Dynamics of Randomly Constrained Nematics

Here r is the distance from the cylinder axis and is the coherence length. The distance that the molecule migrates in the iso-tropic phase during a NMR measurement VD/(5v is in the above case of the order of the cylinder diameter R and the motional averaging by translational self-diffusion must be taken into account. The diffusion coefficient in the surface layer was estimated as Dg 10 ° cm s and is much smaller than the bulk value. The time a molecule resides at the surface is then t IqID = QT s. In the fast exchange regime the effective spin-spin relaxation rate can be expressed as 

Here 5vg is the quadrupole splitting of the 2H NMR line in the bulk, Sg is the corresponding bulk nematic order parameter and So is the nematic order parameter in the surface layer. 

Randomly constrained orientational order has been investigated by H NMR in porous Vycor glass [217] and aerogels [218] of various pore size distributions. The isotropic-nematic transition was found to be replaced by a continuous evolution of orientational order in the pores. The reason for this is that in contrast to classical liquids the surface effect on the orientational molecular order in liquid crystals is of long range. 

Critical dynamics of a nematic liquid crystal in silica gels, as observed by the 

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