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Centrifuge And Centrifugal Force
What
Centrifugation Is and Why It's Used
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).
The modern device traces its
origins to a spinning arm apparatus designed in the 18th century by engineer
Benjamin Robins to determine drag.
In 1864, Antonin Prandtl
applied the technique to separate milk and cream. His brother refined the
technique, inventing a butterfat extraction machine in 1875.
While centrifuges are still
used to separate milk components, their use has expanded to many other areas of
science and medicine.
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 other
purposes than mechanical separation.
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 the forces at work. If the bucket spins fast enough, the
water is pulled into it 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 one chamber rather than individual sample chambers.
Very
high-speed centrifuges and ultracentrifuges spin at such a high rate that they
can be used to separate molecules of different masses or even isotopes of atoms.
For example, a gas centrifuge
may be used to enrich uranium, as the heavier isotope is pulled outward more
than the lighter one. Isotope separation is used for scientific research and to
make nuclear fuel and nuclear weapons.
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. These
machines are used to isolate components of immiscible liquids and
suspensions.
Uses include separating blood
components, isolating DNA, and purifying chemical samples.
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, for example. A similar device spins the water out of
swimsuits.
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. C
entrifuges 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 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.
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 a sample and its volume.
Anne Marie Helmenstine, Ph.D.
· 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.
Anne
Marie Helmenstine, Ph.D.
ThoughtCo
and Dotdash
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