Ultrasound Effects on Nematic 5cb Studied by Field Cycling Relaxometry

F. BONETTO; E. ANOARDO; R. KIMMICH

Torino, Italia

Conferencia; 2nd Conference on Field Cycling NMR Relaxometry; 2001

Universidad de Torino

In this paper we will discuss the influence of ultrasound waves in the molecular dynamics of nematic 5CB (4-n-pentyl-cyanobiphenyl) as revealed by the Larmor frequency dispersion of the spin lattice relaxation time (T1). Experimental and theoretical studies of nuclear magnetic relaxation in nematic liquid crystals indicate that T1 essentially reflects two kind of molecular mechanism: collective motions called Director Order Fluctuation (ODF) and individual molecular motions, such as selfdiffusion and rotations [1,2]. Collective re-orientations modes are mostly contributing to spin-lattice relaxation at low frequencies. Experimental evidences exist that contributions from ODF to the total T1 relaxation parameter are dominant in the kHz range [3-6]. Because the aim of the work was to study the influence of a sonic perturbation in a dynamical region where ODF is a dominant mechanism, experiments were carried out using the fast field cycling technique [7-9]. The ultrasound was coupled to the sample through a 3mm-glass sonotrode matched to a 30 kHz/50W ultrasonic generator. We will present T1 dispersion curves in a Larmor frequency band between 10 kHz and 1MHz at two different temperatures in the nematic range, in presence and absence of ultrasonic waves. Results will be discussed in terms of usual molecular dynamic theory in nematic liquid crystals. References [1] R. Y. Dong, Nuclear Magnetic Resonance of Liquid Crystals, Springer - New York (1997). [2] R. Blinc and I. Muševic, Dynamic Properties of Nematic Liquid Crystals in Handbook of Liquid Crystals, D. Demus, J. Goodby, G. W. Gray, H. W. Spiess and V. Vill Eds., Wiley-VCH - Weinheim (1998). [3] Th. Mugele, V. Graf, W. Wölfel and F. Noack, Z. Naturforsch 35a, 924 (1980). [4] F. Noack, M. Notter and W. Weiss, Liq. Cryst. 3, 907 (1988). [5] D. Pusiol and F. Noack,Liq. Cryst. 5 (1), 377 (1989). [6] R. Koellner, K. H. Schweikert and F. Noack, Liq. Cryst. 13, 483 (1993). [7] Kimmich R, Bull Magnetic Resonance, 1, 195 (1980). [8] Noack F, Progress in NMR Spectroscopy, 18, 171 (1986). [9] R. Kimmich, NMR Tomography Diffusometry Relaxometry, Springer - Berlin (1997).