Ph. Martinot-Lagarde

Tempearture Dependence of Tilt, Pitch and Polarization in Ferroelectric Liquid crystals

We persent tempeature measurements of the importnt parameter defming the properties of the semctic C* Phase in a series of schiff bases. The pitch of the helica texture is measured optically. The m...

Unwinding of the Helical Texture of a Smectic C* Liquid Crystal, Through Ferroelectric and Dielectric Anisotropic Coupling With an Applied Fiel

Abstract A large enough DC or low frequency AC field couples to the permanent polarization P and unwinds the helical texture of a smectic C* liquid crystal.′ From the measurement of the unwinding critical field, one can deduce the value of P. We present here a general model to compute the unwinding critical field; this model takes into account both the effects of P, and of the dielectric anisotropy. This point will be useful to an understanding of the AC high frequency regime.2

Electric Polar Surface Instability in Nematic Liquid Crystals

Nematic cyanobiphenyl molecules are aligned normal to a glass electrode by a polymerized DMOAP silane coating. Under the action of a normal electric field, oriented toward the bulk nematic, a weak diffuse surface splay bend is observed, above a field threshold ~ 0.1 V/μm. This polar instability can be attributed to the coupling of the flexoelectric and possibly a surface polarization with the electric field, as predicted long ago by Helfrich.

On the sign of flexoelectric coefficients in nematic liouid crystals

Abstract We measure the distortion of a nematic liquid crystal placed in a quadrupolar field distribution. This allows the measurement of (e1+e3)/K, where e1 and e3 are the flexoelectric coefficients and K a bend curvature elastic constant. For MBBA, e1+e3 is found negative, contrary to previous estimates. For 80CB, e1+e3 is found positive. This confirms the conformational origin of flexoelectricity in this later compound.

Ion Bump in the Ferroelectric Liquid Crystal Domains Reversal Current

Abstract In ferroelectric liquid crystal cells, variable electric field is applied to obtain the electro-optical switching. The transient currents through the cell show a complex behavior. A bump may appear1 in addition tothe well known bumps corresponding to the induced polarization variation and the reversal of spontaneous polarization in the bulk and at the cell surfaces. We explain different possible models for thisbump. Experimental studies were conducted on the amplitude and the peak time of this bump forvarious compounds in different phases. From the obtained results, we conclude that this bump corresponds to the movement inside the cell of ions stuck on the electrodes. We found ionic impuritiesallowing to control the amount of these stuck ions.

Strength of Memory-Induced Anchoring of 5CB on Bare Untreated ITO

Abstract We realize uniform planar alignment of 5CB on isotropic untreated ITO substrate. The counterplate orientation is “imprinted” on ITO and memorized by adsorption of an oriented nematic layer. We measure the azimuthal and the zenithal anchoring energies. We show that the memory induced anchoring strength is similar to the one usually found for substrates with high anisotropy. Studying rubbed ITO substrate, we show that in this case also the azimuthal anchoring is mainly due to memory effect.

Charged Twist Walls in Nematic Liquid Crystals

The influence of electric fields on nematic liquid crystals has been extensively studied in the past.1 The field can reorient the nematic texture by coupling with the dielectric anisotropy. A curvature distortion can create space charges, through the anisotropy of conductivity, and give rise to electrohydrodynamic instabilities.2 The same curvature also creates a flexoelectric polarization which can be acted upon by an external field in suitable geometries.3 In d.c. or at low frequencies, the applied field is usually decreased by nonohmic electrodes, which become polarized by the ions available in the medium which are responsible for its conductivity. When reversing the field one can observe4 in the transient current a signal which corresponds to the transport of these polarizing ions from one electrode to the other. We describe in this paper an experiment which shows the propagation of these ions, dynamically trapped into a twist wall of the nematic texture. The point is that this charged twist wall cannot exist for small distortions and must be considered as a singular wave