Macroscopic dynamics near the nematic-columnar transition in liquid crystals

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Abstract
We derive the dynamic equations for the macroscopic variables close to the nematic– to–hexagonal-columnar transition in discotic liquid crystals. We discuss especially the physical consequences of a static cross-coupling term between fluctuations of the discotic order parameter and gradients of the nematic director, a term omitted by previous workers. Furthermore, we predict the breakdown of flow alignment in disklike nematics close to the transition to a columnar phase and we discuss the microscopic reasons for this effect and, in general, the same type of behavior for other liquid crystalline phases with positional order such as smectic-A, -B, -C, -G, etc.