# Centrifugation: What It is and Why It's Used

## Understanding the forces that pull rotating objects outward

The term centrifuge can refer to a machine that houses a rapidly rotating container to separate its contents by density (noun) or to the act of using the machine (verb). Centrifuges are most often used to separate different liquids and solid particulates from liquids, but they may be used for gases. They are also used for purposes other than mechanical separation.

## Invention and Early History of the Centrifuge

The modern centrifuge traces its origins to a spinning arm apparatus designed in the 18th century by English military engineer Benjamin Robins to determine drag. In 1864, Antonin Prandtl applied the technique to separate the components of milk and cream. In 1875, Prandtl's brother, Alexender, refined the technique, inventing a machine to extract butterfat. While centrifuges are still used to separate milk components, their use has expanded to many other areas of science and medicine.

## How a Centrifuge Works

A centrifuge gets its name from centrifugal force—the virtual force that pulls spinning objects outward. Centripetal force is the real physical force at work, pulling spinning objects inward. Spinning a bucket of water is a good example of these forces at work.

If the bucket spins fast enough, the water is pulled inward and doesn't spill. If the bucket is filled with a mixture of sand and water, spinning it produces centrifugation. According to the sedimentation principle, both the water and sand in the bucket will be drawn to the outer edge of the bucket, but the dense sand particles will settle to the bottom, while the lighter water molecules will be displaced toward the center.

The centripetal acceleration essentially simulates higher gravity, however, it's important to keep in mind the artificial gravity is a range of values, depending on how close an object is to the axis of rotation, not a constant value. The effect is greater the further out an object gets because it travels a greater distance for each rotation.

## Types and Uses of Centrifuges

The types of centrifuges are all based on the same technique but differ in their applications. The main differences between them are the speed of rotation and the rotor design. The rotor is the rotating unit in the device. Fixed-angle rotors hold samples at a constant angle, swinging head rotors have a hinge that allows sample vessels to swing outward as the rate of spin increases, and continuous tubular centrifuges have a single chamber rather than individual sample chambers.

Separating Molecules and Isotopes: Extremely high-speed centrifuges and ultracentrifuges spin at such high rates that they can be used to separate molecules of different masses or even isotopes of atoms. Isotope separation is used for scientific research and to make nuclear fuel and nuclear weapons. For example, a gas centrifuge may be used to enrich uranium, as the heavier isotope is pulled outward more than the lighter one.

In the Lab: Laboratory centrifuges also spin at high rates. They may be large enough to stand on a floor or small enough to rest on a counter. A typical device has a rotor with angled drilled holes to hold sample tubes. Because the sample tubes are fixed at an angle and centrifugal force acts in the horizontal plane, particles move a tiny distance before hitting the wall of the tube, allowing dense material to slide down. While many lab centrifuges have fixed-angle rotors, swinging-bucket rotors are also common. Such machines are employed to isolate components of immiscible liquids and suspensions. Uses include separating blood components, isolating DNA, and purifying chemical samples.

High-Gravity Simulation: Large centrifuges may be used to simulate high-gravity. The machines are the size of a room or building. Human centrifuges are used to train test pilots and conduct gravity-related scientific research. Centrifuges may also be used as amusement park rides. While human centrifuges are designed to go up to 10 or 12 gravities, large-diameter non-human machines can expose specimens to up to 20 times normal gravity. The same principle may one day be used to simulate gravity in space.

Industrial Centrifuges are used to separate components of colloids (like cream and butter from milk), in chemical preparation, cleaning solids from drilling fluid, drying materials, and water treatment to remove sludge. Some industrial centrifuges rely on sedimentation for separation, while others separate matter using a screen or filter. Industrial centrifuges are used to cast metals and prepare chemicals. The differential gravity affects the phase composition and other properties of the materials.

Everyday Applications: Medium-size centrifuges are common in daily life, mainly to quickly separate liquids from solids. Washing machines use centrifugation during the spin cycle to separate water from laundry. A similar device spins the water out of swimsuits. Salad spinners, used to wash and then spin dry lettuce and other greens, are another example of a simple centrifuge.

## Related Techniques

While centrifugation is the best option for simulating high gravity, there are other techniques that may be used to separate materials. These include filtration, sieving, distillation, decantation, and chromatography. The best technique for an application depends on the properties of the sample being used and its volume.

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