Soft Matter Research Seminar: Dr Boris I. Ostrovskii

How surface ordering affects phase transitions in liquid crystal films - Dr Boris I. Ostrovskii, of the Russian Academy of Science, Moscow will be presenting a seminar on his research.

FSRC ‘‘Crystallography and Photonics”, Russian Academy of Sciences, Moscow, Russia

Institute of Solid State Physics, Russian Academy of Sciences, Chernogolovka, Russia

 

We provide an overview of the influence of the surface ordering and finite size on the phase transitions in liquid crystal (LC) films. The results from the synchrotron studies of free-standing smectic films (FSSF) carried out in recent years are presented. The FSSF are particularly suitable to investigate the above problems: such films are substrate-free; the alignment of the smectic layers is almost perfect, allowing the study of single-domain samples of various thicknesses. Several phase transitions in FSSF involving smectic A (Sm-A), smectic C (Sm-C) and hexatic B (Hex-B) phases are discussed. These phases can be considered as a stack of parallel molecular layers, in which elongated molecules are oriented on average along the layer normals (Sm-A, Hex-B) or tilted with respect to them (Sm-C). The Hex-B is a three-dimensional (3D) analogue of the common hexatic phase predicted by Halperin and Nelson; it exhibits long-range bond-orientational  order and short-range positional order within each layer.

In LCs a free surface stabilizes a higher-ordered phase that is only observed at lower temperatures in the bulk or not observed in the bulk at all. This is in contrast to solids, which exhibit surface-induced disorder that can lead to surface melting. In the case of LCs, surface freezing occurs instead. We discuss two scenarios of the growth of the ordered surface region in FSSF:  a layer-by-layer sequence and continuous penetration into the film interior.

Next, experimental and theoretical studies of a Sm-A to Hex-B transition in FSSFs of the 54COOBC compound are presented. Micro-focused X-ray diffraction in combination with Angular X-ray Cross-Correlation Analysis revealed coexistence of the Sm-A and Hex-B phases at first order transition in these films. The temperature region of two-phase coexistence near the phase transition point diminishes with decreasing film thickness. The width of this temperature region as a function of the film thickness was derived on the basis of a Landau mean-field theory in the vicinity of a tricritical point (TCP). It is shown that close to TCP the surface Hex-B order penetrates anomalously deep into the film interior.