Centrifuge

A laboratory tabletop centrifuge. The rotating unit, called the rotor, has fixed holes drilled at an angle (to the vertical), visible inside the smooth silver rim. Sample tubes are placed in these slots and the motor is spun. As the centrifugal force is in the horizontal plane and the tubes are fixed at an angle, the particles have to travel only a little distance before they hit the wall of the tube and then slide down to the bottom. These angle rotors are very popular in the lab for routine use.

A centrifuge is a piece of equipment that puts an object in rotation around a fixed axis (spins it in a circle), applying a potentially strong force perpendicular to the axis of spin (outward). The centrifuge works using the sedimentation principle, where the centripetal acceleration causes denser substances and particles to move outward in the radial direction. At the same time, objects that are less dense are displaced and move to the center. In a laboratory centrifuge that uses sample tubes, the radial acceleration causes denser particles to settle to the bottom of the tube, while low-density substances rise to the top.[1]

There are 3 types of centrifuge designed for different applications. Industrial scale centrifuges are commonly used in manufacturing and waste processing to sediment suspended solids, or to separate immiscible liquids. An example is the cream separator found in dairies. Very high speed centrifuges and ultracentrifuges able to provide very high accelerations can separate fine particles down to the nano-scale, and molecules of different masses.

Large centrifuges are used to simulate high gravity or acceleration environments (for example, high-G training for test pilots). Medium-sized centrifuges are used in washing machines and at some swimming pools to wring water out of fabrics.

Gas centrifuges are used for isotope separation, such as to enrich nuclear fuel for fissile isotopes.

History

Early 20th-century advertising poster for a milk separator.

English military engineer Benjamin Robins (1707–1751) invented a whirling arm apparatus to determine drag. In 1864, Antonin Prandtl proposed the idea of a dairy centrifuge to separate cream from milk. The idea was subsequently put into practice by his brother, Alexander Prandtl, who made improvements to his brother's design, and exhibited a working butterfat extraction machine in 1875.[2]

Types

A centrifuge machine can be described as a machine with a rapidly rotating container that applies centrifugal force to its contents. There are multiple types of centrifuge, which can be classified by intended use or by rotor design:

Types by rotor design:[3][4][5][6]

Types by intended use:

Industrial centrifuges may otherwise be classified according to the type of separation of the high density fraction from the low density one.

Generally, there are two types of centrifuges: the filtration and sedimentation centrifuges. For the filtration or the so-called screen centrifuge the drum is perforated and is inserted with a filter, for example a filter cloth, wire mesh or lot screen. The suspension flows through the filter and the drum with the perforated wall from the inside to the outside. In this way the solid material is restrained and can be removed. The kind of removing depends on the type of centrifuge, for example manually or periodically. Common types are:

In the sedimentation centrifuges the drum is a solid wall (not perforated). This type of centrifuge is used for the purification of suspension. For the acceleration of the natural deposition process of suspension the centrifuges use centrifugal force. With so-called overflow centrifuges the suspension is drained off and the liquid is added constantly.Common types are:[7]

Though most modern centrifuges are electrically powered, a hand-powered variant inspired by the whirligig has been developed for medical applications in developing countries.[8]

Uses

Laboratory separations

A wide variety of laboratory-scale centrifuges are used in chemistry, biology, biochemistry and clinical medicine for isolating and separating suspensions and immiscible liquids. They vary widely in speed, capacity, temperature control, and other characteristics. Laboratory centrifuges often can accept a range of different fixed-angle and swinging bucket rotors able to carry different numbers of centrifuge tubes and rated for specific maximum speeds. Controls vary from simple electrical timers to programmable models able to control acceleration and deceleration rates, running speeds, and temperature regimes. Ultracentrifuges spin the rotors under vacuum, eliminating air resistance and enabling exact temperature control. Zonal rotors and continuous flow systems are capable of handing bulk and larger sample volumes, respectively, in a laboratory-scale instrument.[1] Another application in laboratories is blood separation. Blood separates into three main components: red blood cells, white blood cells, and plasma. DNA preparation is another common application for pharmacogenetics and clinical diagnosis. DNA samples are purified and the DNA is prepped for separation by adding buffers and then centrifuging it for a certain amount of time. The blood waste is then removed and another buffer is added and spun inside the centrifuge again. Once the blood waste is removed and another buffer is added the pellet can be suspended and cooled. Proteins can then be removed and the entire thing can be centrifuged again and the DNA can be isolated completely.

Isotope separation

Other centrifuges, the first being the Zippe-type centrifuge, separate isotopes, and these kinds of centrifuges are in use in nuclear power and nuclear weapon programs.

Gas centrifuges are used in uranium enrichment. The heavier isotope of uranium (uranium-238) in the uranium hexafluoride gas tends to concentrate at 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 centrifugations to enrich uranium enough for use in a nuclear reactor (around 3.5% enrichment), and many thousands more to enrich it to weapons-grade (above 90% enrichment) for use in nuclear weapons.

Aeronautics and astronautics

The 20 g centrifuge at the NASA Ames Research Center

Human centrifuges are exceptionally large centrifuges that test the reactions and tolerance of pilots and astronauts to acceleration above those experienced in the Earth's gravity.

The first centrifuges used for human research were used by Erasmus Darwin, the grandfather of Charles Darwin. The first largescale human centrifuge designed for Aeronautical training was created in Germany in 1933.[9]

The US Air Force at Brooks City Base, Texas operates a human centrifuge while awaiting completion of the new human centrifuge in construction at Wright-Patterson AFB, Ohio. The centrifuge at Brooks City Base is operated by the United States Air Force School of Aerospace Medicine for the purpose of training and evaluating prospective fighter pilots for high-g flight in Air Force fighter aircraft.[10]

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.[10][11]

Non-Human centrifuge

At the European Space Agency (ESA) technology center ESTEC (in Noordwijk, the Netherlands) an 8-meter diameter centrifuge is used to expose samples in both fields of Life Sciences as well as Physical Sciences. This Large Diameter Centrifuge (LDC)[12] is operational since 2007. Samples can be exposed to a maximum of 20 times Earth gravity. With its 4 arms and 6 freely swing out gondolas it is possible to expose samples with different g-levels at the same time. Gondolas can be fixed at 8 different position. Depending on their locations one could e.g. run an experiment at 5 and 10g in the same run. Each gondola can hold an experiment of maximum 80 kg. (measured at 1g!). Experiments performed in this facility ranged from zebra fish, metal alloys, plasma,[13] cells,[14] liquids, Planaria,[15] Drosophila[16] or plants

Geotechnical centrifuge modeling

Geotechnical centrifuge modeling is used for physical testing of models involving soils. Centrifuge acceleration is applied to scale models to scale the gravitational acceleration and enable prototype scale stresses to be obtained in scale models. Problems such as building and bridge foundations, earth dams, tunnels, and slope stability, including effects such as blast loading and earthquake shaking.[17]

Synthesis of materials

High gravity conditions generated by centrifuge is applied in the chemical industry, casting, and material synthesis.[18][19][20][21] The convection and mass transfer are greatly affected by the gravitational condition. Researchers reported that the high-gravity level can effectively affect the phase composition and morphology of the products.[18]

Commercial applications

Sugar centrifugal machines, to separating sugar crystals from the ntrifugal controls, retrieved on June 5, 2010

Mathematical description

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. During circular motion the acceleration is the product of the radius and the square of the angular velocity , and the acceleration relative to "g" is traditionally named "relative centrifugal force" (RCF). The acceleration is measured in multiples of "g" (or × "g"), the standard acceleration due to gravity at the Earth's surface, a dimensionless quantity given by the expression:

A 19th-century hand cranked laboratory centrifuge.

where

is earth's gravitational acceleration,
is the rotational radius,
is the angular velocity in radians per unit time

This relationship may be written as

or

where

is the rotational radius measured in millimeters (mm), and
is rotational speed measured in revolutions per minute (RPM).

To avoid having to perform a mathematical calculation every time, one can find nomograms for converting RCF to rpm for a rotor of a given radius. A ruler or other straight edge lined up with the radius on one scale, and the desired RCF on another scale, will point at the correct rpm on the third scale.[22] Based on automatic rotor recognition, modern centrifuges have a button for automatic conversion from RCF to rpm and vice versa.

See also

References and notes

  1. 1 2 Susan R. Mikkelsen & Eduardo Cortón. Bioanalytical Chemistry, Ch. 13. Centrifugation Methods. John Wiley & Sons, Mar 4, 2004, pp. 247-267.
  2. ↑ Vogel-Prandtl, Johanna Ludwig Prandtl: A Biographical Sketch, Remembrances and Documents, English trans. V. Vasanta Ram. The International Centre for Theoretical Physics Trieste, Italy, pub. August 14, 2004. pp. 10-11.
  3. ↑ "Basics of Centrifugation". Cole-Parmer. Retrieved 11 March 2012.
  4. ↑ "Plasmid DNA Separation: Fixed-Angle and Vertical Rotors in the Thermo Scientific Sorvall Discovery™ M120 & M150 Microultracentrifuges" (Thermo Fischer publication)
  5. ↑ http://uqu.edu.sa/files2/tiny_mce/plugins/filemanager/files/4250119/lectures/1._instr.pdf
  6. ↑ Heidcamp, Dr. William H. "Appendix F". Cell Biology Laboratory Manual. Gustavus Adolphus College,. Retrieved 11 March 2012.
  7. ↑ "Centrifuges".
  8. ↑ M. Saad Bhamla, Brandon Benson, Chew Chai, Georgios Katsikis, Aanchal Johri & Manu Prakash (10 January 2017). "Hand-powered ultralow-cost paper centrifuge". Nature.
  9. ↑ http://www.dtic.mil/dtic/tr/fulltext/u2/a236267.pdf
  10. 1 2 "The Pull of HyperGravity - A NASA researcher is studying the strange effects of artificial gravity on humans.". NASA. Retrieved 11 March 2012.
  11. ↑ Hsu, Jeremy. "New Artificial Gravity Tests in Space Could Help Astronauts". Space.com. Retrieved 11 March 2012.
  12. ↑ van Loon JJWA, Krause J., Cunha H., Goncalves J., Almeida H., Schiller P. The Large Diameter Centrifuge, LDC, for life and physical sciences and technology. Proc. of the 'Life in Space for Life on Earth Symposium', Angers, France, 22–27 June 2008. ESA SP-663, December 2008.
  13. ↑ Šperka, Jiří; Souček, Pavel; Loon, Jack J. W. A. Van; Dowson, Alan; Schwarz, Christian; Krause, Jutta; Kroesen, Gerrit; Kudrle, Vít (2013-12-01). "Hypergravity effects on glide arc plasma". The European Physical Journal D. 67 (12): 261. ISSN 1434-6060. doi:10.1140/epjd/e2013-40408-7.
  14. ↑ Szulcek, Robert; Bezu, Jan van; Boonstra, Johannes; Loon, Jack J. W. A. van; Amerongen, Geerten P. van Nieuw (2015-12-04). "Transient Intervals of Hyper-Gravity Enhance Endothelial Barrier Integrity: Impact of Mechanical and Gravitational Forces Measured Electrically". PLOS ONE. 10 (12): e0144269. ISSN 1932-6203. PMC 4670102 Freely accessible. PMID 26637177. doi:10.1371/journal.pone.0144269.
  15. ↑ Adell, Teresa; Saló, Emili; Loon, Jack J. W. A. van; Auletta, Gennaro (2014-09-17). "Planarians Sense Simulated Microgravity and Hypergravity". BioMed Research International. 2014: 1–10. ISSN 2314-6133. PMC 4182696 Freely accessible. PMID 25309918. doi:10.1155/2014/679672.
  16. ↑ 1.      Paloma Serrano, Jack J.W. A. van Loon, F. Javier Medina · Ra´ ul Herranz Relation between motility accelerated aging and gene expression in selected Drosophila strains under hypergravity conditions. Microgravity Sci. Technol. (2013) 25:67–72. DOI 10.1007/s12217-012-9334-5.
  17. ↑ C. W. W. Ng; Y. H. Wang; L. M. Zhang (2006). Physical Modelling in Geotechnics: proceedings of the Sixth International Conference on Physical Modelling in Geotechnics. Taylor & Francis. p. 135. ISBN 0-415-41586-1.
  18. 1 2 Yin, Xi; Chen, Kexin; Zhou, Heping; Ning, Xiaoshan (August 2010). "Combustion Synthesis of Ti3SiC2/TiC Composites from Elemental Powders under High-Gravity Conditions". Journal of the American Ceramic Society. 93 (8): 2182–2187. doi:10.1111/j.1551-2916.2010.03714.x.
  19. ↑ Mesquita, R.A.; Leiva, D.R.; Yavari, A.R.; Botta Filho, W.J. (April 2007). "Microstructures and mechanical properties of bulk AlFeNd(Cu,Si) alloys obtained through centrifugal force casting". Materials Science and Engineering: A. 452-453: 161–169. doi:10.1016/j.msea.2006.10.082.
  20. ↑ Chen, Jian-Feng; Wang, Yu-Hong; Guo, Fen; Wang, Xin-Ming; Zheng, Chong (April 2000). "Synthesis of Nanoparticles with Novel Technology: High-Gravity Reactive Precipitation". Industrial & Engineering Chemistry Research. 39 (4): 948–954. doi:10.1021/ie990549a.
  21. ↑ Abe, Yoshiyuki; Maizza, Giovanni; Bellingeri, Stefano; Ishizuka, Masao; Nagasaka, Yuji; Suzuki, Tetsuya (January 2001). "Diamond synthesis by high-gravity d.c. plasma cvd (hgcvd) with active control of the substrate temperature". Acta Astronautica. 48 (2-3): 121–127. Bibcode:2001AcAau..48..121A. doi:10.1016/S0094-5765(00)00149-1.
  22. ↑ Nomogram example Archived December 9, 2013, at the Wayback Machine.

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