Columnar versus smectic order in systems of charged colloidal rods

We study the stability of inhomogeneous liquid crystalline states in systems of monodisperse, stiff, charged rods. By means of a bifurcation analysis applied to the Onsager free energy for charged rods in strongly nematic states, we investigate nematic-smectic and nematic-columnar instabilities as a function of the Debye screening length. While the nematic-smectic transition clearly pre-emts the nematic-columnar one in the regime of strong screening a marked stability of hexagonal columnar order is observed at larger screening lengths. The theoretical results are substantiated by Brownian dynamics computer simulation results based on the Yukawa-site model. Our findings connect to experiments on tobacco mosaic virus rods in particular but might be relevant for soft rod-like mesogens in strong external directional fields in general.