On distingue dans les cristaux liquides les phases smectique, nématique et cholestérique, dont on examine ici les caractéristiques
et les utilisations possibles. On examine plus particulièrement ici les liquides cholestériques dans leur emploi comme indicateurs
des températures ambiantes. De bons résultats ont été obtenus par des mélanges d’esters cholestériques. Les conditions de
mesure sont définies et des exemples d’applications sont donnés.
In some definite temperature intervals some organic substances have properties which partly belong to the solid-crystalline
and partly to the liquid-isotropic state of matter. They are called liquid crystals.
There are three groups, nematic, smectic and cholesteric liquids, the properties and applications of which are to be discussed.
Cholesteric liquids are able to reflect quasimonochromatic light, the wavelength of which strongly depends on temperature.
The BAM is therefore mainly interested in the application of cholesteric liquids to thermal non-destructive testing.
We first used binary mixtures of certain cholesteric esters as temperature indicators, where their sensitivity and measurement
range could be adjusted by different concentrations. For the most interesting temperature interval of 15°C to 35°C we recently
developed some special cholesteric esters.
Using cholesteric liquids, the resolvable temperature interval strongly depends on the wavelength interval of the reflected
light, the colour-discrimination of the human eye as well as on illuminating and viewing angles.
Taking a suitable binary mixture of cholesteric liquids, which reflect all spectral colours within a temperature range of
about 5°C, the smallest, confidently resolvable temperature difference was 0.06 °C.