Liquid crystal alignment at macroscopically isotropic polymer surfaces: Effect of an isotropic-nematic phase transition

Abstract

We study the effect of an isotropic-nematic (I-N) phase transition on the liquid crystal alignment at untreated polymer surfaces. We demonstrate that the pattern at the untreated substrate in the planar cell where the other substrate is uniformly rubbed strongly depends on the temperature gradient across the cell during the I-N phase transition, being macroscopically isotropic if the untreated substrate is cooled faster, but becoming almost homogeneous along the rubbing direction in the opposite temperature gradient. We interpret the observed effect using complementary models of heat transfer and nematic elasticity. Based on the heat transfer model we show that the asymmetric temperature conditions in our experiments provide unidirectional propagation of the I-N interface during the phase transition and determine the initial director orientation pattern at the test's untreated surface. Using the Frank-Oseen model of nematic elasticity, we represent the three-dimensional director field in the nematic cell as a two-dimensional (2D) pattern at the untreated surface and perform 2D numeric simulations. The simulations explain the experimental results: Different initial director orientations at the untreated surface evolve into different stationary patterns.

Polar in-plane surface orientation of a ferroelectric nematic liquid crystal: Polar monodomains and twisted state electro-optics

“…We also probed the in-plane polar ordering and alignment effects of single surfaces in cells using the temperature gradient methods of Aryasova and Reznikov ( 28 ), who studied the structure and phase behavior of nematics in samples where a constant temperature difference, Δ T , was maintained across the thickness of the cell. Upon cooling through the isotropic-to-nematic phase transition under these conditions, they found that the final nematic alignment obtained was predominantly that favored by the cooler cell surface, concluding that in a temperature gradient, the nematic phase appears first at the cooler surface and the I/N interface then moves as a quasi-planar sheet toward the warmer surface.…”

“…RM734 was cooled at −1 °C/min through the N–N F transition in ANTIPOLAR cells while maintaining a ∼2 °C temperature difference between the outer surfaces of the top and bottom cell plates, with the top plate cooler. The temperature difference across the LC-containing cell gap was estimated to be Δ T ∼0.1 °C ( 28 – 30 ). In these cells, the N–N F transition appears as a sharp boundary across which there is a discernably larger birefringence in the N F phase ( 14 ).…”

“…We also probed the in-plane polar ordering and alignment effects of single surfaces in cells with the aid of the temperature gradient methods of Aryasova and Reznick [21]. They studied nematic LC structure and phase behavior with the LC between glass plates, while maintaining a constant temperature difference, ΔT, across the thickness of the cell.…”

Self-Consistent Explanation of the Untwist Alignment of Ferroelectric Nematic Liquid Crystals with Decreasing Cell Thickness and Deviation of the Surface Easy Axis Experimented upon Using the Brewster Angle Reflection Method

“…The LC material was injected into the cell through capillary action at the isotropic phase temperature. During the cooling process after the injection of the LC material, little temperature gradient between the top and bottom substrates occurred in the measurement, which contributed to maintaining the director's alignment symmetry [30,31].…”

Evaluation of polar alignment structure and surface anchoring energy in ferroelectric nematic liquid crystals by renormalized transmission spectroscopic ellipsometry

“…During the cooling process after filling the substance, minimizing the temperature difference between the upper and lower substrates contributed to preserving the director alignment symmetry throughout the cell. 30,31) Figure 4 represents the experimentally measured ( ) l D and ( ) l Y at 18 °C, where the ferroelectric nematic phase was confirmed by POM and a was 174°. In this experiment, ( ) l D and ( ) l Y measurements with the upper substrate as the incident side [Fig.…”