Flow patterns and disclination-density measurements in sheared nematic liquid crystals I: Flow-aligning 5CB

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Abstract
We report the first direct measurements of disclination density during shear flow of a nematic liquid crystal. Samples of 4,4′-n-pentylcyanobiphenyl, or 5CB, are initially aligned in the homeotropic configuration within a torsional shear cell using surface treatment with lecithin. For low shearing rates, the director rotates within the shearing plane, except near the axis of rotation where irregular time-dependent out-of-plane orientation patterns exist. Disclinations form above a threshold shear rate, with the first ‘thick’ and ‘thin’ disclinations appearing near the sample edge, rapidly multiplying, and subsequently filling the sample. We measure the steady-state disclination density, ρA, the length of disclinations per unit projection area. The dimensionless disclination density, ρA h, where h is the sample thickness, is found to depend on two dimensionless quantities: Er, the Erickson number, and [rtilde] = r/h, the scaled radial position. In the limit of large [rtilde]([rtilde]) 100), the dependence of ρA on [rtilde] disappears and we find the empirical relation ρA h ∞ Er0.63.