Each moving unit of a long, flexible, molecule induces in the surrounding solvent a velocity field vhich reacts on the motion of other segments. This long-range hydrodynamic interaction modifies strongly the dynamical form factor S(qv) at low frequencies co and small scattering vectors q. For neutron scattering (qRc ^ where Rq is the radius of gyration of the polymer), the frequency width A c of S(qu>) at fixed q becomes proportional to q*(for an ideal coil). Also the effect of stretching the molecule becomes more dramatic, since stretching greatly reduces the hydrodynamic interactions, asolvent *

We present an extension of the Helfrich theory of hydrodynamic instabilities to the case of alternating electric fields. The electrohydrodynamic effects are described by two coupled equations for the charge density (q) and the local curvature of the molecular alignment (ψ). The relaxation time for q is the dielectric relaxation time τ (∼ 10-2 s in typical samples). The relaxation time T for ψ is strongly dependent on the field magnitude. Provided that the sample thickness d is above a certain limit dc, the nature of the instability is very different, depending on the ratio of the field frequency ω to a critical frequency ω c. For ω < ωc the onset of instability corresponds to a charge q which oscillates at the frequency ω, while the curvature ψ is essentially time independent. For ω > ωc the situation at threshold corresponds to a constant q and an oscillating ψ. These predictions, together with the calculated curves of threshold voltage vs ω and d, are in reasonable agreement with a number of recent experiments

Ellipsometric measurements of a nematic drop spreading on a rough surface are reported. The thickness profile of the drop shows two typical lengths belonging to the microscopic and to the mesoscopic scales (a few tens and some hundreds of Å, respectively). No intermediate thicknesses are observed during the spreading. A simple model involving the balance among elastic, anchoring, and spreading energies is used to interpret the results. [S0031-9007(96)00975-1] 68.45.Gd Nematic liquid crystals (NLC) are uniaxial liquids. In the bulk, molecules tend to be parallel. The director n (unitary vector) gives an average orientation of the molecules. Without external strains n is degenerated in space while if the NLC is in contact with a solid surface the degeneracy breaks down. This distortion of the director orientation gives rise to an elastic torque [1]. Surfaces are usually treated in order to define specific orientation; n perpendicular to the surface is called homeotropic orientation ͑u 0͒, and n in the surface plane is called planar orientation ͑u p͞2͒. Any orientation can be achieved [2]: for example, a clean flat glass leads to planar or homeotropic alignments, depending on the NLC nature; oblique evaporation of SiO x gives planar, tilted, or bistable states, depending on the parameters of the evaporation (angle, SiO x thickness, evaporation rate [3]). Those orientations are defined for "macroscopic" thick film (at least few mm), but do not give any information on the tilting of the molecules close to the surface.Very few wetting studies have been related to the structure of LC at the solid interface either in the case of smectics [4] or in the case of nematics [5,6]. In this paper we report microscopic studies of 5CB ͑4-n-pentyl-4 0 -cyanobiphenyl͒ spreading over a controlled surface of amorphous silicon wafers coated with SiO x evaporation.Samples of 5CB were purchased from BDH Ltd., the purity of this compound announced by the furnisher is better than 99%. All the results are presented at ambient temperature close to 23 ± C at which the 5CB is in a nematic phase. The 5CB molecule has a molecular dipole associated to the cyano group lying roughly parallel to the molecular long axis. In bulk, it is well known that molecules form a quadrupole pair with their polar head facing each other. The apparent length of this bilayer is estimated to be 25 Å [7] while the molecule length is 18.7 Å. We also used other compounds of the same nCB series containing n methylene/methyl groups ͑n 5 11͒.Silicon substrates (type N, dopant PH, orientation 111 purchased from Siltronix) are coated with silicon monoxide ground evaporated at the rate of 4 Å͞s in a vacuum of 10 26 Torr for times ranging from 2 to 60 s. Oblique evaporation (0 ± to 75 ± from the surface normal) leads to anisotropic surfaces where roughness can be controlled. Those surfaces are well known to induce strong molecular alignment, but the mechanism is not fully understood. All substrates are kept at room temperature in N 2 atmosphere before use.A very small drop...

Ia3d bicontinuous cubic phases are interpreted as constituted of a fluid film of constant thickness supported by the gyroid minimal surface. We first compute the gyroid structure factor. Then we decorate the surface by the film. We compute the diffracted intensities and compare to X-ray intensities measured in some lipid systems