The diverse world of liquid crystals

Liquid crystals are all around us: in high-strength plastics, snail slime, laundry detergent, textile fibers such as silk and Kevlar, crude oil, insect wings, mineral slurries, lipstick, Bose-Einstein condensates, and the mantles of neutron stars. We eat them as aligned molecules in gluten and drink them as phospholipids in milk, where they stabilize fat globules. In our bodies they transport fats, make up cell membranes, and affect the functioning of hair cells in the inner ear, and even of DNA. The characteristic useful feature of liquid crystals, exemplified hy liquid-crystal displays (LCDs), is their exceptional responsivity to excitations. The name notwithstanding, liquid crystals are not crystals, and they need not be liquids. They are systems whose constituents possess long-range orientational, but not positional, order. A more apt name would be orientationally ordered soft matter. Positional order—that is, invariance under discrete translations—and its consequences have received a great deal of attention. That symmetry provides much of the foundation of solid-state physics. But orientational order— invariance under discrete rotations—has received far less attention. Today researchers are still making new discoveries and gaining deeper understanding of orientational order and its consequences.