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...
In this letter, we report the results of phototunable lasing in dye-doped cholesteric liquid crystals (DD–CLC). Photoexcitation of DD–CLC films gives rise to laser emission in the violet-UV range. Control of the structure of the chiral dopant driven by UV phototransformation is exploited in order to obtain a permanent variation of the cholesteric pitch. Laser emission wavelength tuning, by means of photoinduced shifting of the selective reflection band of the cholesteric liquid crystals is established. A tuning interval of about 35 nm, in the wavelength range of 385–415 nm, is observed.
At microscale level, the mechanical response of the most anterior stroma is complex and nonlinear. The microstructure (fibers' packing, number of cross-links, water content) and the combination of elastic (collagen fibers) and viscous (matrix) components of the tissue influence the type of viscoelastic response. Efforts in modeling the biomechanics of human corneal tissue at micrometric level are needed.
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