nematic phase
/ ni-MAT-ik fayz /
Imagine a school of fish all facing the same way as they drift, or a crowd of people on an escalator all turned toward the front. Nobody is in an assigned spot, everyone is still milling and shifting, but there is a shared sense of direction. That is the feeling of the nematic phase.
The nematic phase is the simplest kind of liquid crystal. Its rod-shaped molecules agree on a common axis — they all point, on average, the same way — but their centers are scattered at random, free to flow past one another like an ordinary liquid. So it has orientational order, a shared direction, without positional order, without fixed places. That single shared direction is called the director, and it is the one thing the molecules cooperate on.
The nematic phase matters because it is the workhorse inside most flat-panel screens: a small voltage rotates the common direction of the molecules, and that rotation controls how light passes through, switching pixels bright or dark. A common confusion is to picture the molecules as neatly stacked; in a nematic they are not stacked at all, they merely lean the same way while still tumbling and sliding freely.
The 'twisted nematic' cell inside an old calculator display stacks nematic molecules so their common direction gently spirals from top to bottom, guiding light through. Apply a voltage and the spiral unwinds, blocking the light and turning that segment black.
A calculator's black digits are nematic molecules being switched by voltage.
In a nematic the director marks an axis, not an arrow: pointing 'up' and pointing 'down' are equivalent, because the rod-shaped molecules look the same from either end.