Walking on custard: shear-thickening fluid

19 November 2019

Interview with 

Mazi Jalaal and Mike Cates, Department of Applied Maths and Theoretical Physics, University of Cambridge

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Shear-thickening fluids, also known as non-Newtonian fluids, become solid and rigid when pushed

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Did you know that you can actually walk on custard? It’s true! Well, specifically custard made from custard powder. It’s because custard powder is mostly cornstarch, and when you mix cornstarch with a bit of water, you create a liquid that acts like a solid when you try to move it too quickly. The stuff is lovingly called “oobleck” in children’s science demonstrations. It’s a great demonstration of a non-Newtonian fluid, specifically one that is ‘shear thickening’, which means that it becomes thicker and more viscous the more shear force you apply to it. Phil Sansom had a go at making some for himself...

Phil - Oh, that doesn't look good. Oh no!

Mazi - You do not want to walk on this. 

Phil - That's a liquid!

Mazi - That's a very liquidy custard, yes. I don't think you even want eat it.

Phil - That is me with fluid dynamist Mazi Jalaal. And the reason we're so disappointed is that, while it's true you can walk on custard, you can't do it by following the recipe on the back of the tin. Here is Mazi explaining how it's supposed to work.

Mazi - Most of the fluids that we're familiar with, like water or honey, are Newtonian. So basically what it means is that they have one, constant viscosity. But many materials - like many foods - their response depends on how fast and how hard you push them.

Phil - So if you hit it harder it becomes more viscous? Is that right?

Mazi - Again, depends on the material. So let's think about ketchup, for instance, which is a very common example of food for shear thinning materials. Ketchup, if you hold it, it doesn't come out; but if you put enough stress on it, it starts to flow. So we basically somehow yield it, and after that it looks like a shear thinning material, meaning the harder you stress them the faster it flows.

Phil - That's why you hit the ketchup bottle to get the ketchup out.

Mazi - That's exactly what you do.

Phil - That's shear thinning, where the liquid gets runnier the more force you apply. The other type of non-Newtonian fluid is like cornstarch and water: that's shear thickening, getting thicker when you apply force to it. It happens when you have these tiny particles suspended in a liquid.

Mazi - For instance, in cornstarch there are basically starch particles that interact with each other. That gives this very strange behaviour. Basically these suspensions are in water, and if you press them hard enough the solid particles of starch start to somehow make some sort of friction onto each other.

Phil - Is that something that applies to other materials in other places?

Mazi - It is applied to almost any industrial fluid that you can think of. Cosmetic industries, the oil industry... most of the fluids that you're dealing with are not Newtonian.

Phil - The science of dealing with flowing materials like this is called rheology. And if you're a rheologist like Mazi you might want to design a material to have the perfect amount of shear thinning or shear thickening. Now this stuff can get kind of extreme. I went to find out more from Lucasian Professor of Mathematics Mike Cates.

Mike - There's a guy called Norman Wagner who did have a big contract with the US defense funding agency. And they were wondering whether if you actually shot bullets at this fluid it could stop a bullet. And the answer is: not quite. What you have to do is, you have to get some of the shear thickening fluid and you have to laminate it between layers of Kevlar. Then that combination does seem to be capable, certainly of stopping a stab wound, and possibly a bullet.

Phil - Yep, you heard that right. Bulletproof custard. 

Mike - The thing about the dense custard powder suspension is that it is a fluid; we know how it thickens like that, and it jams; nonetheless it can fracture and fly to pieces. But if you put it in that sandwich with the Kevlar layers on the outside, that holds it in. The combination can stop a projectile; maybe a bullet. 

Phil - Is this actual cornstarch, custard powder, that we're talking about here?

Mike - Cornstarch has become actually quite a good benchmark model system. It has just the right interactions; particles which are very hard; quite smooth; and also have some sort of little repulsion between them. And cornstarch, when you put in water, the particles become charged so that they automatically get this repulsion. Of course there are much more sophisticated synthetic systems where you make specified particle size, perfectly spherical particles; and some of those are certainly better than cornstarch in any particular application, like a bulletproof vest. In particular, cornstarch in water, if you came back in a couple of weeks time would be pretty smelly. Bacteria love cornstarch; they're going to eat your clothes.

Phil - So maybe don't expect Bird's to become the number one military contractor. Anyway, back to Mazi and I, where we've managed to perfect our formula.

Mazi - So this one has almost three quarters of the custard powder and one quarter water. So now what you see is a shear thickening fluid.

Phil - Is it working?

Mazi - It is working actually very well.

Phil - Can I try? Oh yeah, wow! It's really hard to mix when you're trying, but then when you just glide it through the liquid it goes easily. That is bizarre.

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