Flow patterns and disclination-density measurements in sheared nematic liquid crystals II: Tumbling 8CB

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Abstract
We examine the behaviour of nematic 4,4′-n-octylcyanobiphenyl (8CB) subjected to steady torsional shear flow at a temperature where this nematic is ‘tumbling’, i.e. the ratio α3/α2 of Leslie viscosities is negative. If the disc rotation speed is gradually increased from zero ( < 10−6 rad s−2), the director eventually departs from the shear plane and becomes oriented radially. However, if the disc rotation speed is increased from rest at higher rates, twist wall defects are created sequentially at the sample meniscus and propagate inward with velocity ≈0.85mm s−1. Eventually, disclinations are nucleated. Disclination density measurements reveal that over similar ranges of values of the Ericksen number Er, samples of 8CB have roughly ten times the disclination density of the flow-aligning nematic, 5CB, reported in the preceding paper. Additionally, a transition from a power-law exponent of 0.5 to 1.0 for the dependence of dimensionless disclination density on Er is observed near Er = 2000, consistent with theoretical arguments.