Dynamic light scattering and viscosity measurements in a ternary and quaternary discotic lyotropic nematic liquid crystal: Tuning the backflow with salt

Abstract

Using a dynamic light scattering technique, we measure the damping rate of thermal fluctuations of the nematic director for the so-called disklike nematic N(D) phase of both the ternary lyotropic K-laurate-1-decanol-H(2)O system and the quaternary one of similar composition except for the addition of salt (K(2)SO(4)). By varying the scattering angle in suitable geometries and polarizations, we are able to measure the orientational diffusivities associated with the pure deformations of splay and twist. A previous study made in the N(D) phase of the same ternary system yielded a large deviation between the splay and twist diffusivities. The effect was then interpreted in terms of the anisotropy between their associated viscosities due to induced flows, or backflow. In the present work we observe a strong increase of the backflow as an effect of the added salt. In addition, we make auxiliary measurements of shear viscosity and magnetic instabilities, which help to characterize the effect of the salt in the orientational diffusivities as they are mixed quantities involving elastic constants and viscosity coefficients.

Shear viscosity and rheology of ternary and quaternary lyotropic liquid crystals in discotic and calamitic nematic phases

“…One could imagine that this assumption eventually breaks down in the case of quaternary samples, as the dramatic increase of η observed there might be associated to an increase of the micelle size, e.g., because of salt-mediated hydration (Figueiredo Neto and Salinas 2005;Galerne et al 1987). However, recent DLS data (Risi et al 2013) have shown a giant backflow effect in these salted mixtures, which implies that γ 1 (N D ) η holds for such systems too. A similar comparison can be made in the ternary N C phase, for which we can quote the DLS result (Lacerda Santos and Amato 1999) D twist = 0.42×10 −12 m 2 /s.…”

Lyotropic nematic liquid crystals: interplay between a small twist elastic constant and chirality effects under confined geometries

“…The viscosity η splay is the largest in the N C phase of chromonic DSCG, whereas in the micellar N D phase η bend is the largest. The reverse behaviour of N C and N D phases regarding η splay vs. η bend can be explained according to the Ericksen-Leslie model with the backflow mechanism [61][62][63][64][65][66][67][68][69]. It seems that backflow lowers η splay but not η bend in lyotropic N D phases.…”