Hypothesis of Dye Aggregation in a Nematic Liquid Crystal: From Experiment to a Model of the Enhanced Light-Director Interaction V. M. Pergamenshchik V. Ya. Gayvoronsky S. V. Yakunin R. M. Vasyuta V. G. Nazarenko Institute of Physics, Kyiv, Ukraine O. D. Lavrentovich Liquid Crystal Institute, Kent State University, Kent, Ohio We report experimental and theoretical arguments showing that the effect of strong enhancement of the light-induced director reorientation in a nematic liquid crystal doped with anthraquinone dye molecules (J  anossy effect) can be related to their aggregation. The torque on the director is due to dependence of the entropy of the aggregate subsystem on the electric field vector relative to the director. Keywords: dye aggregation; Ja ´ nossy effect; nematic director INTRODUCTION Electromagnetic fields produce a mechanical torque on the nematic director and can cause a director reorientation (DR) [1]. About fifteen years ago J anossy with his collaborators discovered that just one per- cent of light-absorbing dye dopands dissolved in a nematic liquid crystal (NLC) enhances this DR by a factor of hundred [2,3]. The intrigue of this discovery was that the polarizability, size, and other parameters of the We are greatful to O. D. Kachkovskiy, Yu. L. Slominski, A. B. Nych, B. I. Lev, Yu. P. Boiko, Yu. P. Piryatinskiy, Yu. A. Nastyshyn, P. M. Tomchuk, P. J. Collings, S. V. Shiyanovskii, and A. S. Zolot’ko for discussions. The work was supported by NAS of Ukraine via Grant #1.4.1B=109 and CRDF award #UK-P1-2617-KV-04. Address correspondence to V. M. Pergamenshchik, Institute of Physics, 46 Nauky ave., Kyiv-39, 03039 Ukraine. E-mail: pergam@i.kiev.ua Mol. Cryst. Liq. Cryst., Vol. 454, pp. 145=[547]–156=[558], 2006 Copyright # Taylor & Francis Group, LLC ISSN: 1542-1406 print=1563-5287 online DOI: 10.1080/15421400600654181 145=[547]