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Centrifugation
What It is and Why It's Used
Understanding the forces that pull rotating objects outward
By Anne Marie Helmenstine, Ph.D.
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.
Anne Marie Helmenstine, Ph.D.
Chemistry Expert
Education
Ph.D., Biomedical Sciences, University of
Tennessee at Knoxville
B.A., Physics and Mathematics, Hastings
College
Introduction
Ph.D. in biomedical sciences from the
University of Tennessee at Knoxville - Oak Ridge National Laboratory.
Science educator with experience
teaching chemistry, biology, astronomy, and physics at the high school,
college, and graduate levels.
ThoughtCo and About Education chemistry
expert since 2001.
Widely-published graphic artist, responsible
for printable periodic tables and other illustrations used in science.
Experience
Anne Helmenstine, Ph.D. has covered
chemistry for ThoughtCo and About Education since 2001, and other sciences
since 2013. She taught chemistry, biology, astronomy, and physics at the high
school, college, and graduate levels. She has worked as a research
scientist and also abstracting and indexing diverse scientific literature for
the Department of Energy.
In addition to her work as a science writer,
Dr. Helmenstine currently serves as a scientific consultant, specializing in
problems requiring an interdisciplinary approach. Previously, she worked
as a research scientist and college professor.
Education
Dr. Helmenstine holds a Ph.D. in biomedical
sciences from the University of Tennessee at Knoxville and a B.A.
in physics and mathematics with a minor in chemistry from Hastings
College. In her doctoral work, Dr. Helmenstine developed ultra-sensitive
chemical detection and medical diagnostic tests.
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