Flow-aligning and tumbling in small-molecule liquid crystals: pure components and mixtures

The results of an experimental study to measure the tumbling parameter, lam bda, for various small-molecule liquid crystals and their mixtures are pres ented. The methods used include textural observations (twist walls), a dire ct method, a rheological method, and the oscillatory method developed by Ma ther, Pearson, and Burghardt in 1995. The single-component results are comp ared with a molecular theory derived in 1995 by Archer and Larson as well a s Kroger and Sellers, which predicts the temperature dependence of lambda, while the results from the binary mixtures are compared to a continuum theo ry derived by Rey in 1996, giving the concentration dependence of lambda. T he results from the four experimental methods agree with each other for sin gle-component liquid crystals, but not for mixtures. This suggests a failur e of the single director Leslie-Ericksen theory to describe the rheology of liquid crystal mixtures.