Order Director Fluctuations Enhancement by Ultrasonication in Nematic And Smectic A Mesophases

F. BONETTO; E. ANOARDO; R. KIMMICH

Torino, Italia

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

Universidad de Torino, Italia

NMR relaxometry was used to study collective motions in standard thermotropic nematic and smectic liquid crystals (5CB, PAA, 8CB and 11CB) in the presence and absence of ultrasonic irradiation. In this presentation we address 3 main points: a)- A realistic interpretation of the T1 relaxation dispersion in both mesophases. b)- The nature of the ultrasound – order director fluctuations (ODF) interaction. c)- The utility of the “ultrasound-assisted” NMR relaxometry for the study of molecular dynamics. Early results in a nematic phase revealed that the acoustic energy transferred to the molecular system at a well defined frequency is efficiently distributed in the whole observed frequency window [1]. Results were explained on the grounds of the interaction between the acoustic field and the nematic ODF dynamics [2]. The orienting effects of the ultrasonic field, scarcely discussed in the literature, were considered for the formulation of a new free energy term that allowed us to successfully interpret the results [1,3]. The same free energy orienting term was also recently proposed on a more rigorous basis in connection with optical experiments [4]. A more complex situation applies to the smectic phase, where even the non-sonicated dispersion is still not yet understood. However, it is now clear that the low-field strong dispersion, usually attributed to smectic ODF (assuming a Larmor frequency dependence nu^(1)) ), is more related with a false dispersion due to experimental pitfalls [5]. Relaxation experiments under sonication are useful to shed some light onto the problem. References [1]- F. Bonetto, E. Anoardo and R. Kimmich, Chem. Phys. Lett. 361, 237 (2002). [2]- S. Nagai, P. Martinoty and S. Candau, J. Phys. 37, 769 (1976). [3]- F. Bonetto, E. Anoardo and R. Kimmich, J. Chem. Phys. 118, (2003) in press. [4]- J. Selinger, M. Spector, V. Greanya, B. Weslowsky, D. Shenoy and R. Shashidhar, Phys. RE66, 51708 (2002). [5]- E. Anoardo and R. Kimmich, XXXII National Congress on Magnetic Resonance GIDRM, ia – Italy (2002). Submitted.