First-order isotropic-smectic-A transition in liquid-crystal-aerosil gels

The short-range order which remains when the isotropic to smectic-$A$ transition is perturbed by a gel of silica nanoparticles (aerosils) has been studied using high-resolution synchrotron x-ray diffraction. The gels have been created in situ in decylcyanobiphenyl, which has a strongly first-order isotropic to smectic-$A$ transition. The effects are determined by detailed analysis of the temperature and gel density dependence of the smectic structure factor. In previous studies of the continuous nematic to smectic-$A$ transition in a variety of thermotropic liquid crystals the aerosil gel appeared to pin, at random, the phase of the smectic density modulation. For the isotropic to smectic-$A$ transition the same gel perturbation yields different results. The smectic correlation length decreases more slowly with increasing random-field variance in good quantitative agreement with the effect of a random pinning field at a transition from a uniform phase directly to a phase with one-dimensional translational order. We thus compare the influence of random fields on a freezing transition with and without an intervening orientationally ordered phase.