Editing Liquid crystal (section)

====Nematic phase====
{{see also|Biaxial nematic|Twisted nematic field effect}}
[[File:LiquidCrystal-MesogenOrder-Nematic.jpg|thumb|left|120px|Alignment in a nematic phase]]
[[File:Smectic nematic.jpg|thumb|Phase transition between a nematic (left) and smectic A (right) phases observed between crossed [[polarizers]]. The black color corresponds to isotropic medium.]]

The simplest liquid crystal phase is the nematic. In a nematic phase, {{linktext|calamitic}} (rod-like) organic molecules lack a crystalline positional order, but do self-align with their long axes roughly parallel. The molecules are free to flow and their center of mass positions are randomly distributed as in a liquid, but their orientation is constrained to form a long-range directional order.<ref>{{cite journal |url=http://www.csupomona.edu/~jarego/pubs/RD2_LC.pdf |title=Asymmetric synthesis of a highly soluble 'trimeric' analogue of the chiral nematic liquid crystal twist agent Merck S1011 | vauthors = Rego JA, Harvey JA, MacKinnon AL, Gatdula E |s2cid=95102727 |journal=Liquid Crystals |volume=37 |issue=1 |date=January 2010 |pages=37–43 |doi=10.1080/02678290903359291 |url-status=dead |archive-url=https://web.archive.org/web/20121008152304/http://www.csupomona.edu/~jarego/pubs/RD2_LC.pdf |archive-date=October 8, 2012 |df=mdy-all }}</ref> 

The word ''nematic'' comes from the [[Greek language|Greek]] {{lang|el|νήμα}} (''{{langx|el|nema}}''), which means "thread". This term originates from the [[disclination]]s: thread-like [[topological defect]]s observed in nematic phases.  

Nematics also exhibit so-called "hedgehog" [[topological defect]]s. In two dimensions, there are topological defects with [[topological charge]]s {{math|+{{sfrac|1|2}}}} and {{math|-{{sfrac|1|2}}}}. Due to hydrodynamics, the {{math|+{{sfrac|1|2}}}} defect moves considerably faster than the {{math|-{{sfrac|1|2}}}} defect. When placed close to each other, the defects attract; upon collision, they annihilate.<ref>{{cite journal |vauthors=Géza T, Denniston C, Yeomans JM|title=Hydrodynamics of Topological Defects in Nematic Liquid Crystals |journal=Physical Review Letters |date=26 February 2002 |volume=88 |issue=10 |pages=105504 |doi=10.1103/PhysRevLett.88.105504|pmid=11909370 |arxiv=cond-mat/0201378 |bibcode=2002PhRvL..88j5504T |s2cid=38594358 }}</ref><ref>{{cite journal|vauthors=Géza T, Denniston C, Yeomans JM|title=Hydrodynamics of domain growth in nematic liquid crystals |journal=Physical Review E |date=21 May 2003 |volume=67 |issue=5 |pages=051705 |doi=10.1103/PhysRevE.67.051705|pmid=12786162 |arxiv=cond-mat/0207322 |bibcode=2003PhRvE..67e1705T |s2cid=13796254 }}</ref> 

Most nematic phases are uniaxial: they have one axis (called a directrix) that is longer and preferred, with the other two being equivalent (can be approximated as cylinders or rods). However, some liquid crystals are [[biaxial nematic]], meaning that in addition to orienting their long axis, they also orient along a secondary axis.<ref>{{cite journal | vauthors = Madsen LA, Dingemans TJ, Nakata M, Samulski ET | title = Thermotropic biaxial nematic liquid crystals | journal = Physical Review Letters | volume = 92 | issue = 14 | pages = 145505 | date = April 2004 | pmid = 15089552 | doi = 10.1103/PhysRevLett.92.145505 | bibcode = 2004PhRvL..92n5505M }}</ref> Nematic crystals have fluidity similar to that of ordinary (isotropic) liquids but they can be easily aligned by an external magnetic or electric field. Aligned nematics have the optical properties of uniaxial crystals and this makes them extremely useful in [[liquid-crystal display]]s (LCD).<ref name=castellano>{{cite book| last = Castellano | first = Joseph A. | name-list-style = vanc |title =Liquid Gold: The Story of Liquid Crystal Displays and the Creation of an Industry| publisher=World Scientific Publishing|date =2005| isbn = 978-981-238-956-5}}</ref>

Nematic phases are also known in non-molecular systems: at high magnetic fields, electrons flow in [[charge density wave|bundles or stripes]] to create an "electronic nematic" form of matter.<ref>{{cite journal|department=Letters to Nature|title=Electronic liquid-crystal phases of a doped Mott insulator|first1=SA|last1=Kivelson|first2=E|last2=Fradkin|first3=VJ|last3=Emery|journal=Nature|volume=393|date=11 June 1998|issue=6685 |publisher=Macmillan|pages=550–553|doi=10.1038/31177 |arxiv=cond-mat/9707327 |bibcode=1998Natur.393..550K |s2cid=4392009 |url=https://www.nature.com/articles/31177.pdf|name-list-style=vanc}}</ref><ref>{{cite journal|title=Nematic Fermi Fluids in Condensed Matter Physics|first1=Eduardo|last1=Fradkin|first2=Steven A|last2=Kivelson|first3=Michael J|last3=Lawler|first4=James P|last4=Eisenstein|first5=Andrew P|last5=Mackenzie|name-list-style=vanc|journal=Annual Review of Condensed Matter Physics|date=May 4, 2010|volume=1|pages=153–178|doi=10.1146/annurev-conmatphys-070909-103925|arxiv=0910.4166|bibcode=2010ARCMP...1..153F|s2cid=55917078|url=https://authors.library.caltech.edu/20497/|access-date=August 5, 2022|archive-date=September 14, 2020|archive-url=https://web.archive.org/web/20200914003558/https://authors.library.caltech.edu/20497/|url-status=dead}}</ref>