K₁₃-lnduced Deformations in a Nematic Liquid Crystal: Experimental Test of the First-Order Theory

Abstract

We consider predictions of the so-called “first-order elastic theory” with non-zero elastic constant K ₁₃ for a nematic cell with tilted molecular orientation. The geometry has been suggested by Faetti to test the validity of different theoretical approaches to the problem of the divergence K ₁₃ elastic term. According to the first-order elastic theory, in which only the first and second derivatives in the elastic free energy are retained, the K ₁₃ term causes deformations at the cell surface when a magnetic field is applied along the initial director n. These deformations can be observed in the measurements of the optical phase retardation; the distinctive feature of the K ₁₃ effect is non-monotonous behavior of the phase retardation when the magnetic field increases. Despite a number of difficulties, the effect can be tested experimentally. We report experimental data on the behavior of the nematic cell with a high pretilt angle and weak anchoring in a tilted magnetic field. These results show no significant deviation in the monotonous behavior of the phase retardation; therefore, the experiment does not confirm the predictions of the first-order theory when K ₁₃ ≠ 0.

Key Words:

• Nematic liquid crystals
• elastic constants
• splay-bend elasticity
• magneto-optics
• K ₁₃
• experimental test