However, the size of the CNCs is orders of magnitude bigger than their counterparts in molecular liquid crystals, which leads to much larger electric dipoles and slower relaxation times, contributing to making apolar CNC dispersions an original system when exposed to strong AC electric fields (Frka-Petesic et al. 2017), following the behavior initially predicted theoretically and observed experimentally for molecular cholesteric systems and under magnetic fields when the molecules tend to align parallel to the magnetic field (De Gennes 1968 Rondelez and Hulin 1972), and later also experimentally observed in colloidal systems (Dogic and Fraden 2000). Above a critical field value typically about 0.4–0.6 kV/cm, the cholesteric completely unwinds and exhibits a uniform monodomain nematic order (Frka-Petesic et al. In cholesteric suspensions, the application of an electric field tends to first reorient the cholesteric domains so that their helical axes point away from the field direction, and then the helical order distorts to progressively align the CNCs along the field direction, leading to a pitch increase. 2006), and that their alignment dynamics is dominated by a permanent dipole contribution when the electric field is suddenly reversed (Frka-Petesic et al. Previous works established that in dilute suspensions, CNCs align parallel to the electric field (Bordel et al. Moreover, this allows for the investigation of the individual and collective behavior of CNCs in suspensions in presence of strong electric fields (> 0.1 kV/cm) on centimeter-scale samples, without triggering the electrochemical complications due to water electrolysis. The extension of their colloidal stability range to apolar solvents, achieved using suitable surface functionalization or surfactants, allows for the exploration of the formation of cholesteric phases where electrostatic interactions do not play a major role (Heux et al. This work concludes with possible future experimental investigations to clarify the exact regime of instability responsible for these observations.Ĭellulose nanocrystals (CNCs), as extracted from cotton by sulfuric acid hydrolysis, form colloidally stable aqueous suspensions capable of forming spontaneously a cholesteric liquid crystalline phase above a threshold concentration (Revol et al. However, the typical geometry where these instabilities were most documented across the literature differs from the geometry used in this work. These instabilities usually present complex regimes varying with the field, the voltage, the frequency and the geometry. We ascribed this pattern to electrohydrodynamic convection instabilities. In this work, we show that at much higher electric fields ( \(\ge\) 4.6 kV/cm at 1 kHz) the sample develops a periodic pattern that varies with the field amplitude. When they are suspended in apolar solvents such as toluene using surfactants, the application of an AC electric field leads to the reorientation and then distortion of the cholesteric order until the cholesteric structure completely unwinds into a nematic-like order, typically above 0.4–0.6 kV/cm at 1kHz. Cellulose nanocrystals are slender, negatively charged nanoparticles that spontaneously form a cholesteric liquid crystal in aqueous suspension above a critical concentration.
0 Comments
Leave a Reply. |
AuthorWrite something about yourself. No need to be fancy, just an overview. ArchivesCategories |