Two Novel Cholesteric Liquid Crystal Systems: Microfluidics-Produced Droplets and Self-Assembled Cellulose Nanocrystal Films

Abstract

Cholesteric (N*) liquid crystals have attracted much attention over a broad research field thanks to their peculiar optical properties, rendering them exquisite soft photonic materials. The period of their self-assembled helical structure can be tuned to reflect light in any part of the visible spectrum by adjusting chemical composition or temperature. The helical arrangement of the building blocks of the phase also gives the reflected light circular polarization with same handedness as the helix. Here we focus on two modern cholesteric systems and the unique optical properties that they develop: monodisperse cholesteric liquid crystal droplets and dried films of cellulose nanocrystals with a vitrified internal cholesteric structure.

With a coaxial microfluidic set-up we produce monodisperse droplets of suitable cholesteric mixtures, dispersed in a dilute aqueous polyvinylalcohol (PVA) and glycerol solution as continuous phase. The droplets are collected at such high volume fraction that they spontaneously organize into a 2D colloidal crystal arrangement. The continuous phase composition ensures planar alignment, i.e. the helix adopts a radial orientation. This leads to omnidirectional selective reflection from the cholesteric droplets giving rise to peculiar and attractive optical phenomena. We demonstrate the specific way of photonic cross communication between droplets, resulting in multicolored and symmetric patterns. Apart from being fascinating from a fundamental optics perspective, the system holds extensive potential for applications, e.g. in sensors or in anti-counterfeiting tags.

Cellulose nanocrystal (CNC) is a novel bionanomaterial that currently attracts considerable international attention. It is produced from natural cellulose sources by extracting the smallest structural element in the form of crystalline nanorods. At certain concentrations a CNC suspension develops a cholesteric liquid crystal phase with consequent helical modulation of the nanorods. We developed a new method of drying CNC suspensions under controlled shear flow, yielding a thin CNC film with an unusually high degree of structural control. A key element is to delay and slow down the glass formation which competes with liquid crystalline ordering as the CNC concentration is increased when the water evaporates. Films dried under appropriate conditions exhibit large-scale uniform iridescent properties, a result of a uniformly aligned helix with pitch in the submicron scale. By increasing the circular shear flow the helix can however also be unwound and a uniformly birefringent film is produced.