Application of field-cycling NMR relaxometry to the study of ultrasound-induced effects in the molecular dynamics and order of mesomorphic materials

E. ANOARDO

Conferencia; HK 2010 - Humboldt Kolleg International Conference on Physics Argentina-Alemania: un siglo de cooperación científica en Física.; 2011

A salient characteristic of nuclear magnetic resonance (NMR) techniques is the possibility to scan nuclear spins evolutions within a broad Larmor frequency range. Special instrumentation was developed to extend nuclear spin relaxation studies up to proton Larmor frequencies in the sub-kilohertz regime, a technique known as field-cycling NMR relaxometry. This presentation refers to an experimental version where the sample under study is simultaneosly subjected to ultrasonic irradiation. The fact that ultrasound couples slecetively to the collective dynamics of liquid crystals, offers new insights for the study of the molecular dynamics in these materials using NMR relaxation.  The experimental technique was successfully used to detect different ordered states created in a nematic specimen through magnetoacoustic manipulation. Depending on the initial order imposed to the molecular system, the acoustic and magnetic cycles of the experiment and the sample temperature, metastable states displaying long-term memory effects can be created within the volume. It was shown that NMR relaxometry at low fields is highly sensitive to the presence of such metastable states in the volume.