Planar and Tilted Uniform Alignment of Liquid Crystals by Plasma Treated Substrates
Liquid Crystal Institute, Kent State University, Kent, OH 44242, USA
Dept. of Crystals, Institute of Physics of NASU, pr. Nauki 46, 03028 Kiev, Ukraine
We describe the 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 the 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 plasma beam; (2) with the easy axis perpendicular to the plane of incidence. The mode 1 transforms into the mode 2 when 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, particle energy, etc. In mode 2, the pretilt angle is zero (planar alignment). The azimutal anchoring coefficient is relatively weak for the first type of alignment and strong, (comparable to the rubbed polymer substrates) for the second type. The two-mode alignment feature can be used to control the alignment properties and to create alignment patterns. The method is free of shortcomings known for the traditional rubbing technique. We discuss the possible mechanisms of the two-mode alignment and show that the plasma-induced modifications of the substrate topography might be an important factor in liquid crystal alignment.