Ultrasound - Order Director Fluctuations interaction liquid crystals: a NMR relaxometry study

F. BONETTO; E. ANOARDO; R. KIMMICH

Caxambú, Minas Gerais, Brasil

Congreso; XXVI Encontro Nacional de Fisica da Materia Condensada; 2003

Sociedade Brasileira de Fisica

In this work field-cycling nuclear magnetic resonance (NMR) relaxometry was used to study the spin-lattice relaxation dispersion of selected standard nematic liquid crystals at different temperatures, in presence and absence of a monochromatic ultrasonic field. Experimental and theoretical studies of nuclear magnetic relaxation in nematic liquid crystals indicate that T_1 essentially reflects two kind of molecular mechanism: collective motions called Director Order Fluctuation (OFD) and individual molecular motions, such as self-diffusion and rotations. Collective re-orientations modes are mostly contributing to spin-lattice relaxation at low-frequencies. It was found that sonication changes the relaxation dispersion and, in some cases, it shortens the spin-lattice relaxation time on a broad frequency range. The theoretical interpretation is based on the interaction between the ultrasound and the nematic director field. A model previously developed and tested in preliminary experiments, is now successfully applied in a broader temperature range for different compounds. The model predicts a change in the relaxation dispersion based in an  acoustic-induced enhancement of the ODF modes in the whole frequency spectrum. We found a remarkably good agreement with earlier experiments in pentylcyanobiphenyl (5CB). Nonetheless, new interesting features and stronger effects were observed in p-azoxyanysole (PAA). The analysis of our results clearly suggests a sound-induced enhancement of order director modes. A characterization of such changes is done and physical explanations are proposed. Finally, some new but preliminary theoretical results concerning possible changes caused by an acoustic wave as revealed by NMR T_1 relaxometry are presented. These results can be applied to nematic as well as smectic compounds.