Resumen

NMR relaxometry can be used to probe the molecular dynamics in a variety of fluids (bulk and confined), soft and solid materials [1]. The technique became of relevance for the study of the molecular dynamics in thermotropic mesophases, mainly after the success of F. Noack´s group (Stuttgart, 1975) in identifying the strong influence of collective order fluctuations in driving the spin-lattice relaxation (T₁) within the low-field regime. It was recently suggested that the collective dynamics in thermotropic smectic specimens can only be observed when the molecular system is coupled to an ultrasonic field [2]. Then, relaxometry experiments combined with acoustic stimulation showed to be a potential technique to discriminate dynamical processes that cannot be identified in the absence of sound [3]. As an additional feature, the proposed methodology was successfully used to study the influence of sonication in the molecular dynamics of the system. The acoustic field can be tuned and/or "shaped" in order to strongly modify the involved molecular dynamics and the order state. An important result derived from this approach was the acoustic induction of metastable nematic states in the bulk. As a general conclusion, it can be stated that the elastic and hydrodynamic properties of mesomorphic materials can be manipulated through the combined action of an acoustic field and a second orienting field that can be of magnetic or electric nature. In the near future, this knowledge will be used to investigate the permeability control of lamellar molecular walls in spherical closed structures.