Planar and tilted uniform alignment of liquid crystals by plasma-treated substrates

Abstract

We describe a new aligning technique that yields uniform planar or tilted orientation of a nematic liquid crystal at different organic and inorganic substrates. The method is based on the oblique irradiation of an aligning substrate with a partially collimated flux of accelerated plasma. The sheet-like plasma flux is produced by an anode layer thruster with a race track geometry of the discharge channel. For liquid crystals with a positive dielectric anisotropy the technique produces two modes of uniform alignment: (1) with the easy axis in the incident plane of the plasma beam; (2) with the easy axis perpendicular to the plane of incidence. Mode 1 transforms into mode 2 as the irradiation dose increases. In mode 1, the pretilt angle can be controlled by changing the parameters of irradiation such as incidence angle, current density and particle energy. In mode 2, the pretilt angle is zero (planar alignment). The azimuthal anchoring coefficient is relatively weak (W _a∼10⁻⁶ J m⁻²) for the first type of alignment and strong (W _a≥10⁻⁴ J m⁻², comparable to rubbed polymer substrates) for the second type. The two-mode alignment feature can be used to control alignment properties and to create alignment patterns. The method is free from the usual shortcomings of the traditional rubbing technique. We discuss possible mechanisms of the two-mode alignment and show that plasma-induced modifications of the substrate topography might be an important factor in liquid crystal alignment.

Notes

^†More precisely, the anode potential U determines the maximum energy of the Ar⁺ ions. The energy spectrum of generated ions is broad with the maximum at approximately  eU. In the following we consider maximum ion energy, i.e. E=eU.