How do centrifuges separate samples?

1. This tube holds muddy pond water. Sand, fine clay and water are all mixed together. How can we pull them apart?
2. If the tube just stands still, gravity pulls the heavy sand down quite quickly. But the tiny clay bits sink so slowly that it takes hours.
3. So we put the tube in a centrifuge and close the lid. A motor spins it thousands of times a minute, and the tube holders swing out sideways.
4. Spinning pushes everything toward the outer end of the tube. The big, heavy sand grains win the race. The fine clay follows, and clear water is left near the middle.
5. One tube on its own makes the rotor heavier on one side, so it wobbles. A matching tube straight across balances it, and it spins smoothly.
6. After a few minutes the rotor stops. The mixture has become neat layers: sand at the bottom, clay above it and clear water on top.

Tap each part of the centrifuge. Every one moves when you find it!

• Lid. The strong lid covers the spinning rotor. It keeps fingers out and keeps everything inside if a tube leaks.
• Lid lock. The lock holds the lid shut. The motor will not start until it is locked, and the lid will not open until the rotor has stopped.
• Rotor. The rotor is the spinning part in the middle. It carries the tubes around in a circle, thousands of times each minute.
• Swing buckets. Each tube sits in a bucket on a hinge. When the rotor spins, the buckets swing out sideways, so the bottom of each tube points outward.
• Sample tubes. The tubes hold the mixture. They are loaded in matching pairs, straight across from each other, so the rotor stays balanced.
• Motor. An electric motor under the rotor turns a shaft. The shaft spins the rotor up to speed and then slows it down again.
• Springy mounts. The motor sits on springy mounts. They soak up small wobbles so the machine does not shake or walk across the bench.
• Control panel. On the panel you set the speed and the time. The screen shows how fast the rotor is turning.

Tap the holes to balance the rotor, choose a speed and press Spin. Watch the close-up tube. What does spinning faster do?

Experiment: The run always lasts 5 minutes. What happens to the sample if you spin it faster?

1. Tap the hole straight across from the sample to add a matching tube.
2. Set the speed to Slow, press Spin and look at the close-up tube when it stops.
3. Press Shake it up again, set the speed to Fast and spin once more.

What we found: A slow spin only settles the big sand grains. A fast spin pushes much harder, so even the tiny bits reach the outer end and the sample ends in neat layers.

Turn on JavaScript to try it yourself.

Load 2 tubes, then 3, then 4. Each time, place them so the rotor is balanced and spin it.

Have you ever been on a playground roundabout that was spinning fast? You feel pushed toward the outside edge. Really, your body is trying to keep going in a straight line, and the roundabout keeps pulling you around the bend. Everything inside a spinning tube feels the same thing.

In a mixture, the heavy bits push outward harder than the liquid around them, so they sink toward the outer end of the tube. Light things, like oil or cream, are squeezed the other way, toward the middle. Big, dense bits such as sand move fast. Tiny bits such as clay move slowly, because the water drags on them.

Gravity would do the same job if you waited long enough, but a centrifuge can push hundreds or thousands of times harder than gravity. A job that takes hours in a jar takes minutes in a rotor. Spinning twice as fast pushes four times as hard.

Centrifuges are everywhere. Hospital labs spin blood to separate the red cells from the pale yellow liquid called plasma. Dairies spin milk to take off the cream. A washing machine's spin cycle and a salad spinner use the same idea to fling water out. The tubes must always be balanced, or the machine shakes itself and stops.

Key words

Centrifuge

A machine that spins tubes very fast to separate a mixture.

Mixture

Two or more things mixed together that can be separated again, like sand in water.

Density

How heavy something is for its size. A stone is denser than a cork.

Rotor

The spinning part of a centrifuge that carries the tubes.

Relative centrifugal force (RCF)

How many times stronger than gravity the outward push is.

Pellet

The packed layer of solid bits that collects at the outer end of a tube.

Check your thinking

1. When a tube of muddy water spins, where do the heaviest bits end up?

   Show the answer

   At the outer end of the tube. Spinning pushes everything outward. The heavy bits push hardest, so they reach the outer end first.

2. You have one sample tube to spin. What should you do first?

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   Put a tube of the same weight straight across from it. A matching tube on the opposite side balances the rotor, so it does not wobble.

3. Why does a centrifuge separate muddy water faster than just waiting?

   Show the answer

   It pushes the bits much harder than gravity does. Spinning gives an outward push that can be hundreds of times stronger than gravity, so the bits settle in minutes.

4. Salad dressing is spun in a centrifuge. Oil is lighter than vinegar. Where does the oil go?

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   To the end nearest the middle. The heavier vinegar pushes outward and squeezes the lighter oil toward the middle.

5. Three tubes of equal mass must be spun in a rotor with six holes. How can they be balanced?

   Show the answer

   Space them evenly, with one empty hole between each. Three equal masses 120 degrees apart cancel each other, so the center of mass stays on the axis.

6. Why will a centrifuge not let you open the lid while it is spinning?

   Show the answer

   The rotor is moving very fast, so the lock keeps hands safely out. A spinning rotor has lots of energy. The lock stays shut until it has stopped completely.