Reorientation dynamics in a nematic liquid crystal studied by fast field cycling nuclear magnetic resonance

R. H. ACOSTA; D. J. PUSIOL

JOURNAL OF CHEMICAL PHYSICS

Año: 2003 vol. 119 p. 3461 - 3467

0021-9606

We propose a novel approach for studying the reorientation process of nematic liquid crystals in thepresence of a variable external magnetic field based on the fast field cycling nuclear magneticresonance ~NMR! technique. Thermotropic 4-cyano-48-octylalkylbiphenyl was aligned as a nematicmonodomain with an external magnetic field that is suddenly turned off, inspection of the NMR lineshape shows that the nematic monodomain disorders in nematic domains that have a continuousdistribution of local orientations. The dynamics of the orientation from a mesoscopic-disorderedsystem were determined by the application of a constant magnetic field. Both experiments weredone as a function of temperature for the whole nematic phase and as a function of the intensity ofthe external magnetic field. The technique allows a clear identification of two regimes, a highfrequency one where the magnetic torque governs the dynamics of reorientation and a lowfrequency one where thermal fluctuations are responsible for the dynamics.