Surface-induced orientational phase transition in a lyotropic liquid crystal observed by nonlinear optical techniques.

We have observed a phase transition from a uniaxial to a biaxial nematic phase in a lyotropic liquid crystal as a function of decreasing film thickness. The results, obtained by optical second-harmonic generation experiments in a wedged cell geometry, are supported by additional Z -scan measurements and can be interpreted by wall-induced ordering effects.

Manipulation of anchoring strength in an azo-dye side chain polymer by photoisomerization.

The generation of surface anisotropy by photochemical means has been proposed as an attractive method to align liquid crystals. In this paper, we present an experimental study of the alignment induced on a liquid crystal by a polymer film containing azo-dye groups in the side chain. Optical measurements were performed in nematic liquid crystal cells to determine the azimuthal and zenithal anchoring strengths as a function of the irradiation energy and chromophores concentration. It was observed that the director tends to align perpendicular to the polarization direction of the incident light and the orientation process consists essentially of a rotation of the director in the plane parallel to the boundary surfaces. However, the concentration of azo-dye groups in the polymer film must exceed a minimum value to get a macroscopical effect on the liquid crystal alignment. It is shown that the azimuthal anchoring strength can be varied two orders of magnitude by controlling the irradiation energy and azo-dye concentration.

Dynamical behavior of a nematic lyotropic liquid crystal in flat confined samples.

We investigate the dynamical behavior of the director in the surface layer of lyotropic nematic liquid crystals in an external magnetic field. The characteristic time of the orientation process is obtained from optical measurements for samples of different thickness (200, 50, and 10 microm). The boundary surfaces are glass plates coated with rubbed polymer [polymethylmethcrylate (PMMA)], to introduce an easy axis. Drastic changes in the dynamical behavior are observed when the thickness is decreased and the experimental results indicate a uniaxial to biaxial transition due to the confinement of the nematic sample.

Surface breaking in lyotropic nematic liquid crystals induced by a magnetic field.

The twist deformation induced by a magnetic field H in a lyotropic liquid crystal is analyzed. Our results show the existence of three regimes for the surface deformation related to two critical values for the external magnetic field. The first, for low magnetic field, is an elastic one in which the surface deformation disappears when the field is removed. In a second regime, a permanent deformation of the lyotropic liquid crystal is induced by the distorting field. This regime is characterized by a threshold for H, called H(*)(c). In the third regime, the sample is uniformly oriented along H. Also this regime has a well defined threshold value, H(**)(c), and the alignment remains even when the distorting field is removed. To interpret the experimental data recently published, we proposed a phenomenological model according to which there is an elastic coupling between the surface layer of thickness l and the bulk. This elastic coupling has a saturation for a given value of the deformation of the bulk with respect to the surface layer.

Probing soft polymeric coatings of a capillary by atomic force microscopy.

Atomic force microscopy (AFM) has been used to probe the surface of a capillary after coating with "soft" polymers, notably polyacrylamides. The aim was the investigation of the efficiency of coverage of the silica surface, so as to reduce or eliminate the electroosmotic flow (EOF), particularly noxious in the separation of macromolecules. The quality of such coating is strongly dependent on two variables: temperature and pH. In the first case, progressively higher temperatures produce open silica patches, where no polymer seems to be bound. The transition from coated to largely uncoated surfaces occurs at 50 degrees C. Also the pH of the polymerizing solution strongly affects the coating efficiency. Since in all coating procedures the monomer solution is not buffered, addition of accelerator (TEMED, N,N,N'N'-tetramethylethylendiamine) induces polymer growth at pH 10-11. These pH values generate hydrolysis of the siloxane bridge anchoring the bifunctional agent (Bind Silane, onto which the polymer chain should grow) to the wall. Thus, coating and de-coating occur simultaneously. Low temperatures during polymer growth (typically 10 degrees C) and buffered solutions (pH 7, titrated after TEMED addition) ensure a most efficient and thorough coating, with virtual elimination of EOF: well coated capillaries exhibit residual EOF values, at pH 10, of the order of 10(-7) cm2 V-1 s-1 vs. a standard value for uncoated capillaries of the order of 10(-4) cm2 V-1 s-1. The AFM data have been fully confirmed by direct measurement of EOF in coated and uncoated capillaries under an electric field.