Phase transitions pretransitional fluctuations

When a nematic phase is cooled towards a smectic A phase, fluctuations of smectic order build up. These pretransitional fluctuations were called cybotactic clusters in the early literature (Fig. 5.21). Regardless of a physical picture of such fluctuations, it has been observed that the cluster size grows as the transition is approached. Furthermore, these clusters are anisotropic, being elongated along the director. They also grow faster along this direction as the transition is approached from above. 

Figure 1. Schematic variation of the enthalpy H and specific heat capacity Cp with temperature T for transitions (at T ) that are (a) strongly first-order, (b) weakly first-order with pretransitional fluctuation behavior, (c) mean-field second-order (CP indicates the critical point on the enthalpy curve for the Landau second-order transition temperature Tc=T ), (d) and (e) are critical fluctuation dominated second-order transitions with a diverging (d) or large but finite (e) specific heat capacity at the critical temperature T =T . For the first-order transitions the latent heats AWl correspond with the steps in H(T) at T=T . 5W represent the fluctuation induced enthalpy change associated with the phase transition.

---