A centrifuge is a piece of equipment, generally driven by a motor, that puts objects in rotation around a central, fixed axis, applying a force perpendicular to the axis. The equipment consists of a fixed base and a rotating component, called a rotor, that holds the objects or samples to be spun. The spinning action, called centrifugation, subjects the samples to forces that far exceed the force of gravity.
In general, a centrifuge is useful for separating materials in certain types of mixtures. If a liquid (or solution) contains materials of different densities or widely differing molecular weights, these materials may be separable by using a centrifuge. For example, a centrifuge may be used to separate cream from milk, or to separate biological cells or virus particles from suspension in a liquid. A household washing machine acts as a centrifuge during the spin cycle, when the liquid (water) is separated from the solids (clothing). Specialized gas centrifuges are employed for enrichment of the isotope uranium-235, for use in nuclear reactors or nuclear weapons. Some centrifuges are designed to accommodate humans or animals to test the effects of high gravitational forces on their bodies.
English military engineer Benjamin Robins (1707-1751) invented a whirling arm apparatus to determine drag. In 1864, Antonin Prandtl invented the first dairy centrifuge to separate cream from milk. In 1879, Swedish engineer Gustaf de Laval demonstrated the first continuous centrifugal separator, making its commercial application feasible. In the 1920s, Swedish chemist Theodor Svedberg built the ultracentrifuge, using it to determine the molecular weights of viruses and proteins.
During the process of centrifugation, the centrifugal force acts to separate various components of a mixture. More-dense components of the mixture migrate away from the axis of the centrifuge, while less-dense components migrate toward the axis. The rate of centrifugation is specified by the acceleration applied to the sample, typically quoted in revolutions per minute (RPM) or in multiples of g, the acceleration due to gravity at the Earth's surface. The particles' settling velocity during centrifugation is a function of their size and shape, centrifugal acceleration, the volume fraction of solids present, the density difference between the particles and the liquid, and the viscosity of the liquid.
Protocols for centrifugation typically specify the amount of acceleration to be applied to the sample, rather than specifying a rotational speed such as revolutions per minute. This distinction is important because two rotors with different diameters running at the same rotational speed will subject samples to different accelerations. The acceleration is often quoted in multiples of g, the standard acceleration due to gravity at the Earth's surface.
The acceleration can be calculated as the product of the radius and the square of the angular velocity.
There are at least five types of centrifuge:
Industrial centrifuges may otherwise be classified according to the type of separation of the high density fraction from the low density one :
Centrifuges are often used in chemistry, biology, and biochemistry laboratories for isolating and separating materials of differing densities or molecular weights. These centrifuges vary widely in speed and capacity. They usually consist of a rotor containing two, four, six, or more numbered wells that carry centrifuge tubes containing the samples.
Gas centrifuges are used in uranium enrichment. The heavier isotope of uranium (uranium-238) in uranium hexafluoride gas tends to concentrate near the walls of the centrifuge as it spins, while the desired uranium-235 isotope is extracted and concentrated with a scoop selectively placed inside the centrifuge. It takes many thousands of centrifuges to enrich uranium enough (around 3.5 percent enrichment) for use in a nuclear reactor, and many thousands more to enrich it to weapons-grade (around 90 percent enrichment) for use in nuclear weapons.
Human centrifuges are exceptionally large, designed to test the reactions and tolerance of pilots and astronauts to accelerations much higher than those experienced in the Earth's gravitational field.
The U.S. Air Force at Holloman Air Force Base, New Mexico, operates a human centrifuge. The centrifuge at Holloman AFB is operated by the aerospace physiology department for the purpose of training and evaluating prospective fighter pilots for high-g flight in Air Force fighter aircraft. It is important to note that the centrifuge at Holloman AFB is far more difficult for a pilot to tolerate the high-g environment in the centrifuge than in a real fighter aircraft. This well-known fact is based on countless accounts from experienced operational fighter pilots.
The use of large centrifuges to simulate a feeling of gravity has been proposed for future long-duration space missions. Exposure to this simulated gravity would prevent or reduce the bone decalcification and muscle atrophy that affect individuals exposed to long periods of freefall. An example of this can be seen in the film 2001: A Space Odyssey.
Relative centrifugal force is the measurement of the force applied to a sample within a centrifuge. This can be calculated from the speed (RPM) and the rotational radius (cm) using the following calculation.
All links retrieved April 28, 2013.
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