FLOW PATTERNS AND DISCLINATION-DENSITY MEASUREMENTS IN SHEARED NEMATIC LIQUID-CRYSTALS .1. FLOW-ALIGNING 5CB

We report the first direct measurements of disclination density during shear how of a nematic liquid crystal. Samples of 4,4'-n-pentylcyanob iphenyl, or 5CB, are initially aligned in the homeotropic configuratio n within a torsional shear cell using surface treatment with lecithin. For low shearing rates, the director rotates within the shearing plan e, except near the axis of rotation where irregular time-dependent out -of-plane orientation patterns exist. Disclinations form above a thres hold shear rate, with the first 'thick' and 'thin' disclinations appea ring near the sample edge, rapidly multiplying, and subsequently filli ng the sample. We measure the steady-state disclination density, rho(A ), the length of disclinations per unit projection area. The dimension less disclination density, rho(A)h, where h is the sample thickness, i s found to depend on two dimensionless quantities: Er, the Erickson nu mber, and (r) over tilde = r/h, the scaled radial position. In the lim it of large (r) over tilde((r) over tilde) > 100), the dependence of r ho(A) on (r) over tilde disappears and we find the empirical relation rho(A)h proportional to Er-0.63.