Cholesteric to cholesteric phase transitions in liquid crystals

We discuss the phase transitions which can occur between two different cholesteric phases and we present general symmetry arguments why this transition has to be always of the first order. We derive the macroscopic dynamics close to the phase transition and we propose an experiment to shift the critical temperature which makes use of a novel reversible cross coupling between the order parameter of the transition and inhomogeneous rotations Discussion sur les transitions pouvant apparaitre entre deux phases cholesteriques. On montre par des arguments de symetrie generale que cette transition doit toujours etre du premier ordre