Self-organized arrays of dislocations in thin smectic liquid crystal films

Abstract

Combining optical microscopy, synchrotron X-ray diffraction and ellipsometry, we studied the internal structure of linear defect domains (oily streaks) in films of a smectic liquid crystal 8CB with thicknesses in the range of 100-300 nm. These films are confined between air and a rubbed PVA polymer substrate which imposes hybrid anchoring conditions (normal and unidirectional planar, respectively). We show how the presence or absence of dislocations controls the structure of highly deformed thin smectic films. Each domain contains smectic layers curved in the shape of flattened hemicylinders to satisfy both anchoring conditions, together with grain boundaries whose size and shape are controlled by the presence of dislocation lines. A flat grain boundary normal to the interface connects neighboring hemicylinders, while a rotating grain boundary (RGB) is located near the axis of curvature of the cylinders. The RGB shape appears such that dislocation lines are concentrated at its summit close to the air interface. The smectic layers reach the polymer substrate via a transition region where the smectic layer orientation satisfies the planar anchoring conditions over the entire polymer substrate and whose thickness does not depend on that of the film. The strength of planar anchoring appears to be high, larger than 10(-2) mJ m(-2), compensating for the high energy cost of creating an additional 2D defect between a horizontal smectic layer and perpendicular ones of the transition region. This 2D defect may be melted, in order to avoid the creation of a transition region structure composed of a large number of dislocations. As a result, linear defect domains can be considered as arrays of oriented defects, straight dislocations of various Burger vectors, whose location is now known, and 2D nematic defects. The possibility of easy variation between the present structure with a moderate amount of dislocations and a structure with a large number of dislocations is also demonstrated.

Persistence of Smectic-A Oily Streaks into the Nematic Phase by UV Irradiation of Reactive Mesogens

“…In well-aligned regions a series of dark and light stripes was observed, characteristic of the well-known oily streak texture [21]. From the color of the reflected light, we estimated the film thickness in the Sm-A phase based upon Newton tint tables [21,24], this measurement appearing in agreement with the relationship between thickness and period of the oily streaks previously established [27]. The samples were then irradiated in a nitrogen atmosphere with UV light of 0.15 mW cm −2 intensity at wavelength 365 nm for 20 min at a temperature T = 29.5, 23.5, and 23.5 • C for the 3, 7, and 10 wt % mixtures, which is slightly below their respective nematic-Sm-A transition temperatures (before irradiation) of 30, 24.5, and 24.5 • C, respectively (T NA = 32 • C for pure 8CB).…”

Liquid Crystals and Their Defects

“…A somewhat similar situation to the order reconstruction problem occurs in the experiments on smectic A thin films carried out by the research group of Emmanuelle Lacaze (see [32,33,59,70,71,95,96,97]). Here there is parallel anchoring on the substrate, with homeotropic anchoring on the free surface of the film, leading to interesting configurations of the smectic layers in which their normals m, and thus the director n also, suffer jump discontinuities on surfaces.…”

Fluorescence correlation spectroscopy in thin films at reflecting substrates as a means to study nanoscale structure and dynamics at soft-matter interfaces

“…But also there the mismatch of anchoring conditions on the substrate and on the air-interface, causes deformations of the smectic LC film. The authors suggest a transition zone to form close to the substrate that ranges between 27 and 49 nm into the film, while it may contain a number of dislocations or a melted nematic area [31]. On silicon with native oxide, for very thin 8CB films a trilayer conformation is reported [36].…”

“…For example, recently, LC materials have been investigated for enhancement of organic solar cell performance [29]. The here employed 4n-octyl-4'-cyanobiphenyl (8CB) is in its smectic-A state at room temperature and allows us to compare the results with literature reports on investigations using different experimental methods [30,31]. LC materials are very sensitive not only to interface structures, but also to interaction forces with the substrate, influencing their anchoring conditions [32,33].…”