Measurement of Coherence-Length Anisotropy in the Isotropic Phase of Nematics

A phase-sensitive, differential, light-scattering method is used to measure the shape of correlated regions in the isotropic phase of nematics. In MBBA ($p$-methoxybenzylidene $p-n$-butylaniline) these regions are elongated along the molecular-axis direction and the critical behavior is close to that predicted by mean-field theory; de Gennes's parameter $\frac{L_2}{A}$ is positive. In the isotropic phase of the smectic OBBA ($p$-octoxybenzylidene $p-n$-butylaniline) the correlated regions are flat and cannot be described by a constant $L_2$.