Twisting with a twist: supramolecular helix fluctuations in chiral nematics

摘要

Most theoretical descriptions of lyotropic cholesteric liquid crystals todate focus on homogeneous systems in which the rod concentration, as opposed tothe rod orientation, is uniform. In this work, we build upon theOnsager-Straley theory for twisted nematics and study the effect of weakconcentration gradients, generated by some external potential, on thecholesteric twist. We apply our theory to chiral nematics of nanohelices inwhich the supramolecular helix sense is known to spontaneously change sign uponvariation of particle concentration, passing through a so-called compensationpoint at which the mesoscopic twist vanishes. We show that the imposed fieldoffers exquisite control of the handedness and magnitude of the helicoidaldirector field, even at weak field strengths. Within the same framework we alsoquantify the director fluctuation spectrum and find evidence for a correlationlength diverging at the compensation point.