Simultaneous characterization of optical and thermal parameters of liquid-crystal nanocolloids with high-temperature resolution

Abstract

We report on the high-temperature resolution measurements of the optical and thermal parameters of a liquid-crystal-silica nanoparticle colloid, as well as its video inspection, simultaneously performed in an upgraded photopyroelectric calorimeter. Over the nematic-isotropic coexistence region, the determined nematic correlation length, obtained from turbidity measurements, showed the characteristic two-step nematic nucleation process previously reported only for the specific heat.

Invited Review Article: Photopyroelectric calorimeter for the simultaneous thermal, optical, and structural characterization of samples over phase transitions

“…Two laser beams, modulated at different frequencies, were directed to the sample through the bottom window. 25 The light from one laser was absorbed at a metallic opaque layer, which covered part of the bottom of the sample cell, to produce the sample ac heating. The light from the second laser was driven through the transparent part of the cell to cross the sample and probe its transparency, and it was then directed onto a photodiode.…”

“…24 Moreover, in the case of transparent samples, it was possible to also probe the sample turbidity and/or birefringence, to evaluate the sample morphology evolution in a quantitative way. 25 Finally, an improved procedure for the PPE signal data analysis was introduced which allows also the detection of the latent heat exchanged across the two-phase coexistence of first order-phase transitions. 26 Other PPE configurations were also used to measure directly the specific heat or the thermal conductivity.…”

Dielectric relaxation dynamics in liquid crystal – dye composites

“…Owing to their large optical anisotropy (i.e., birefringence) and dielectric anisotropy, NLCs have also been used to modulate refractive index by molecular reorientation with an external field or under the action of light [1]. Nowadays, to obtain materials with eutectic physical properties for displays or other optical devices, different additives such as carbon nanotubes [2][3][4], metal nanoparticles [5][6][7] and dyes [8][9][10] are doped in pure liquidcrystalline materials. Two main classes of aromatic dyes have enjoyed use as guests in LC mixtures as azo dyes have been employed as dopants in condensed optical materials intended for novel optoelectronic devices.…”

Strain induced homeotropic alignment in the smecticA phase of liquid crystals

“…This is related to the fact that the thermal transport parameters are largest when measured with the heat flow direction parallel to the molecular long axis [9]. All these observations can be performed simultaneously in a photopyroelectric (PPE) calorimetric set-up [9][10][11][12], where the LC samples were contained in quartz cells of calibrated thickness ranging between 10 and 100 µm. The cell side in contact with the LC was coated with a 300-nm-thick optically opaque Ti layer.…”

“…The power density and the temperature rate change were adjusted to minimise, in the sample, the thermal gradients which can induce smearing of the vs temperature profiles of the observed quantities. Thanks to the transparent ITO electrodes of the pyroelectric transducer, the light from a lamp, transmitted through a polariser, could be introduced in the sample and, after the reflection from the metalised side of the cell, it could be conveyed through a crossed analyser to reach a CCD camera, so that polarisation microscopy observations could be recorded [10][11][12].…”