NEIL LIN: Now, we're going to do a simple experiment. Here's a simple experiment. We have a bag of cornstarch. 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 cornstarch in the beaker, a lot of cornstarch. Then I'm going to add a little bit of water into the same beaker. Now, the next thing you should do is we're going to mix this to make a mixture.
So the cool thing about the mixture of cornstarch and water is, if you stir the cornstarch 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 like solid, and it breaks. So you can really take out a chunk of cornstarch.
We poured some mixture of cornstarch and water into a Petri dish. And as you can see, if you stir the mixture slowly, gently, it flows like a liquid. It drips. But now, if you try to scrap it more vigorously, the cornstarch actually breaks like solids. It cracks.
Yes, I'm going to just take out some cornstarch and put it on the speaker. And as you can see, the cornstarch-- actually, this is like liquid, because it flows. And I think this is good enough.
And we're going to switch on the amplifier. And as we can see, we're vibrating the cornstarch. But at this moment, the cornstarch is not really doing anything, because it's still like a liquid. So how do we shear thicken the suspension or the cornstarch, is we're going to vibrate at a higher frequency.
And with even larger displacement, and as we can see, the suspension is actually already shear thickened. So to prove that, I'm going to scrap it. Now, you've got this cornstarch monster. Oh, it's pretty messy.
ITAI COHEN: And as Neil was showing you, cornstarch has these very bizarre properties that involve the material becoming more viscous as it's stirred more vigorously. And actually, the reason behind this thickening behavior has been a mystery for a very long time.
And there have been two competing camps. One camp claims that these particles, which are just floating around in the liquid, as they're being pushed together, they're basically squeezing a film of liquid from out in between them, and that as it becomes harder and harder to squeeze this film, the particles lock into structures called hydroclusters that sort of move together and make it more difficult for the cornstarch to flow.
But an alternative hypothesis has been suggested recently, which states that these particles aren't just squeezing liquid from in between them, they're actually coming into contact. And it's the frictional properties of the particles that are determining this clustering behavior and how hard it is for these clusters to flow through the suspension.
So the question is, how do we decide between these two hypotheses? And to give you a better idea of our strategy, I want to walk you through some of the basic differences between the symmetries associated with each of these mechanisms-- the hydrodynamic versus the contact.
What I have here are two concentric cylinders. There's an inner cylinder that I can rotate with this lever and an outer cylinder that stays fixed. And in between the cylinders, there's very viscous Karo corn syrup. And here, I have some more of that corn syrup. And the difference between the stuff that I have here is that it's dyed.
And what I'm going to do is I'm going to take up some of the corn syrup into the syringe. And it's viscous, so it's going to take me a little bit of time to get it up there. And then what I'm going to do is I'm going to inject the dyed syrup from in between the two cylinders. So here I go.
Can't get too much further in. And there is the dyed spot. And now, what I'm going to do is I'm going to rotate the cylinder and mix the dye in. So the cylinder is being rotated once, being rotated twice, being rotated three times. And you can see-- and four times-- you can see the dye has been mixed in.
But what time reversibility tells us is that, if I now reverse the flows, then everything returns to exactly the same spot. So if I go backwards once, twice, three times, and four times, the dye returns to exactly the same place.
In contrast to this symmetric contribution from the hydrodynamics, if two of the particles that we have in our suspension actually touch each other, they form a frictional contact. And that force is asymmetric. What that means is that if I'm pushing the particles together, they feel a force.
But the minute I release that pushing and separate the particles, they feel no force at all. So in contrast to the hydrodynamic contribution, where the force to push particles together and pull them apart is symmetric, for the contact contribution where they're touching, that force is asymmetric.
NEIL LIN: So the idea of the experiment is pretty simple. We use a rheometer. Basically, it's a cone with a plate. And we sandwich a suspension or the sample of shear thickening fluid in between this geometry.
And the idea of the experiment is I'm going to put the cone in the suspension, the shear thickening fluid, and work to turn this clockwise until it reaches a steady state. Then immediately, I'm going to reverse the direction of the stirring. And then you immediately record the instantaneous response of the sample.
And so the hypothesis is if the response is hydrodynamic, which means multiple contribution comes from the background liquid, your supposed to get exactly the same viscosity for the response of the sample. But if it's because of a contact force, since the stress is being released, you're supposed to see a sudden drop of the force. That indicates it is really because of the frictional force between particles that gives you the shear thickening.
And what we really found was really striking. We find immediately after we reverse the stir interaction, we saw a sudden drop of the force. That's actually a smoking gun evidence, showing you that the frictional force is the dominant role in shear thickening mechanism.
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When you mix cornstarch and water you get an unusual result. If you treat it gently, it behaves like a liquid. But if you are rough with it, it behaves like a solid. How can this be? Itai Cohen, associate professor of physics, and his graduate student, Neil Lin, demonstrate the phenomenon and explain the reasons behind it.