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An in depth study has been made of the optical properties of a series of non-thermochromic chiral nematic materials to determine the influence of refractive index dispersion on pitch measurements. The materials used were mixtures of a chiral nematic material, BL131a, and a nematic material BL130 (from Merck Ltd.) in which the pitch is changed by varying the concentration of the nematic component. Detailed measurements were made of the ordinary and extraordinary refractive indices as a function of wavelength across the visible spectrum. Dispersion effects changed the birefringence by approximately 10% across the visible spectrum. Reflection spectra were obtained and used to deduce the helicoidal pitch. Failure to include dispersion in the calculation led to a significant error in calculations of both the bandwidth and pitch of the material. Finally, as SSCT devices are electro-optic in nature, the influence of the conducting surface of the device on the optical properties is examined.

The isothermal formation of concentric colour domains has been observed in a cholesteric mixture doped with 10 wt % of a photochromic nematic material. The host cholesteric mixture included the Merck materials BL131a and BL130, while the photochromic dopant was a mixture of 4-n-butyl-4′-n-alkoxyazobenzenes (BAAB). The helical pitch of the host cholesteric mixture was increased, as expected, on addition of the photochromic nematic material prior to irradiation with the molecules in the purely trans-configuration. On irradiation with low power (0.6 mW) argon ion laser light, cis-isomers formed within the interaction region and concentric colour domains appeared. Selective reflection from the colour domains occurred in the 400–560 nm spectral range. The coloured domains persisted in the time period following irradiation and extended beyond the interaction region because of diffusion of the cis-isomers. Using the diffusion equation, an expression has been obtained for the average concentration of cis-isomers ...

Diffusing wave spectroscopy (DWS) in the backscattering geometry was employed to observe the evolution of the intensity correlation function during the acidification of skimmed milk by gluconic-δ-lactone (GDL). At the stage when the formation of casein particle gel is largely complete the correlation function at shorter decay times reveals the local structural arrest of the casein micelles, whereas at longer delay times it illustrates the hindered slow motion of casein micelle aggregates. We use the principles of the approach suggested by Mason, Gang and Weitz, linking the optically measured mean square displacement, <Δr2(t)>, of the microscopic particles in a dense colloid to its viscoelastic properties, to provide an estimate of the frequency dependent viscoelastic modulus of the acidified milk gel (AMG). We compare the viscoelastic moduli measured by the conventional mechanical rheometry with the optically measured ones. The results of the two different experimental methods are found to be in reasonable agreement.

Diffusing wave spectroscopy (DWS) provides access to information about the internal dynamics of optically dense colloids, which in turn is known to reflect their viscoelastic properties. The rheological properties of optically dense particle gels, such as acidified milk gels (AMG), are of considerable fundamental and applied interest. A cell was specially designed to allow DWS investigations of dense colloids under the condition of shear, in both continuous and oscillatory modes. We demonstrate here the results of the rheo-optical approach to the diagnostics of the condition of an AMG, produced by the addition of glucono-δ-lactone to skimmed milk, under shear. The response of the AMG to shear flow, the structural transitions induced by shear and the effects on the intensity correlation function of the scattered light are discussed qualitatively.