able to sustain this cluster. But the minute you pull them apart, the particles are no longer in contact and that makes the force keeping those contact chains together asymmetric.
This asymmetry allows us to come up with a strategy for taming oobleck. So here's how-- here's the idea. The idea is that, in addition to the main shearing direction that we use in order to create these clusters that are aligned along the direction of flow, what we're going to do is we're going to put in a slight oscillatory shear in the orthogonal direction. So we're going to move the bottom plate. In addition to moving it forward, we're also going to move it side to side at the same time.
What that does is, that as the-- as the shearing plates move to the side, it pulls these chains apart. OK? And that breaks up the clusters and allows us to change the viscosity of the oobleck on demand.
Now, in the case of the experiment that we showed you earlier, what happens is that the flywheel is creating the initial shearing force that's creating these clusters. And what the speaker does is it vibrates it perpendicularly to the flywheel. So that's the same as creating the clusters in this direction and then vibrating it side to side in order to break these clusters up. And that vibration has to be a very low amplitude and just high enough frequency so that you can break these clusters up without forming new clusters in the orthogonal direction.
Now, why is this so important? Many industrial processes deal with pastes. These are particles that are suspended into fluid and because of that they shear thicken just like the corn starch does. And to demonstrate this to you, I have here some corn starch that we've put in. And I'm literally going to like this-- fluid really-- I mean, it flows terribly. And you can see it's just not a normal material and the more I shake it the harder it is to get it to flow.
And this is the kind of problem that industry faces every day. So what if we could figure out a way of now tuning the behavior of the cornstarch to make it flow more easily? And that's what this strategy of shearing orthogonally does. It allows us to control the viscosity of the corn starch by something like a factor of 100. And if we can reduce the viscosity by a factor of 100, then we can make it easy to flow and solve many of these industrial problems.
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Cornstarch and water make a remarkable mix. Treat it gently and it flows, handle it roughly and it shatters like a solid. In this video you will see a rotating cylinder being placed in cornstarch and water. The cornstarch mixture is so thick that the cylinder stalls; however, vibrations from a loudspeaker cause it to become sufficiently fluid that the cylinder will once again rotate. When the loudspeaker is on, the cylinder rotates; when it is off, the cylinder stalls. Prof. Itai Cohen explains what is going on.