Defects in liquid crystals

The most striking feature of liquid crystals is the wide variety of visual patterns they display. These patterns, such as those shown in figure 1 and on the cover, are due almost entirely to the defect structure that occurs in the long‐range molecular order of the liquid. Indeed, historically, the underlying structure of the liquid‐crystal phases known as nematic and smectic‐A was discovered from a study of stable defects that characterize these phases. Such defects are easily visible in the optical microscope. By examining the thin and thick thread‐like structures observed in nematic liquid crystals, Otto Lehman and Georges Friedel deduced that this phase involves long‐range orientational ordering of the long axis of the rod‐like molecules. (The direction of this orientational ordering is what we now denote by the unit vector n̂, called the director.)