External and intrinsic anchoring in nematic liquid crystals: a Monte Carlo study

Liquid crystal quenched orientational disorder at an AFM-scribed alignment surface

A polyimide substrate was scribed using the stylus of an atomic force microscope, then covered with a nematic liquid crystal. The fiber from a near field scanning optical microscope was immersed into the liquid crystal and rastered approximately 80 nm above the surface, thereby obviating smearing effects that occur in thicker samples. By appropriate averaging of multiple data sets, a histogram of the "frozen-in" director deviation Δφ from the average easy axis was obtained, having a full-width-half-maximum of ∼0.02 rad. Additionally, the spatial autocorrelation function of Δφ was extracted, where the primary correlation length was found to be comparable to, but larger than, the liquid crystal's extrapolation length. A secondary characteristic length scale of a few μm was observed, and is thought to be an artifact due to material ejection during the scribing process. Our results demonstrate the utility of nanoscale imaging of the interface behavior inside the liquid crystal.

Nonmonotonic behavior of the nematic tilt angle in a temperature-induced surface transition

We report on a nonmonotonic temperature behavior of the pretilt in cells containing polymers as alignment layers. The latter are very promising since they enable control of the anchoring of the liquid crystal. We show that such a behavior cannot be interpreted by the standard Landau or mean-field theory. We propose a generalization of the mean-field model, including the temperature dependence of the anchoring due to the polymer layer that enables a description of the nonlinear behavior. The agreement between the predictions of the model and the experimental data is good.

Anchoring of a nematic liquid crystal on a wettability gradient

We have studied the anchoring of the nematic liquid crystal 5CB (4'-n-pentyl-4-cyanobiphenyl) as a function of the surface wettability, thickness of the liquid crystal layer, and temperature by measuring the birefringence of a hybrid aligned nematic cell where the nematic material was confined between octadecyltriethoxysilane-treated glass surfaces, with one surface linearly varying in its hydrophobicity. A homeotropic-to-tilted anchoring transition was observed as a function of the lateral distance along the hydrophobicity gradient, typically in a region corresponding to a water contact angle of approximately 64 degrees. The effect of the nematic layer thickness was measured simultaneously by preparing a wedge cell where the thickness varied along the direction perpendicular to the wettability. The detailed behavior of the onset of birefringence was found to be consistent with a dual-easy-axis model that predicts a discontinuous anchoring transition from homeotropic to planar. The anchoring was independent of temperature, except within 1 degrees C of the nematic-to-isotropic transition temperature (T(NI)). As the temperature approached T(NI), the tendency for planar anchoring gradually increased relative to that for homeotropic anchoring.

Using particle shape to induce tilted and bistable liquid crystal anchoring

We use Monte Carlo simulations of hard Gaussian overlap (HGO) particles symmetrically confined in slab geometry to investigate the role of particle-substrate interactions on liquid crystalline anchoring. Despite the restriction here to purely steric interactions and smooth substrates, a range of behaviors are captured, including tilted anchoring and homeotropic-planar bistability. These macroscopic behaviors are all achieved through appropriate tuning of the microscopics of the HGO-substrate interaction, based upon nonadditive descriptions for the HGO-substrate shape parameter.

Computer simulation of a liquid-crystal anchoring transition

We present a study of the effects of confinement on a system of hard Gaussian overlap particles interacting with planar substrates through the hard-needle-wall potential. Using geometrical arguments to calculate the molecular volume absorbed at the substrates, we show that both planar and homeotropic arrangements can be obtained using this model. Monte Carlo simulations are then used to perform a systematic study of the model's behavior as a function of the system density and the hard-needle-wall interaction parameter. As well as showing the homeotropic to planar anchoring transition, the anchoring phase diagrams computed from these simulations indicate regions of bistability. This bistable behavior is examined further through the explicit simulation of field-induced two-way switching between the two arrangements.

Phase behavior of liquid crystals confined by smooth walls

Monte Carlo simulations for a simple model liquid crystal are presented. The influence of flat walls on the phase behavior is analyzed for two different anchoring mechanisms, one favoring homeotropic alignment and one simulating a twisted nematic cell without external fields, e.g., two walls with different homogeneous planar alignment. The simulations are performed in the constant pressure ensemble. The box volume may change in the directions perpendicular to the wall normal. The isotropic-nematic phase transition in the bulk system is first studied for different isobars. For the weak first order transition we do not observe any hysteresis down to a temperature accuracy of deltaT=0.001. The isotherm T=1 is then studied in the bulk as well as in the confined geometries. The walls stabilize the positional order in the systems due to the formation of layers. The orientational order is weakly stabilized.