Anomalous dielectrophoresis and electrohydrodynamic effects of liquid crystal microdroplets

Abstract

Liquid crystal (LC) and monomer mixture is a significant composite material with a high potential in application After phase separation through change in temperature applying an electric field or light illumination many LC droplets can disperse in the monomer or polymer matrix (without or with the reaction of the polymerization) These systems are both interesting and attractive systems for the sake of comprising homogeneous randomly oriented LC droplets distributed in a soft monomer or hard polymer matrix which has an optically isotropic property They have attracted wide attention and led to studies on their fundamental characteristics and applications such as tunable light valves (displays) gratings and lasers due to the presence of isotropic monomer/polymer and anisotropic LCs and electrically controlled electro-optic (EO) ability The dissertation is entitled “Anomalous dielectrophoresis and electrohydrodynamic effects of liquid crystal microdroplets” Two studies are included in the dissertation and described in brief as follows: (1) The first study is entitled “Electrohydrodynamic-induced abnormal electro-optic characteristics in a polymer-dispersed liquid crystal film” This study demonstrates for the first time abnormal EO characteristics induced by electrohydrodynamics (EHD) in a polymer-dispersed liquid crystal (PDLC) film in the presence of a low-frequency (1 kHz) AC voltage Large LC droplets buried in the film can be obtained after the illumination of one UV light with a weak intensity (~0 96 mW/cm2) for 12 h This film exhibits abnormal EO features including the transmittance’s decay at a high voltage regime at normal incidence and the conversion between polarization independence and polarization dependence for the transmittance-voltage curve at normal and oblique incidences respectively of which properties are different from those shown in traditional PDLC films with small droplets The abnormal EO characteristics of the large-droplet PDLC at the high voltage regime are attributed to a strong scattering effect associated with the formation of the foggy LC droplets in the cell This effect is induced by a vortex-like LC director field with a rotational axis normal to the cell substrates in each dome-like droplet of the cell at the high voltage regime The vortex-like director field is induced by a vortex-like turbulence of charged impurity generated by the EHD effect under the action of the AC electric field along the cell normal and the confinement of the dome-like boundary of the droplet on the charged impurities in each droplet The scattering is decided by the degrees of mismatch between the refractive indices of the LC droplet and polymer and the local fluctuation of the vortex-like director field in the droplet resulting in the abnormal EO behaviors of the large-droplet PDLC This investigation provides novel insight into the EHD effect in three dimensional (3D) microdroplets with anisotropic fluid Such a large-droplet PDLC has potential in photonic applications such as electrically controlled polarization-based optical components or optical converters between polarization independence and polarization dependence (2) The second study is entitled “External-voltage-free dielectrophoresis of liquid crystal droplets” This work reports for the first time a dielectrophoresis (DEP) effect-induced motion of LC droplets in an LC/monomer mixture sample with a poly-(N-vinyl carbazole) PVK-coated substrate without an external voltage With the UV pre-irradiation of the PVK layer through a binary mask a laterally non-uniform electric field can be induced between the pre-illuminated regions and the neighboring non-pre-illuminated PVK regions near the borders of the two regions The phase separation occurs once the temperature is lower than 50 ?C and the LC droplets can form in the sample The pre-formed non-uniform field provides a DEP-like force to manipulate the small LC microdroplets in the pre-illuminated regions to effectively migrate to the adjacent non-pre-illuminated regions The continuous supply of the LC from the pre-illuminated regions to the adjacent non-pre-illuminated regions significantly increases the diffraction efficiency of the grating sample This study provides an insight into developing new external-voltage-free DEP-based devices that can be applied on various fields such as photonics displays and biomedicines

Date of Award	2017 Sept 4
Original language	English
Supervisor	Chia-Rong Lee (Supervisor)