S-Space College of Engineering/Engineering Practice School (공과대학/대학원) Dept. of Materials Science and Engineering (재료공학부) Theses (Ph.D. / Sc.D._재료공학부)
Application of Nematic Liquid Crystal to Tunable Optofluidic Birefringent Lens and Thermo-sensitive Smart Film : 가변형 광유체 복굴절 렌즈와 열감응형 스마트 필름에 대한 네마틱 액정의 응용
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- 공과대학 재료공학부
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- polymer dispersed liquid crystal film ; phase transition ; transmittance ; anomalous diffraction approach ; Joule heating ; silver nanowire ; nematic liquid crystal ; optofluidics ; tunable birefringent lens ; liquid lens ; Ericksen-Leslie theory
- 학위논문 (박사)-- 서울대학교 대학원 : 재료공학부, 2016. 2. 윤재륜.
- Applications of liquid crystals (LCs) have been focused on displays using orientation of them under an electric field, optical films such as retardation films and reflective polarizers using cholesteric LCs for liquid crystal displays (LCDs), and polymer-dispersed liquid crystal (PDLC) films. Among outstanding features of LCs, we note hydrodynamic properties in a microchannel and phase changes over transition temperature.
In Chapter II, a tunable birefringent lens using interfaces between an isotropic and nematic liquid crystal (NLC) stream in a microchannel, referred to as L2 interfaces here, is demonstrated. When the NLC is subjected to an electric field and flows along the direction orthogonal to the field, the alignment of NLC molecules is determined according to a balance between the flow-induced orientation by a viscous torque and the field-induced orientation by the electric field. In this study, we propose electric and hydrodynamic conditions for NLC molecules to be aligned perpendicular to the direction of flow under a strong electric field using a two-dimensional finite element method based on Ericksen-Leslie (E-L) dynamic theory for NLC and a dimensionless analysis. In addition, we observe the orientation of NLC, the L2 interfaces, and birefringent characteristics of the device at the proposed conditions experimentally. At this time, the flow rates of the NLC stream are controlled by a N2 gas pressure pump instead of a conventional syringe pump to supply a sufficiently small amount of fluid in a stable manner, and the exact flow rate at each applied pressure is confirmed by measuring the inflow volume of the NLC. If the NLC molecules are uniformly aligned under the strong electric field, simplified E-L equations can be derived and coupled with Navier-Stokes equation for the isotropic fluid by the volume of fluid (VOF) method. Thus, we can describe the L2 interfaces numerically, and calculate focal lengths using a ray-tracing.
In Chapter III, a thermal-induced PDLC film using the phase transition of LCs is investigated. A conventional PDLC film, which consists of a continuous polymer matrix and a number of LC droplets, is positioned between two transparent electrodes, and shows a transparency due to refractive index matching between the LC droplets and the polymer matrix when an electric field is applied to the film. On the contrary, the LC droplets act as scattering particles in the absence of the electric field, so that the film turns to be opaque. Since the conventional PDLC film utilizes the field-induced orientation of LCs, it necessarily requires two transparent electrodes and a thin thickness of the film for low driving voltage. In general, ITO (Indium-tin oxide) coated glasses, which are fabricated by an electro-deposition process, are used as the transparent electrodes for the conventional PDLC film, but it costs high for a large-area production. In order to overcome the mentioned problems, we propose a novel PDLC thermo-sensitive smart film (TSF), which controls the optical transmittance through not the field-induced orientation of LCs but the thermal-induced phase transition of PDLCs. The TSF is fabricated by coating the PDLC layer on silver nanowire (AgNW) networks coated on a film-substrate. Also, the operation mechanism is investigated from measurements of thermo-optical properties and phase transition characteristics of the TSF. Based on these results, numerical analyses are carried out to predict thermal performances and temperature dependent transmittances using anomalous diffraction approach (ADA) as a scattering model. Furthermore, an effective operation method of the TSF is introduced by adopting a dynamic power control to reduce response time and consumed energy.
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