Optical gratings formed in thin smectic films frustrated on a single crystalline substrate

Abstract

Through the combination of three different, complementary techniques (optical microscopy, x-ray diffraction and atomic force microscopy), we reveal the deformations inside thin smectic films frustrated between two interfaces imposing antagonistic anchorings. We study the model system, 4-n-octyl-4'-cyanobiphenyl (8CB) between MoS2 and air, which is characterized by the competition between homeotropic anchoring at air and planar unidirectional anchoring on the substrate, with thicknesses varying around 0.3 microm. Optical microscopy and x-ray diffraction demonstrate the continuous topology of smectic layers between the interfaces, which are stacked into periodic flattened hemicylinders. These latter are one-dimensional (1D) focal conic domains which form an optical grating in the smectic film, of a period ranging from 1 to 2.5 microm. The interpretation of our results through an energetic model, associated with the atomic force microscopy (AFM) measurements, shows the presence below a critical thickness of a new type of curvature wall between neighboring hemicylinders.

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

“…Each sample was observed under a polarizing microscope in reflection mode using a 50× objective. 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].…”

“…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).…”

Ordered interfaces for dual easy axes in liquid crystals

“…This allows to determine the disorientations between the planar anchoring directions of the domains within liquid crystal films with a 90° degeneracy. This degeneracy can be overcome in smectic phase, since defects associated with an homeotropic anchoring at the air interface appear in the liquid crystal film, leading to linear textures perpendicular to the planar uniaxial anchoring direction (figure 1c) [19]. Measurements of the textures consequently directly evidence the uniaxial planar anchoring directions within each domain.…”

Mechanical properties of nanotubes of polyelectrolyte multilayers

“…Layer-by-layer (LbL) assembly of polyelectrolytes was recently demonstrated within nanoporous templates such as track-etched or alumina membranes: LbL is performed by repeatedly passing through the membranes solutions of polyelectrolytes of opposite charge [1][2][3][4][5][6][7]. After dissolving the templating membrane, LbL nanotubes and nanowires are readily obtained, which could be of interest for applications such as drug encapsulation, gene delivery, or sensing.…”