RESUMO
Previous experimental evidence has suggested the existence of a transient planar state during the transition from a homeotropic state to a focal conic state. This does not appear at first to be physically reasonable. Here we show a quantitative numerical model of this transition that shows the existence of the transient planar state and agrees well with measured data. We compared this simulation with two experimental samples: one with parallel boundary conditions, and one with perpendicular boundary conditions. Both samples had thickness to pitch ratios of 13.9. The agreement of the simulation with the experimental data gives further evidence that the system passes through the transient planar during the transition. Also shown here are preliminary results for the second part of the transition, the transition from the transient planar state to the focal conic state, that suggest a Helfrich-like undulation deformation.
RESUMO
The complete time evolution of a Helfrich-type instability that is driven by twist energy in a cholesteric liquid crystal is presented. The modulation is investigated using an initially uniform planar director configuration of a cholesteric liquid crystal (with helical axes uniformly aligned along the cell normal) whose pitch is substantially longer than the equilibrium value.
RESUMO
We report the first observation of anti-Stokes fluorescence cooling in a thulium-doped solid with pump excitation at 1.82 &mgr;m
RESUMO
A study has been carried out of the conformations of triisopropyl(aryl)silanes (i-Pr)(3)SiAr, (Ar = phenyl, 1-naphthyl, and 2-naphthyl) both as to the orientation of the three isopropyl groups and the conformation about the silicon-aromatic bonds. The report comprises dynamic NMR studies of conformational interconversions in solution and in the solid state as well as molecular mechanics calculations. The barriers for the stereomutation processes measured in the crystalline state were found significantly higher than those in solution.