Stokes’ Law – explaining the science of separation
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Stokes’ Law – explaining the science of separation


Could you pass the yoghurt, please? Mum, do you know what these products have in common? No, what? I’ve learned about something called separation. Separation is an important process in dairies that makes many dairy products possible. I’m going to tell you why. Okay. Separation is a natural process. Imagine if we milked a cow and let the milk stand it would eventually separate by itself into cream and skimmed milk. This is because the fat globules – that’s cream – are lighter, and the remaining milk is heavier. The milk has been separated by gravity. How quickly this happens depends on a few things. Right… Look at these dollops of honey. Their size will affect how fast they move
through a liquid. A big dollop will drop to the bottom faster
than a small one will. So mum, in the same way, when milk
separates by gravity the heavier skimmed milk is held down by gravity while the cream, which is lighter, floats to the top. The speed a particle travels also depends
on how dense it is – that means compact – compared to the liquid around it. A particle that is more dense travels more quickly to the bottom of a liquid than a less dense particle. I’m going to show you what I mean. If we took two spoons, one wooden, and one metal, the denser metal spoon would fall through the tea faster than the wooden one, which is less dense. So you see, mum, the combination of size and density of a particle will affect the speed a fat globule moves through milk during separation. Okay, I’m with you. Now imagine if the liquid the particle is
traveling in was thicker, like yummy yoghurt. The thicker the liquid is, the more time it takes for a particle to travel through it. Because there is resistance in the liquid, which is also known as viscosity. Do you see? The speed a particle travels through a liquid
depends on several things. The size of the particle, its density, and the viscosity of the liquid it’s traveling through.’ These all come together in a really cool formula, known as Stokes’ Law. It describes the speed a particle travels through a liquid. So, if we look at Stokes’ Law, we can see that if a particle is very large and dense compared to the liquid it is in the speed of separation will be faster than if the same particle was traveling in a thick liquid, such as yoghurt. You see? It’s cool, isn’t it? That’s most of Stokes’ Law but check this out! Stokes’ Law is good for explaining what happens when we leave milk to separate under the Earth’s gravity. It’s a slow process. But, there is a way to speed it up. Okay… Imagine if we spin the beaker around like this. Centrifugal force will speed up the journey of the cream through the liquid. The milk separates here, too. The lighter fat particles, the cream, are at the top while the heavier skimmed milk is at the bottom. So centrifugal force also affects separation, mum. And we can add it to the other factors in Stokes’ Law. The same things happened as with separation by gravity but it’s happened much faster, thanks to centrifugal force. The dairies that produce the milk we buy are using Stoke’s Law and big centrifugal separators. They separate the cream from the milk which are then mixed back together in the right amounts, depending on the type of milk or yoghurt, or cheese, or whatever dairy product they want to make for us. So mum, now you know some of the science behind the stuff on our breakfast table. Don’t you think that’s cool?

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