A centrifuge is a device used to separate liquids from solids by spinning. Long ago, people saw that gravity could eventually separate a sediment from a liquid or separate two liquids which do not mix. The heavier element within a container would descend, while the lighter element would rise to the surface. This process was extremely slow if left up to nature alone and was also wasteful, as evidenced by the way farmers used to separate cream from milk. They would let whole milk stand for several hours until the lighter cream rose to the top. They then skimmed off the cream with a wooden spoon, but as much as 40 percent of the cream was left in the milk. Later, small strainer dishes were used to extract the cream, yet this too was a slow and tedious process.
In 1877, Swedish inventor Carl Gustaf Patrik de Laval introduced a high-speed centrifugal cream separator. Milk was placed in a chamber where it was heated and then sent through tubes to a container that was spun at 4,000 revolutions per minute by a steam engine. The centrifugal force separated the lighter cream, causing it to settle in the center of the container. The heavier milk was pushed to the outer part and forced up to a discharge pipe. Thus, only the cream was left in the container. Several years later an improved cream separator was introduced with the capability for self-skimming and self-emptying. Other separators can extract impurities from lubricating oils, beer and wine, and numerous other substances. Other types of centrifuges were created in which spin dryers were used for filtering solids: a perforated drum was spun, driving any separated liquids to the outside where they were collected.
These spin dryers can now develop accelerations of up to 2,000 times the force of gravity. They are used in the food, chemical, and mineral industries to separate water from all sorts of solids. Other centrifuges remove blood serum (plasma) from the heavier blood cells.
However, some scientists needed faster rotations for separating smaller particles. Such particles, like DNA (deoxyribonucleic acid), proteins, and viruses, are too small to settle out with normal gravity; the banging of water molecules is enough to keep the particles from separating. The key was to build an ultracentrifuge that could spin fast enough to cause these small particles to settle out. In 1923 a Swedish chemist, Theodor Svedberg, developed a device that could spin fast enough to create gravity over 100,000 times normal. It could take small samples in glass containers, balance them on a cushion of air, and send jets of compressed air that touched the outer surface. By 1936 Svedberg had produced an ultracentrifuge that spun at 120,000 times per minute and created a centrifugal force equal to 525,000 times that of normal gravity. Newer models can accelerate samples to 2,000,000 times the force of gravity. This machine enabled biologists, biochemists, physicians, and other life scientists to examine viruses; cell nuclei; small parts within cells; and individual protein and nucleic acid molecules. Thus, genetic engineering became a field ripe with possibility. Centrifuges are also commonly used in the petroleum industry to separate oil components. In 1997, state of the art centrifuges, such as those built by U.S. Centrifuge, use a finite-element-analysis software made by Algor Inc. to design advanced centrifuge systems.
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