On the Electric Field Induced Stripe Instability Threshold of Twisted Nematic Layers

Most liquid crystal displays make use of the elect roopt ic properties of twisted nematic layers. Both the s tandard twisted nematic displays of 90° twist [1] as well as the recently developed 'supertwist ' displays of more than 180° twist [2] require director fields which depend only on one spatial coordinate, the coordinate z parallel to the layer normal. Director fields which depend on more than one spatial coordinate may, however, also be stable in twisted layers. They appear as periodic structures, such as parallel stripes [3], which make them unsuitable for display applications. Recent experiments on polymer liquid crystals in magnet ic fields have shown that stripe format ion may not be restricted to layers of relatively high twist, but can even occur when the twist is zero [4], F o r stripes to form with zero twist, the twist to splay elastic constant rat io k22/kll must be smaller than the critical value [4-8]