液晶Liquid crystal

按形成条件分为热致液晶（随温度变化）和溶致液晶（随浓度变化，如表面活性剂和生物膜）。热致液晶按有序程度分为：向列相（分子长轴大致平行，但位置无序）、近晶相（分子排成层状）和胆甾相（向列层逐层扭转形成螺旋，能选择性反射特定颜色的光）。液晶分子的介电各向异性使其取向能被电场改变，从而调制偏振光——这是液晶显示的原理。

Thermotropic liquid crystals change phase with temperature, and lyotropic ones with concentration, as with surfactants and biological membranes. By degree of order thermotropic phases are nematic (long axes roughly parallel but positions random), smectic (molecules arranged in layers) and cholesteric (nematic layers twisting into a helix that selectively reflects particular colours). Because the molecules are dielectrically anisotropic, an electric field can reorient them and so modulate polarised light, the principle of liquid-crystal displays.

1888 年奥地利植物学家莱尼策尔发现苯甲酸胆甾醇酯有两个熔点：145.5 °C 时熔化成浑浊液体，178.5 °C 时才变清亮。德国物理学家莱曼研究后命名为“液晶”。1968 年美国 RCA 公司的海尔迈耶制成第一个液晶显示器。1973 年格雷合成了室温稳定的氰基联苯类液晶（如 5CB），使液晶显示实用化。德热纳因液晶与高分子有序现象研究获 1991 年诺贝尔物理学奖。

In 1888 the Austrian botanist Friedrich Reinitzer found that cholesteryl benzoate has two melting points: at 145.5 °C it melts to a cloudy liquid, which turns clear only at 178.5 °C. The German physicist Otto Lehmann studied it and named it a liquid crystal. In 1968 George Heilmeier at RCA built the first liquid-crystal display, and in 1973 George Gray synthesised the room-temperature cyanobiphenyls such as 5CB, which made LCDs practical. De Gennes received the 1991 Nobel Prize in Physics for work on order in liquid crystals and polymers.

液晶显示器（LCD）曾长期主导手机、电脑和电视屏幕；胆甾相液晶用于温度计和防伪油墨；液晶高分子（如凯夫拉纺丝时的溶致液晶态）用于高强度纤维；生物体中的细胞膜和 DNA 浓溶液也具有液晶性质。

Liquid-crystal displays long dominated phone, computer and television screens; cholesteric liquid crystals are used in thermometers and anti-counterfeiting inks; liquid-crystalline polymers, such as the lyotropic solution from which Kevlar is spun, give high-strength fibres; and cell membranes and concentrated DNA solutions also have liquid-crystalline character.