Non-Newtonian Fluids on a Particle Level

Recently I found this new concept–Non-Newtonian Fluids. Many people have tried out and played with this kind of “new” matter inside their houses or shot videos about it.

I remember that I saw some military videos about putting this kind of tech onto bullet-proof vests 4 or 5 years ago. Yeah at the time I thought it was a tech.

However, this physics phenomenon exists since the Genesis.

The phenomenon is fairly simple. If you apply a sudden force onto the surface of the Oobleck or cornstarch water, it would suddenly become a solid and you would feel the force back at you at your hand. But if you put the force slowly, and gently, you would slowly sink in it just like any liquid.

The thing with Non-Newtonian Fluids is that they don’t behave the way normal liquids do. And not all Non-Newtonian Fluids behave the same. Technically, viscosity is what matters in this case. “The rate of liquid’s flow.” Normal liquids’ viscosities are constant, whereas, NNF’s viscosities are variable.

The classic example of NNF is the simple cornstarch and water mixture, Oobleck. But there are also other suspensions out there are NNF, e.g. ketchup, blood, toothpaste, quicksand. But they don’t all behave the way Oobleck does. Not our subject here today, as we are going to find out why NNF works the way we don’t expect it to.

To understand a physical phenomenon, the best way is to scale down the scope to the “indivisible” level–particle level. This way we are going to be much easier to find out why particles perform what we’ve seen in the macro-scale.

Not molecules though, I think this question lies onto the particle level, a bit bigger than molecules.

OK, so here comes the questions. Why Oobleck would suddenly become solid when we apply quick force onto the surface? And why it would auto dissolve to the original state after the force disappeared?

I’ve tried to find out the answers on Internet, and actually, I’ve found some answers. They are rather similar, but all of them are theoretical. Two answers, precisely. And it seems like nobody has yet to crack the mysteries behind this phenomenon in current times.

Answer 1 is a broad, but practical guess. The scientist believes that: On a microscopic level, the particles of Oobleck are still solid form. They didn’t change form after they merged with water. However, water is still liquid form. So when these solid particles suspended in the water/liquid, and tons of them (The first rule of creating a Oobleck is that you have to pour twice or three times as much cornstarch into one water so that it can form a gel-like substance.) can barely move around. It’s like in an overcrowded subway carriage, there are little space between each one of particles. So they can move, but very slowly. Water is in the space of these cornstarch particles.

Here comes the tricky part. When you apply a sudden force onto the surface of Oobleck, the particles of cornstarch barely have any time to move away from the force. So that they would simply “become” a solid to respond back to your force.

And do remember, sudden force always means a big force with direction. Since acceleration is the factor of force.

While you put it in a more subtle, gentle and “kind” way, the fluid would welcome you and keep the liquid properties. The particles of cornstarch have enough time and the force is not violent and big so that they can move away from it.

Answer 2, delves into more of the particles. Identical to answer 1, it also supports that simply because particles are too dense to move.

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