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A centrifuge is a device that spins liquid samples at high speeds and thus creates a strong centripetal force causing the denser materials to travel towards the bottom of the centrifuge tube more rapidly than they would under the force of normal gravity.
The theory behind centrifugation.
The idea here is pretty straight forward and mechanical. If you want the more dense materials to be separated from the less dense materials, you need a force that differentiates between particles of different density. Think about a swimming pool with a rock and a piece of styrofoam. The rock is denser than water and thus it sinks. The styrofoam is less dense than water, and thus it floats. Density is of course mass per unit volume. So, if you have a bag full of rocks and styrofoam and you want to separate one from the other, just dump the mixture into some water under the influence of the earth's gravity. The rocks will displace the water because they have greater mass for a given volume and gravity will pull them through the water. On the other hand, the water will displace the styrofoam because a certain volume of water weighs more than the same volume of styrofoam.
However, there are many things that are much closer in density than rocks and styrofoam and it is much harder to separate them just under the Earth's gravity. In addition, diffusion is always at work as random motion smears out small differences due to density. To overcome this, or sometimes just to make the separation process faster, it would be nice to come up with a way of generating larger mass (density) dependent forces than are available from the Earth's gravity alone. Another way to generate a mass dependent force is to spin something. As you know from physics, a body in motion tends to continue in motion along a straight path unless some force is exerted on it to change its path. Thus in order to force something to go in a circle, we must exert force on it pulling it in towards the center. An equal and opposite force will always result, pushing out from the center. This is cetripital force, and it is just the mass of the object times the acceleration required to keep it from flying outward along a straight line. Thus, things with larger mass (for a given volume) will have a greater force exerted on them and they will move towards the outer edge of the container more quickly than the things with a lower mass per volume.
The acceleration required to keep the object from flying outward along a straight path is given by w2r where the greek letter omega stands for the speed of revolution (see below for units) and r is the distance from the axis of the revolution to the position of the sample. To get a the force felt by the sample, we multiply this by the mass of the sample: F = mw2r, where F is the force and m is the mass. Please notice two things about this equation. It is linearly proportional to the mass of the object, but the force increases with the square of the rotation speed. Thus, going from 1000 rpm to 10,000 rpm increases the forces involved by a factor of 100.
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