Orientation and rotation of cholesteric liquid crystals relative to a heat flow studied by molecular dynamics simulation and implications for the Lehmann effect

Alignment effects caused by a heat flow in the cholesteric liquid crystal phase of three coarse grained molecular model systems based on the Gay-Berne potential have been studied by molecular dynamics simulation. In order to keep the systems homogeneous, the Evans heat flow algorithm, where a fictitious mechanical heat field rather than a temperature gradient drives the heat flow, was used. It was found that the cholesteric axis orients in such a way that the heat flow and thereby the irreversible energy dissipation rate are minimized. This is in accordance with a theorem stating that the irreversible energy dissipation rate is minimal in the linear regime of a nonequilibrium steady state. In two of the studied systems this means that the cholesteric axis orients parallel to the heat field and the heat flow. Then the heat field induces a torque that rotates the director around the cholesteric axes which is the basis of the Lehmann effect. However, in one of the systems, the cholesteric axis orients perpendicularly to the heat field and a torque is exerted that rotates the cholesteric axis around the heat field. This is a transport phenomenon that has not been studied before.