1,720,968 research outputs found
Distortion in the surface layers on nematic liquid crystals: an elastic constant approach
No full textA strong boundary distortion of the director field [ncirc](R) near the boundary surface of nematics is evidenced and theoretically discussed in terms of elastic constants. The sources of the distortion are the diffuse nature of the surface tension, the spontaneous splay, which is different from zero in a boundary layer of thickness comparable with the range of molecular interactions, and the surface-like free energy term k 13 div([ncirc] div [ncirc]). In the distorted layer a drastic change of [ncirc](R) with respect to the surface normal [kcirc] occurs. As a consequence, the surface alignment may appear homeotropic from a macroscopic point of view, even in the case where the short range nematic-substrate interactions give an obliquely oriented surface layer, as suggested by recent experiments of P. Guyot-Sionnest, H. Hsiung and Y. R. Shen
On the K13-dependent energy term in nematic liquid crystals
No full textThe difficulties arising from the inclusion of term k 13 (div n div n) in the expression of the free energy density are discussed. It is shown that they are related to the fact that in this term second order spatial derivatives ni,jk of the nematic director n appear. Each term containing a second order derivative would give a distorted configuration of the director field. The global effect is zero in the bulk, owing to the divergence structure of the term, but different from zero near to the boundaries
COMPARISON BETWEEN MEASURED AND CALCULATED REFLECTANCE SPECTRA FROM A MONODOMAIN CHOLESTERIC LIQUID CRYSTAL
No full textTHEORETICAL AND EXPERIMENTAL STUDY OF THE STATIC BEHAVIOUR OF A NEMATIC LIQUID CRYSTAL NEAR THE FREEDERICKSZ TRANSITION.
No full textDirect measurement of the two principal refractive indexes of a nematic liquid crystal slab
No full textA new technique to measure the principal refractive indexes of a nematic liquid crystal is presented. The method is based on an indirect measurement of the refraction angle by determining the direction of a magnetic field which minimizes the light scattering. With this technique one can perform different light scattering experiments, which require a knowledge of the refractive indexes, on the same crystal slab. The accuracy is better than ±0.15%. A discussion about the systematic error due to the distortion of the director profile near the holder edges and the relative correction is give
FRANK ELASTIC CONSTANT RATIOS EVALUATION IN MBBA BY INTENSITY MEASUREMENT OF THE SCATTERED LIGHT
No full textAnalisi della transizione di Frederiks mediante misure interferometriche in incidenza obliqua.
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Optical study of the molecular alignment in a nematic liquid crystal in an oblique magnetic field
No full textThe light intensity I transmitted through a homeotropically oriented nematic liquid crystal slab held between crossed polarizers, as a function of the angle 03B8H of an applied magnetic field, is calculated for oblique light incidence. The presence of singular points in the I vs. 03B8H curve which are related to the elastic constants of the liquid crystal is demonstrated One of these points, found here for the first time, for small angles of incidence depends only on the ratio K33/~a, and allows a direct determination of this ratio. The comparison of the calculated curve I vs. 03B8H with the experimental points taken in a sufficiently wide range of 03B8H values, using K11/~a, K33/~a and An = ne2014 no as best fit parameters, allows a very precise determination of all these constants. The method turns out to be much more sensitive than the usual ones, mostly based on capacitance measurements and it is thus very suitable for determining the behaviour of the elastic constants when the temperature is changed, particularly near the transition point
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