The electro-optic properties of colloidal silica filled nematics

Abstract

The optical and electro-optical properties of lled nematic liquid crystals, nematic systems with added colloidal silica nanoparticles (< 6%), have been studied. The macroscopic near IR birefringence of cells constructed from these materials was measured for wavelengths between 2 and 5 mm, and a wavelength independent value of 0.16 was obtained. The visible optical behaviour of cells formed with untreated ITO substrates using both lled nematic, and lled nematic and dichroic dyes was similar to those observed in polymer dispersed liquid crystals. At an electric eld of 1-2 V mmÕ 1, the cells were highly transmitting, while at low elds they were highly scattering. The eOE ects of colloidal silica nanoparticle concentration, cell thickness, electric eld and substrate preparation (rubbed polyimide versus no surface treatment) on the electro-optical behaviour of these cells were studied. Introductioning situations, these devices are similar to PDLCs. In this mode of operation, lled nematics may be easier to Over the past decade there has been considerable interest in liquid crystal dispersions such as polymer fabricate than standard PDLCs. Moreover, because of their high liquid crystal content, dichroic dyes that are dispersed liquid crystals (PDLCs) [1] and ' lled' nematics, nematic liquid crystals that contain a small percentage soluble in the liquid crystal may be added to produce coloured devices. The switching speeds of such devices (typically 1-5%) of small (<10 nanometer diameter) dispersed particles [2][3][4][5]. These mixed materials allow are also important. Filled nematics may provide both long optical path length and fast switching times. the material near the dispersed particles to be structured diOE erently than the bulk. These systems are of continuing This paper describes experiments that study the optical properties of lled nematics constructed with colloidal theoretical and experimental interest [6-10 ], and are of interest in displays. In one common version of a lled silica nanoparticles. These materials exhibit switching behaviour but no memory eOE ect for drive frequencies nematic display, fumed silica (aerosil), which is manuof 60 Hz and 1 kHz. When appropriate concentrations of factured by ame hydrolysis of SiCl 4 , is added to the silica are added, the optical behaviour is similar to that liquid crystal. These particles generate defects in the liquid of PDLCs. These materials are largely liquid crystal, crystal which scatter light making the lled liquid crystal containing less than 6% silica; for this reason they may medium appear cloudy. The application of an electric be doped with birefringent dyes and coloured cells made. eld to this medium causes reorientation of the liquid Both dye-containing and dye-free cells have been concrystal and transformation of the defects so that a clear, structed. A wide range of colloid sizes and concentrations weakly scattering state results. The behaviour of the in pure liquid crystals and liquid crystal mixtures of the medium after removal of the eld is somewh...

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