Effect of high electric fields on the nematic to isotropic transition in a material exhibiting large negative dielectric anisotropy

• Published in: The European physical journal. E, Soft matter and biological physics Soft matter and biological physics, 2007-02, Vol.22 (2), p.139-149
• Main Authors: DHARA, S, MADHUSUDANA, N. V
• Format: Article
• Language: English
• Subjects: Anisotropy; Biomedical Engineering; Condensed matter: structure, mechanical and thermal properties; Electromagnetic Fields; Equations of state, phase equilibria, and phase transitions; Exact sciences and technology; Physics; Polymers - chemistry; Specific phase transitions; Thermodynamics; Transitions in liquid crystals
• ISSN: 1292-8941; 1292-895X
• DOI: 10.1140/epje/e2007-00020-3

We report the experimental high electric field phase diagram of a nematic liquid crystal which exhibits a large negative dielectric anisotropy. We measure simultaneously the birefringence (Deltan) and the dielectric constant (epsilon( perpendicular)) at various applied fields as functions of the local temperature of an aligned sample. We also measure the higher harmonics of the electrical response of the medium. The following experimental results are noted: (i) enhancement of orientational order parameter S in the nematic phase due to both the Kerr effect and quenching of director fluctuations; (ii) enhancement in the paranematic to nematic transition temperature (T(PN)) with field; (iii) divergence of the order parameter susceptibility beyond the tricritical point as measured by third harmonic electrical signal; (iv) a small second harmonic electrical signal which also diverges near T(PN), indicating the presence of polarised domains. Our measurements show that DeltaT(PN)(= T(PN)(E)-T(NI)(0)) varies linearly with |E| whereas the Landau de Gennes theory predicts a dependence on E(2). It is argued that the quenching of director fluctuations by the field makes the dominant contribution to all the observations, including the thermodynamics of the transition.