Response of a homeotropic nematic liquid crystal to rectilinear oscillatory shear

The response of a homeotropically aligned nematic liquid crystal layer to o scillatory rectilinear shear (Couette flow) was investigated for frequencie s f between 0.01 and 200 Hz and layer thickness d between 10 and 130 mu m. Below the onset of instability the cell was placed between crossed polars a nd light transmission was studied using a parallel light beam. The experime ntal results for the transmitted light intensity agree quantitatively with numerical solutions of the nematodynamic equations for different cell thick nesses, oscillation frequencies, and amplitudes. For frequencies between 25 and 150 Hz the critical oscillation amplitude for the onset of a spatial p attern, observed in polarized white Light, could be reached. The pattern co nsisted of stationary rolls perpendicular to the direction of the oscillati on. The experimentally obtained frequency dependence of the critical shear amplitude for the roll instability for different cell thicknesses is compar ed with an existing theory and the results of numerical calculations. [S106 3-651X(98)09411-2].