RESUMEN
We study the elastic properties of thermotropic smectic liquid crystals with focal conic domains (FCDs). After the application of the controlled preshear at different temperatures, we independently measure the shear modulus G' and the FCD size L. We find out that these quantities are related by the scaling relation G' ≈ γ(eff)/L, where γ(eff) is the effective surface tension of the FCDs. The experimentally obtained value of γ(eff) shows the same scaling as the effective surface tension of the layered systems â(KB), where K and B are the bending modulus and the layer compression modulus, respectively. The similarity of this scaling relation to that of the surfactant onion phase suggests an universal rheological behavior of the layered systems with defects.
RESUMEN
We measure the non-linear relation between the shear stress and shear rate in the lyotropic lamellar phase of C12E5/water system. The measured shear thinning exponent changes with the surfactant concentration. A simple rheology theory of a lamellar or smectic phase is proposed with a prediction gamma approximately sigma3/2, where gamma is the shear rate and sigma is the shear stress. We consider that the shear flow passed through the defect structure causes the main dissipation. As the defect line density varies with the shear rate, the shear thinning arises. The defect density is estimated by the dynamic balance between the production and annihilation processes. The defect production is caused by the shear-induced layer undulation instability. The annihilation occurs through the shear-induced defect collision process. Further flow visualization experiment shows that the defect texture correlates strongly with the shear thinning exponent.