Résumé
Quenched disorder refers to a disorder which is frozen; i.e., it does not change in time. In most experimental situations, this noise has a peculiar statistical signature with correlations decaying on some finite length as opposite to uncorrelated and scale invariant white noise [1]. In liquid crystals the orientational quenched disorder (OQD) profoundly affects the phase order close to a phase transition [2,3]. Unfortunately a direct characterization of these surface random fields necessary for an overall understanding of the measured phenomena is still in its early stages [4]. In this paper we show a complete set of measurement embracing different anchoring layers and liquid crystals. The obtained results support the universality of the measured OQD. We propose a possible physical mechanism which accounts for the measured OQD. In experiments the easy axis distribution was measured via the transmitted intensity trough a hybrid cell made by a hometropic counter-plate and filled with the nematic liquid crystal. Using a polarizing microscope equipped with a high resolution CCD camera we are able to measure the easy axis orientation at the pixel size (0.22 µm). The OQD on solid substrates as rubbed and photo-aligned polymers and SiO, presents the same statistical signature consisting in a compressed spatial auto-correlation function C(R) with unusually large (µm) correlation length (points in the Figure). In the model we consider the adsorption of the director fluctuation modes on the surface. The model main idea is that all fast modes fluctuating with characteristic times lower than a typical adsorption time T are averaged to zero. The model predictions agree with the experimental results by adjusting T ~= 1msec (lines in the Figure). The microscopic origin of the adsorption time will be discussed. References [1] M. A. Rubio, C. A. Edwards, A. Dougherty, and J. P. Gollub, Phys. Rev. Lett. 63, 1685 (1989). [2] T. Bellini, M. Buscaglia, C. Chiccoli, F. Mantegazza, P. Pasini, and C. Zannoni., Phys. Rev. Lett. 85, 1008 (2000). [3] M. Marinelli, F. Mercuri, S. Paoloni, and U. Zammit, Phys. Rev. Lett. 95, 237801 (2005). [4] M. Nespoulous, C. Blanc, and M. Nobili, Phys. Rev. Lett. 104, 9 (2010).