That is, light that enters the crystal is broken up into two oppositely-polarized rays that travel at different velocities. The anisotropy of liquid crystals causes them to exhibit birefringence. This light scattering is a consequence of fluctuating regions of non-uniformity as small groups of molecules form and disperse.
Liquid crystal phases are generally cloudy in appearance, which means that they scatter light in much the same way as colloids such as milk. The side chain is commonly a hydrocarbon chain that serves to elongate the molecule.The terminal group is often (but not always) one that is somewhat polar, giving rise to intermolecular attractions along the long axis.This planarity is enhanced when the linkage group contains a double bond such as -(HC=N)- which keeps the rings in the same plane. The two benzene rings confer a degree of planarity on the molecule that promotes attractions between neighboring molecules.Although a large variety of molecules are known to form liquid crystals, the simplest and most common structures can be represented by the following generic scheme: Over a certain range of temperatures, these attractive forces can lead to a degree of self-organization in which crystal-like order persists in some directions even though it is lost in other directions. The structural units capable of forming liquid crystals are always molecules, usually rather elongated organic ones that possess dissimilar local structural regions that can interact in an organized way with their neighbors.
For this and for related studies on polymers, de Gennes was awarded the 1991 Nobel Prize in Physics.
But science was not quite ready to accept this concept, and despite a number of confirming experiments between 19, the field remained largely dormant until the mid-1960s when the French physicist Pierre-Gilles de Gennes (1932-2007) developed a thorough theoretical model for the properties of liquid crystals, particularly their ability to scatter light. At 145° C the crystalline solid first melted into a cloudy liquid, and then at 178° the cloudiness suddenly disappeared, leaving the clear, transparent liquid that one ordinarily expects after melting.Ī physicist who examined this material recognized that the cloudy liquid had a certain degree of order he proposed that it was a hitherto unknown state of matter, and suggested the name "liquid crystal". To his surprise, he found that this substance appeared to have two melting points. In 1888, an Austrian plant physiologist, working in Prague, attempted to measure the melting point of a cholesterol derivative that he had extracted from a plant. A liquid crystal phase has many of the physical attributes of a liquid, but its molecular units are sufficiently ordered to give rise to some anisotropy, most notably in their optical properties.Īs with so many scientific discoveries, it all started with an unexpected observation. Crystalline solids, in contrast, are anisotropic optical- and other properties such as thermal and electrical conductivity vary with direction. \( \newcommand\)Ī true liquid is isotropic, meaning that its properties are uniform in all directions- the result of its molecules being in constant random motion.
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