the smectic phase
/ SMEK-tik /
The smectic is a liquid crystal that has taken one more step toward being a crystal than the nematic has. Its molecules not only point the same way (as in a nematic) but also sort themselves into distinct layers, like a neat deck of cards or a stack of paper sheets. Within each layer the molecules can still slide around like a liquid, and whole layers can glide over one another, so the material still flows — but it flows in sheets. The name comes from the Greek smegma, soap: soapy, layered smectic textures were first seen in the greasy phases of soaps.
In structural terms the smectic keeps the orientational order of the nematic and adds one dimension of positional order: the molecular centres are organized into equally spaced layers (a periodic density that repeats along the layer-stacking direction), while remaining liquid-like within each layer. There are several kinds. In smectic A the director stands perpendicular to the layers; in smectic C the director tilts at an angle within each layer. The layer spacing is roughly one molecular length, and because that spacing is a genuine periodicity it produces a sharp small-angle X-ray diffraction peak — a clean way to tell a smectic from a nematic, which has no such peak. The smectic sits between the nematic (orientation only) and the true crystal (orientation plus full three-dimensional positional order): it has orientation plus one-dimensional layering, but no order for where a molecule sits within its layer.
Smectics matter both as a rung on the order ladder and in devices. Because a smectic already has layers, its molecules are more constrained and it can hold order more stubbornly. The tilted smectic C, when made of chiral molecules (smectic C*), becomes ferroelectric: it carries a switchable electric polarization that flips far faster than a nematic re-orients, which is exploited in fast ferroelectric LCDs. Smectic layering is also the same architecture that lipid bilayers and cell membranes use, so the smectic is a bridge between display science and the structure of living matter.
Cool a rod-like liquid crystal down from its nematic phase and at some temperature a small-angle X-ray peak suddenly appears: the molecules have organized into layers about one molecule thick, and the sharp peak is the diffraction from that layer spacing. The material still flows like a greasy fluid, layers sliding over layers — but the new periodicity marks it unmistakably as a smectic, not a nematic.
A smectic is a layered liquid crystal: orientation plus one-dimensional layers, giving a sharp small-angle X-ray peak.
A smectic still flows (layers slide over one another), so it is a liquid crystal, not a solid. What sets it apart from a nematic is the layering — a smectic has one dimension of positional order that a nematic lacks.