Director field patterns of nematic polymer solutions at asynchronous rotation in a magnetic field

Abstract

A variety of different patterns is observed when a magnetic field rotating with an angular frequency is applied on a nematic cell with homeotropic boundary conditions. To study the role of backflow in the formation of these dissipative structures, we compared the behaviour of dilute solutions of a mesogenic side group polymethacrylate in a low molar mass nematic liquid crystal (5CB) with that of the pure solvent. The dissolved polymer molecules lead to a strong reduction of the existence range of the so-called VRL-TI (viscosity reduction lattice transverse instability) and complex states. For mass fractions w > 2 % of the polymer these states were found to be suppressed completely due to the efficient reduction of backflow. The growth rate of the viscosity reduction lattices, which can still be observed, is in addition dependent on the polymer concentration. Numerical calculations were performed to explain the behaviour.