Planar Anchorings Research Articles

Interactions of carbon nanotubes in a nematic liquid crystal. II. Experiment.

Multiwall carbon nanotube (CNT) colloids with different anchoring conditions are dispersed in pentyl-cyanobiphenyl (5CB), a thermotropic liquid crystal (LC) that exhibits a room-temperature nematic phase. The experiments make use of CNTs treated for strong planar, homeotropic, or Janus anchorings. Observations with a polarizing microscope show that the CNTs placed in a uniform nematic field stabilize parallel or perpendicular to n depending on their anchoring conditions. In the presence of a splay-bend disclination line, they are first attracted toward it and ultimately, they get trapped on it. Their orientation relative to the line is then found to be parallel or perpendicular to it, again depending on the anchoring conditions. When a sufficient number of particles are deposited on a disclination line, they form a micro- or nanonecklace in the shape of a thin thread or of a bottle brush, with the CNTs being oriented parallel or perpendicular to the disclination line according to the anchoring treatment. The system exhibits a rich versatility, even if until now the weak anchorings appear to be difficult to control. In a next step, the necklaces may be glued by means of pyrrole electropolymerization. In this manner, we realize a true materialization of the disclination lines, and we obtain nanowires capable of conducting the electricity in the place of the initial disclinations that just worked as templates. The advantage of the method is that it finally provides nanowires that are automatically connected to predesignated three-dimensional (3D) electrodes. Such a 3D nanowiring could have important applications, as it could allow one to develop electronic circuits in the third dimension. They could thus help with increasing the transistor density per surface unit, although downsizing of integrated circuits will soon be limited to atomic sizes or so. In other words, the predicted limitation to Moore's law could be avoided. For the moment, the nanowires that we obtain are not completely satisfactory, particularly because they are thickened by aggregates in some places. However, the method is far from being optimized. A few electric charges deposited on the CNTs could remedy those aggregates.

Optical detection of anchoring at free and fluid surfaces using a nematic liquid crystal sensor

Polarizing micrographs of the free and fluid surfaces in contact with nematic liquid crystals were collected and compared with simulated textures to quantitatively understand surface anchoring direction and strength. Wells with square cross section and an open upper surface were filled with a nematic liquid crystal, and free energy calculations were performed to determine the existing structures and their corresponding simulated optical textures. Homeotropic, uniform planar, and degenerate planar anchorings were differentiated based on comparisons with simulations, and surface anchoring strength was estimated.

Control of the orientation of nematic liquid crystal on iron surfaces: Application to the self-alignment of iron particles in anisotropic matrices

We report on the interactions between nematic liquid crystal (NLC) and iron surfaces. Our aim is to apply, to iron particles, the new colloidal interactions evidenced by Poulin et al. [P. Poulin, H. Stark, T.C. Lubensky, D.A. Weitz, Novel colloidal interactions in anisotropic fluids, Science 275 (1997) 1770–1773]. This approach makes it possible to self-align water droplets suspended in NLCs. For so new type of interactions, the driving forces are related to the NLC distortions induced by the anchoring at the droplet surface. So, to generate self-alignment of iron particles, first, it is essential to control the NLC anchoring at iron surfaces. The anchoring of NLCs on various surfaces has been largely described; however, few results concern iron surfaces. In a first step, cleaned and reproducible iron surfaces were well characterized using reflection adsorption infrared (RAIR) and X-ray photoelectron spectroscopy (XPS). A reproducible hydrated native oxide was formed on iron plates. In a second step, these plates were modified with organic molecules. We describe planar and normal anchorings easily and reversibly obtained from adsorbed molecules such as lecithin and methanol. We also report on very stable planar and normal anchorings induce by the grafting of alkyltriethoxysilane derivatives. The experimental conditions used to form stable polymerised films on the plate surface were successfully adapted to particles. The formation, around isolated particles suspended in homogeneous NLC, of a hyperbolic hedgehog (a point defect) with normal boundary conditions, and the formation of boojums with planar boundary conditions, were evidenced. All this goes to show that these planar and normal anchorings were strong enough to self-align iron particles in homogeneous NLC. For homeotropic anchoring beautiful alignments of particles were evidenced; these chains of particles present the original property of being constituted of particles separated by a distance related to their diameters.

Investigation of Alignment of Nematic Liquid Crystal on Micropatterned Polyimide Surface

The alignment properties of a nematic liquid crystal in contact with a micropatterned polyimide surface were theoretically analyzed and experimentally investigated. The patterns with a period of 1 µm were formed by stripes of alternating planar and homeotropic anchorings. The theoretical and experimental results implied that domains with the opposite tilt angle of liquid crystal directors were formed inside the cell. The average tilt angle was nearly 45° in the two alignment states and slightly varied from this equilibrium value depending on the treatment method, cell configuration and liquid crystal.

Maier-Saupe Nematic Isotropic Phase Transition in the Presence of an External Field

The nematic-isotropic phase transition in an external field is studied in the frame of Maier-Saupe model, where a global phase diagram including both cases of positive and negative susceptibility anisotropic is obtained. In the extreme case of infinitely large negative anisotropy, the system is reduced to the classical XY model because the rotational degree of freedom is restricted to lie in the plane perpendicular to the field, in which the second order phase transition occurs. The crossover of the directional degree of freedom between 3-dimension and 2-dimension is depicted. On the other hand, the critical point appears at a certain strength of the positive anisotropy, beyond which the transition disappears. The theory can be applied to the phase transition of very thin systems with homeotropic and planar anchorings.

Surface structure and anchoring properties of modified self-assembled monolayers

The surface structure of plasma-modified self-assembled monolayers is studied by x-ray photoelectron spectroscopy. The measurements show that the oxygen postplasma gas produces gradual chemical modifications in a pure polyethylene-type monolayer. Different oxygenated functions are observed and quantitatively, they are measured to be denser at shorter distances from the plasma source. Used as liquid crystal alignment layers, these surfaces present a fast transition between the homeotropic and planar anchorings. The average distance between the oxygen atoms on the organic layer is found to be ∼6–10 Å at the anchoring transition.

Planar degenerated anchoring of liquid crystals obtained by surface memory passivation

Weak anchoring of liquid crystals on solid substrates is usually dominated by the surface memory. We show that the memory can be removed by surface passivation by silane molecules. We realize planar, azimuthally degenerated, and memory-free anchorings without pretilt.

Surface controlled nematic bistability

One discusses phenomenologically the statics and dynamics of surface anchoring of nematic liquid crystals. In statics, for symmetrical situations, one can induce, under the action of an external field, a surface breaking with a bifurcation between two opposite orientations of the director. In dynamics, one shows that the surface director relaxation is intrinsically fast, and accompanied by a transient surface shear flow. In thin cells with planar anchorings, filled by cyanobiphenyl materials which orient normal to the plates under a strong electric field, there are now two effects which couple the relaxation of the surface director: a static one, which favors parallel orientation and a uniform cell texture, and a transient dynamical coupling from the surface flow, which forces π-twisted texture. With suitable electric signals, one can control the surface bifurcations to give at will one of these two bistable textures. Results are presented which show that these mechanisms could lead to a new class of surface controlled, fast nematic bistable displays.

Write and erase mechanism of surface controlled bistable nematic pixel

Recently a new nematic bistable pixel, using simple monostable planar anchorings, has been proposed [4−6]. This pixel can be switched between two textures, having the same boundary conditions but different optical properties: one texture ( U ) is undistorted, whereas the other one ( T ) is characterized by the presence of a half-turn twist. The U → T switching was obtained by abruptly dropping the driving electric field. A slow decrease of the driving electric field favors the T → U transition. The electric field fall rate discriminates the kind of coupling between the boundary surfaces, hydrodynamic or elastic. Here we show the same behavior of this system by using square electric field pulses. We present experimental results concerning the critical values (amplitude and duration) of the driving signals, for which the elastic and the hydrodynamic effects are balanced and we measure their optimized values. Our results are in reasonable agreement with the theoretical predictions obtained by a simplified model.

First-order breaking transition of tilted nematic anchoring

We propose a mechanism for breaking tilted nematic anchorings using an electric field normal to the plates. This breaking is a discontinuous first-order transition, unlike the known second-order breaking of planar anchorings. The elastic coupling of the tilted orientations on both plates is used to initialize the first-order transition. We observe the breaking experimentally by fast (in the \ensuremath{\mu}s range) bulk texture changes, resulting in an additional \ensuremath{\pi} twist in the final state.

Fast bistable nematic display using monostable surface switching

We present a novel surface-controlled bistable nematic liquid crystal display. This black and white device uses simple monostable planar anchorings and a dielectrically positive nematic. The two bistable textures are, respectively, quasiuniform U and almost half-turn twisted T. Short electric pulses break the surface anchorings. Hydrodynamically coupled breaking of both anchorings creates T. U is obtained by breaking only one anchoring. Write and erase times for high resolution video are achieved at voltages compatible with the usual drivers.

Anchoring transitions in the nematic-substrate system: Study of the Landau–de Gennes model

We study the phenomenon of anchoring and anchoring transitions in the nematic-substrate system using the Landau–de Gennes formalism. The free-energy functional is expanded around the bulk tensor nematic order parameter up to the second-order terms. This quadratic approximation is used to find an explicit condition for the anchoring direction in a semi-infinite system, and then the phase boundaries between the homeotropic, planar, and conical anchorings are determined. In the cases that we have studied the quadratic approximation predicts a first order homeotropic–conical transition and a first order or continuous planar–conical transition. It also predicts a simple asymptotic expression for the free energy of a finite system, when the sample thickness is large and the deviation of the director from the anchoring direction is small. This asymptotic formula leads in a natural way to the definition of the geometrical measure of the anchoring strength b̄. However, the quadratic approximation is insufficient to predict the correct behavior of b̄ close to a continuous anchoring transition, although it can serve as a rough estimate of b̄ far from the transition.