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系統識別號 U0026-3008201714573800
論文名稱(中文) 液晶微粒奇特之介電泳與電流體動力效應之研究
論文名稱(英文) Anomalous dielectrophoresis and electrohydrodynamic effects of liquid crystal microdroplets
校院名稱 成功大學
系所名稱(中) 光電科學與工程學系
系所名稱(英) Department of Photonics
學年度 105
學期 2
出版年 106
研究生(中文) 吳聲廣
研究生(英文) Sheng-Kuang Wu
學號 l78961165
學位類別 博士
語文別 英文
論文頁數 85頁
口試委員 口試委員-傅永貴
口試委員-李偉
口試委員-羅光耀
口試委員-黃啟炎
共同指導教授-莫定山
指導教授-李佳榮
中文關鍵字 電流體動力學  介電泳  液晶  單體  聚(N-乙烯基咔唑)  聚合物分散液晶 
英文關鍵字 Electrohydrodynamic  dielectrophoretic  crystal  monomer  poly (N- vinylcarbazole)  polymer dispersed liquid crystal 
學科別分類
中文摘要 液晶與單體混合物是一個相當有趣且深具應用潛力之複合式材料,當我們藉由某些方法將此兩物質做相分離,例如變溫、外加電場或照光,可以得到眾多液晶顆粒無序且均勻地散佈於單體或高分子中(前後者分別無與有進行聚合反應),這些皆為非常有趣且吸引人的系統,因為這些系統不僅同時擁有兩種不同的狀態 ‒ 固相與液相,而且也同時包含均向性(高分子或單體)與非均向性(液晶)物質,所以過去吸引非常多科學家進行基礎研究與發展光電應用元件,例如可調控式光閥(顯示器)、光柵、雷射等。
因此,本論文著重於研究與探討有關上述系統中(液晶顆粒散佈於單體或高分子複合材料),所出現之介電泳與電流體動力效應之動態研究。本論文題目訂為『液晶微粒奇特之介電泳與電流體動力效應之動態研究』,可分為下列兩個主題做研究探討:
(一) 第一個主題為『聚合物分散液晶薄膜中的電流體動力學誘導之異常電光特性』。本研究利用低強度紫外光曝光下,製備具有液晶大顆粒分散於聚合物中之薄膜,在外加低頻(1kHz)電場之下,首次觀察到聚合物分散液晶薄膜具有異常之電光行為,包括正入射時外加電壓下,穿透率會先快增爾後慢衰減,並分別在正入射和斜向入射時,電光特性具有偏振獨立性和偏振依賴性之轉換性,其特性與傳統具有小液晶顆粒之聚合物分散液晶薄膜特性相差甚大。
實驗證明,上述大顆粒PDLC所具備的異常電光特性(主要在高電壓區)主要是歸因於樣品的強烈散射所致。此強烈散射又跟樣品內之大顆粒液晶於高電壓區會顯現出霧狀外觀有關。此效應經過證明乃是因為具有類半球形液晶顆粒內部產生一個類漩渦狀液晶導軸場;此場之旋轉軸接近垂直於基板平面。又此場主要是由液晶顆粒內部之雜質電荷之漩渦狀擾流所引起的;此擾流乃由於外加低頻AC電場引起的電流體動力效應與類半球體的微米三維空間邊界限制所致。上述的散射主要由液晶球與聚合物間的折射率不匹配程度與顆粒內部的漩渦狀導軸場的局部動態微擾動程度所決定,進一步決定了大顆粒PDLC之異常電光行為。此研究的成果提供了一個新穎的電流體動力效應機制,其乃發生於一個三維異向性微液顆粒內。此大顆粒PDLC薄膜有淺能可應用在光子學領域,例如電控與偏振相依光學元件或偏振相關與不相關之光學轉換器。
(二) 第二個主題為 『液晶顆粒在無須外加電壓下具有介電泳動行為之研究』。本研究事先利用UV光經過二維黑白光柵光罩後對光導材料PVK薄膜週期選擇性照光,使得PVK膜具有二元區 ‒ 預照區和非預照區之分。之後,將液晶與單體混合物灌入一邊為此二元PVK膜基板之空樣品中。實驗發現,在預照區和非預照區之間的邊界附近自我產生一個內部電場,此場進一步造成介電泳動力,使得液晶微粒可有效地從預照區遷移至非預照區,有效地遞增此光柵樣品之繞射效率。此研究結果提供了發展無須外加電壓式介電泳元件之可能性,這些元件可應用於光子學、顯示器與生醫領域上。
英文摘要 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.
論文目次 摘要 I
Abstract III
Acknowledgements VI
Contents VII
List of Figure X
List of Table XV
Chapter 1 Introduction 1
Chapter 2 Properties of Liquid Crystals 5
2.1 Liquid Crystals Phase 5
2.2 Classification of Liquid Crystals 6
2.2.1 Lyotropic Liquic Crystals 7
2.2.2 Thermotropic Liquic Crystals 7
2.3 Physical and Electro-optical Properties of Liquid Crystals 10
2.3.1 Optical birefringence 11
2.3.2 Dielectric Anisotropy 13
2.3.3 Elastic Continuum Theory of Liquid Crystals 15
Chapter3 Properties of Polymer Dispersed Liquid Crystal, Electrohydrodynamics, Electrophoresis, and Dielectrophoresis 16
3.1 Properties of Polymer Dispersed Liquid Crystal 16
3.1.1 Structure of polymer 16
3.1.2 Production process of polymer-dispersed liquid crystal 17
3.1.3 Application and working principle of polymer-dispersed liquid crystal 21
3.2 Electrohydrodynamics, electrophoresis, and dielectrophoresis 23
3.2.1 Electrohydrodynamics 23
3.2.2 Electrophoresis 27
3.2.3 Dielectrophoresis 30
Chapter 4 Sample Fabrication and Experimental Setups 34
4.1 Materials used 34
4.2 Fabrication of Sample 37
4.2.1 Preparation of Glass Substrates 37
4.2.2 Fabrication of Empty Cells 38
4.3 Experimental Setups 39
Chapter 5 Electrohydrodynamic-induced abnormal electro-optic characteristics in a polymer-dispersed liquid crystal film 41
5.1 Introduction 42
5.2 Sample preparation and experimental setup 44
5.3 Results and Discussion 45
Chapter 6 External-voltage-free dielectrophoresis of liquid crystal droplets 62
6.1 Introduction 62
6.2 Sample preparation and experimental setup 64
6.3 Results and Discussion 66
6.3.1 Formation of LC droplets with significantly different sizes between the pre-illuminated and non-pre-illuminated regions in the grating sample at the cooling step 66
6.3.2 Phenomenon of migration of small LC microdroplets from the pre-illuminated regions to the non-pre-illuminated regions of the grating sample at the constant-TR step 69
6.3.3 Further discussion about key role of the discovered non-uniform internal field playing at the cooling step 74
Chapter 7 Conclusion and Future Works 77
7.1 Conclusion 77
7.2 Future Works 78
List of Reference 80

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