Ultrasonic Field Influence on the Spin-lattice Dispersion in Nematic and Smectic A Liquid Crysrtals: a Theoretical Analysis

F. BONETTO; E. ANOARDO

Torino, Italia

Conferencia; 3rd Conference on Field Cycling NMR Relaxometry; 2003

Universidad de Torino, Italia

Experimental and theoretical studies  of nuclear magnetic relaxation in liquid crystals indicate that nuclear spin relaxation dispersion 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 the Zeeman spin-lattice relaxation at low-frequencies, where the field cycling technique is the appropriated one to get such information. It was theoretically found that sonication affect order director fluctuations [1], and experimentally showed that, in nematics, it changes the relaxation dispersion and, in some cases, it shortens the spin-lattice relaxation time on a broad frequency range [2,3].  In the present work, the nuclear spin relaxation dispersion caused by order director fluctuations in presence of ultrasonic waves is calculated for the nematic and smectic A mesophases. The model is essentially based on the coupling between the ultrasonic waves and the director field. For the sake of simplicity, two kind of ultrasonic waves are analyzed: ultrasonic waves parallel and perpendicular to the director. In the smectic A case, two different treatments of the problem are presented. A comparison and a discussion between both models are also offered. Simulated curves are presented for both sort of waves and for different ultrasound intensities. It was found that, independent of the kind of the ultrasonic waves involved (but fixed ultrasonic power), changes in the T1 frequency dispersion becomes more appreciable as the Larmor frequency decreases. This result holds valid for the nematic as well as for the smectic A mesophases. This fact turns the field cycling technique crucial to observe them. REFERENCES [1] J. Selinger, M. Spector, V. Greanya, B. Weslowsky, D. Shenoy and R. Shashidhar, Phys. Rev. E, 66, 051708 (2002). [2]  F. Bonetto, E. Anoardo and R. Kimmich, Chem. Phys. Let., 361, 237 (2002). [3]  F. Bonetto, E. Anoardo and R. Kimmich, J. Chem. Phys. , 118, (2003), in press.