液晶混奈米高分子之角速度響應與液晶陀螺儀理論基礎之研究

本論文主要在討論奈米物質混液晶的角速度響應，並且試著探討液晶陀螺儀之理論基礎。液晶材料本身具有液體的流動性，同時兼具了類似晶體的分子方向秩序，這樣的特性使液晶具有物理非等向性，由於液晶分子可簡單經由外力場改變其方向，如電磁場、壓力、溫度梯度等，因此被廣泛運用在各種感測器上，以及與我們生活最貼近的液晶面板，因應發展所需，越來越多研究開始關注其他物質混入液晶的特性，大多數致力於靜態的物理特性改善，但是由於非等向性流體本身的流動較一般流體複雜許多，研究其他物質混入液晶之後的動態更顯重要。本實驗研究的奈米物質以常見的奈米碳管與奈米鐵粉為主，此兩種不同密度但是體積相近的顆粒在受到角速度產生的離心力影響後，分別在不同的臨界角速度下開始使奈米物質混液晶的電容值產生變化，這說明了兩件事，第一是物體顆粒要在液晶材料裡運動之前，必須先克服一束縛力，第二是在同大小的物體所受的束縛力幾乎是相同的，為了探討其束縛力的成因，我們做了改變奈米物質混液晶溫度的實驗，實驗結果顯示較高溫的樣品比低溫的樣品除了減少了束縛力，也增加了電容值變化的幅度，由於先前的研究顯示溫度會改變液晶的彈力常數與黏滯係數，因此束縛力與電容值改變的幅度與液晶的彈力常數與黏滯係數有關。為了再詳細探討微觀中的物理特性，我們將物體顆粒運動時對液晶方向秩序破壞的因素加以探討，結果發現當物體顆粒的半徑大於某一特徵長度時，物體顆粒與液晶之間表面能的改變量應小於液晶的彈力能變化量，以滿足能量最小化。在垂直兩偏振片中觀察奈米鐵粉於液晶裡的運動後我們發現，奈米鐵粉的運動會對周圍的液晶產生角度的變化，進而改變奈米物質混液晶的電容值，因此奈米鐵粉的移動速度越快，奈米物質混液晶的電容值改變量就越大，另外，奈米鐵粉的移動速度同時也與遲滯電容成正比，而隨著轉動的次數越多，越多奈米鐵粉集中在液晶盒的邊緣，造成臨界角速度增加與電容改變量下降，因此文章的第六章我們探討液晶陀螺儀的理論基礎，期望保有整體系統的重複性，然而結果發現液晶本身有黏彈特性，過阻尼的特性影響了可工作頻率範圍，而且高黏滯性也使得液晶分子角度變化不大，因此要實現液晶陀螺儀必須降低黏滯係數或是使旋轉黏滯係數接近流動黏滯係數，同時，增加彈力常數或是縮短液晶層厚度也能增加工作頻率範圍。

關鍵字

液晶；角速度響應；奈米物質；黏滯力；液晶陀螺儀

並列摘要

This dissertation focuses on the angular velocity response of nanoparticles dispersed in liquid crystal, and tries to investigate the theoretical foundation study of liquid crystal-based gyroscope. Liquid crystals can move freely like normal liquid, but exhibit the orientational order, which makes liquid crystal as the anisotropic liquid. Owing to the orientation of liquid crystal molecules can simply alter by external field, e.g. electromagnet field, pressure and temperature gradient, liquid crystals have been widely used in many sensors, and popularly been used in display technology. Recently, a number of investigators have reported on the behavior of particles dispersed in this interesting material, most of them focus on the improvement of bulk properties of liquid crystals, since the hydrodynamics of liquid crystals is more complicated than isotropic fluid, it is worthy to investigate the dynamic of particles dispersed in liquid crystals. The nanoparticles we used are carbon nanotubes and Fe2O3 nanoparticles. When these two types of particles with similar volume experienced centrifugal force, the capacitance of liquid crystal cell started to change at different threshold angular velocities. This phenomenon indicates that, first, particles should overcome some trapping force before they move, second, the trapping forces exert on these two types of particles are almost the same. In order to investigate the trapping force in detail, we change the temperature of liquid crystal cell. The result indicated the trapping force was reduced as well as the difference capacitance was increased while raising the temperature of liquid crystal cell. Hence, the threshold angular velocity is related to the elastic constant. In microscope level, we found that, when the radius of particle is larger than Kleman-de Gennes length, the difference of surface energy between particle and liquid crystals should less than the difference of elastic energy. By observing the motion of particles in liquid crystal within cross-polarized light microscopy, we found that, the orientations of liquid crystal molecules were altered by the nearby motion of particles, so the faster the movement of particles is, the large difference capacitance we will get. Besides, the mobility of particles is in relation to the hysteresis capacitance, while particles are stack at the edge of liquid crystal cell, the threshold angular velocity increased and the difference capacitance decreased. In Chapter 6, the theoretical foundation study on liquid crystal-based gyroscope was being discussed; due to the verdamped system of liquid crystals, the range of working frequency is limited at low frequency, and the orientation of liquid crystal molecules driven by Coriolis force is slightly changed. In order to improve the working frequency and sensitivity, one should reduce viscosity coefficients or adjust rotational viscosity as close to translational viscosity as possible. In addition, while increasing elastic constant or reducing the thickness of liquid crystal layer, the working frequency of liquid crystal-based gyroscope could be raised.

並列關鍵字

liquid crystal ； angular velocity response ； nanoparticles ； viscosity force ； liquid crystal-based gyroscope

參考文獻

[1] S. Mohanty, “Liquid crystals — the ‘fourth’ phase of matter,” Resonance, vol. 8, pp. 52–70, 2003.

[2] V. Freedericksz and A. Repiewa, “Theoretisches und Experimentalles zur Frage nach der Natur der anisotropen Flussigkeiten,” Physik. Zeitschr., vol. 42, pp. 532-546, 1927.

[3] V. Freedericksz and V. Zolina, “Forces causing the orientation of an anisotropic liquid,” Trans. Fardday Soc., vol. 29, pp. 919-928, 1933.

[4] H. Kawamoto, “The history of liquid-crystal displays,” P. IEEE, vol. 90, pp. 460–500, 2002.

[5] G. W. Gray and S.M. Kelly, “Liquid crystals for twisted nematic display devices,” J. Mater. Chem., vol. 9, pp. 2037–2050, 1999.

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液晶混奈米高分子之角速度響應與液晶陀螺儀理論基礎之研究

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