向列型液晶三明治結構的聲導波波傳

Abstract

液晶具有單光軸雙折射的光學性質，受到電場、磁場或聲波等外界擾動，液晶分子排列會偏離原來的平衡狀態，改變光學特性。液晶與聲波的耦合關係稱為液晶的聲光效應。本研究將液晶簡化為具單一黏滯係數的等向性液體，推導玻璃□液晶□玻璃之三明治結構的聲導波波傳，數值模擬聲導波相速度頻散曲線及各模態的位移、應力與應變分佈，推估液晶分子排列所受的影響。數值結果發現 、 與 模態之聲導波在液體層具有明顯的壓力梯度，能夠影響液晶分子的排列。蘇裕為(2007)的實驗也發現上述模態聲導波會改變液晶的光學特性，與本文的結果具有一致性。 數值模擬之相速度頻散曲線隨著液體層厚度減少，往高頻偏移；液體層密度的減小則使頻散曲線朝向低頻微量偏移。在6 MHz的頻率範圍內，液體層的黏滯係數對於相速度頻散曲線無明顯影響。本研究發現三明治結構聲導波的流體模態與高頻的 模態擁有界面波的特性，固體層的變形侷限於固體與液體的界面附近，超音波的壓力梯度及能量主要分佈於液體層，界面波具有操控液晶分子排列的應用潛力。

Nematic liquid crystals (NLCs) are optically uniaxial, birefringent materials. Alignments of the NLC molecules are altered from their equilibrium states by disturbance of external fields such as electric, magnetic, and acoustic fields. Variations in the refractive indices of NLCs are induced in consequence. The coupling effect between liquid crystals and acoustic waves is called the acousto-optical effect of NLCs. The thesis presents an investigation on acoustic guided wave propagation in a liquid crystal cell, in which a liquid layer sandwiched between two glass plates. The liquid layer is filled with NLC, and it is modeled as a hypothetical isotropic solid with a single viscosity. Phase velocity dispersion curves and displacement, stress, and strain distributions in the cells are numerically simulated. Numerical evidence shows that A0, S0, and S1 modes of acoustic guided waves have significant pressure gradients in the liquid layer. The pressure difference plays an important role on realignment of NLCs in the cells. This prediction agrees with the experimental results observed by Su (2007), in which light transmits through NLCs in sandwich cells subjected to oblique ultrasound insonification. The simulated phase velocity dispersion curves move to higher frequency region as thickness of liquid decreases. Those curves slightly shift to the opposite direction as the mass density of liquid decreases. Variation in viscosity has no influence on the dispersion curves in the frequency range below 6 MHz. It is found that the fluid mode and high-frequency A0 mode feature the properties of interface waves. Most deformation of the solid layer is confined in a region near the interface. Pressure gradient and wave energy are localized in the liquid layer. It is expected that those interface waves have a great application potential in manipulation of NLC molecules with ultrasound.