Complex superstructures in chiral liquid crystals: surface-induced helix destruction.

The influence of surface anchoring in thin chiral ferroelectric liquid crystals on helical superstructures imposed onto smectic layers is studied by means of a simple model allowing surface anchoring through the self-depolarization effect. It is shown that, for narrow temperature ranges close to temperatures of tilting phase transitions, these interactions can destroy helices, leading to the emergence of complex, intertwined undulatory, and helical substructures with different, mainly short, pitches. Regions with both undulated and helical orderings are demonstrated to reveal different sizes and random distributions along the smectic layer normal. Assuming values of material model parameters, typical for liquid crystals being close to tilting phase transitions, the free energy of thin helical smectic liquid crystal systems is determined as a function of two variables. One of them specifies the helix period of the appropriate bulk system (unperturbed by surfaces), while the second describes the interplay between the surface anchoring and the liquid crystal elasticity. It is shown that the free energy exhibits a very complicated behavior within variable regions for which the regular helices or complex superstructures occur. The resulting sensitivity of the free energy on changes of these variables corresponds to the appearance of a large variety of complex superstructures with coexisting helices of different periods. This gives an explanation of the occurrence of intricate superstructures and provides new insight into the interplay between molecular and surface interactions near the tilting phase transitions.