Surface effects in nucleation and growth of smectic B crystals in thin samples

TL;DR

This study investigates how surface interactions with container walls influence the nucleation, growth, and orientation of smectic B crystals in thin samples, revealing effects like homoepitaxy, recrystallization, and grain boundary dynamics.

Contribution

It provides new insights into surface-induced phenomena affecting smectic B crystal growth, including orientation control and defect formation, in thin sample geometries.

Findings

Surface effects mediate crystal orientation in PI and PTFE coatings.

Recrystallization partly destroys initial crystal orientation distribution.

Grain boundary groove dynamics influence grain selection during solidification.

Abstract

We present an experimental study of the surface effects (interactions with the container walls) during the nucleation and growth of smectic B crystals from the nematic in free growth and directional solidification of a mesogenic molecule ($C_4H_9-(C_6H_{10})_2CN$) called CCH4 in thin (of thickness in the 10 $\mu$m range) samples. We follow the dynamics of the system in real time with a polarizing microscope. The inner surfaces of the glass-plate samples are coated with polymeric films, either rubbed polyimid (PI) films or monooriented poly(tetrafluoroethylene) (PTFE) films deposited by friction at high temperature. The orientation of the nematic and the smectic B is planar. In PI-coated samples, the orientation effect of SmB crystals is mediated by the nematic, whereas, in PTFE-coated samples, it results from a homoepitaxy phenomenon occurring for two degenerate orientations. A recrystallization phenomenon partly destroys the initial distribution of crystal orientations. In directional solidification of polycrystals in PTFE-coated samples, a particular dynamics of faceted grain boundary grooves is at the origin of a dynamical mechanism of grain selection. Surface effects also are responsible for the nucleation of misoriented terraces on facets and the generation of lattice defects in the solid.