Slow Dynamics of Isotropic-Nematic Phase Transition in Silica Gels

We report a quasielastic light scattering measurement of the isotropic-nematic phase transition of a liquid crystal in silica gel. The normalized intensity autocorrelation function, $G_2\mathrm{}\left(t\right)\mathrm{}$, which samples the order-parameter fluctuations, is consistent with the form $G_2\mathrm{}\left(x\right)\mathrm{}\sim \frac{1}{(1+x^2)}$, where $x=\frac{\ln t}{\ln \tau }$. The relaxation time, $\tau$, diverges near a temperature $T^{*}$ and can be quantitatively described by the Vogel-Fulcher law, $T={\tau }_0$ exp[const/($T-T^{*}$)]. The measurements suggest that the silica gel imposes a weak random anisotropy on the system, with the bulk nematic phase being replaced by a "glassy" state. However, hysteresis, normally associated with random-"field" behavior, is absent.