Liquid crystal anchoring transitions on aligning substrates processed by plasma beam

TL;DR

This study investigates how plasma beam treatment induces sequential anchoring transitions in nematic liquid crystals on polyimide substrates, revealing continuous and in-plane reorientation transitions explained by a phenomenological competing easy axes model.

Contribution

It demonstrates the occurrence of multiple anchoring transitions induced by plasma treatment and provides a phenomenological model to interpret these transitions.

Findings

Continuous zenithal transition with increasing plasma dose.

In-plane azimuthal reorientation at higher doses.

Monotonic decrease in contact angle with exposure dose.

Abstract

We observe a sequence of the anchoring transitions in nematic liquid crystals (NLC) sandwiched between the hydrophobic polyimide substrates treated with the plasma beam. There is a pronounced continuous transition from homeotropic to low tilted (nearly planar) alignment with the easy axis parallel to the incidence plane of the plasma beam (the zenithal transition) that takes place as the exposure dose increases. In NLC with positive dielectric anisotropy, a further increase in the exposure dose results in in-plane reorientation of the easy axis by 90 degrees (the azimuthal transition). This transition occurs through the two-fold degenerated alignment characteristic for the second order anchoring transitions. In contrast to critical behavior of anchoring, the contact angle of NLC and water on the treated substrates monotonically declines with the exposure dose. It follows that the surface concentration of hydrophobic chains decreases continuously. The anchoring transitions under consideration are qualitatively interpreted by using a simple phenomenological model of competing easy axes which is studied by analyzing anchoring diagrams of the generalized polar and non-polar anchoring models.