25 research outputs found
Smectic ordering in liquid crystal - aerosil dispersions I. X-ray scattering
Comprehensive x-ray scattering studies have characterized the smectic
ordering of octylcyanobiphenyl (8CB) confined in the hydrogen-bonded silica
gels formed by aerosil dispersions. For all densities of aerosil and all
measurement temperatures, the correlations remain short range, demonstrating
that the disorder imposed by the gels destroys the nematic (N) to smectic-A
(SmA) transition. The smectic correlation function contains two distinct
contributions. The first has a form identical to that describing the critical
thermal fluctuations in pure 8CB near the N-SmA transition, and this term
displays a temperature dependence at high temperatures similar to that of the
pure liquid crystal. The second term, which is negligible at high temperatures
but dominates at low temperatures, has a shape given by the thermal term
squared and describes the static fluctuations due to random fields induced by
confinement in the gel. The correlation lengths appearing in the thermal and
disorder terms are the same and show strong variation with gel density at low
temperatures. The temperature dependence of the amplitude of the static
fluctuations further suggests that nematic susceptibility become suppressed
with increasing quenched disorder. The results overall are well described by a
mapping of the liquid crystal-aerosil system into a three dimensional XY model
in a random field with disorder strength varying linearly with the aerosil
density.Comment: 14 pages, 13 figure