Surface-frustrated periodic textures of smectic- a liquid crystals on crystalline surfaces

Using polarizing optical microscopy we studied thin films and droplets of smectic- A 4-cyano- 4;{'} -n-octylbiphenyl (8CB) liquid crystal deposited in air on crystalline surfaces of muscovite mica that induce monostable planar anchoring. The competition with the homeotropic anchoring at the 8CB-air interface leads to the formation of one-dimensional (1D) patterns composed of straight, parallel defect domains that are organized in periodic arrays over areas as large as several mm;{2} . We have developed a simple model which identifies the arrays with self-assembled "oily streaks," comprising straight disclination lines and curvature walls. The model reproduces the observed monotonic increase of the period p with the film thickness h in the range p=1-4 microm and h=0.8-17 microm. For higher values of h we observed a sharp transition to a 2D lattice of fragmented focal conic domains. Despite the apparent generality of our model for hybrid planar-homeotropic anchoring conditions, periodic arrays of straight oily streaks have been observed so far only for 8CB on crystalline surfaces such as mica or MoS2 . Our model indicates that this specificity is due to a particularly strong anchoring of the liquid crystal on such surfaces.