Pore size dependence of the dipolar-correlation effect on the stimulated echo in liquid crystals confined in porous glass

The dipolar-correlation effect on the stimulated echo of nematics probes (a) the order parameter and (b) the distribution of the director fluctuations. This effect was employed for the study of orientational director fluctuations in a nematic liquid crystal confined in Bioran porous glasses with mean pore diameters of 30, 70, and 200 nm. No macroscopic preferential orientation of the director could be detected in the confined material. The reason is, that the local director is oriented by surface interactions rather than by the magnetic field. As already shown in our previous work, the attenuation of the dipolar correlation quotient (stimulated-echo amplitude divided by the primary echo amplitude) is able to monitor director fluctuations in the time scale of the pulse sequence. The echo attenuation rate exhibits a strong dependence on the pore size. It is determined by the mean squared fluctuation of the dipolar coupling constant and characterises the distribution of the director fluctuation modes. The mean squared fluctuation decreases with decreasing pore size. The dependence on the inverse pore diameter decays exponentially and suggests a critical mean pore size of about 120 nm for the crossover to bulk behavior. The dipolar-correlation effect thus may be suitable for the determination of mean pore diameters in porous materials.