沒水式超音波對於向列型液晶聲光效應的影響

Abstract

向列型液晶為單光軸晶體，具有雙折射效應的光學性質，除了電場、磁場之外，還可以藉聲場改變液晶分子排列的平衡狀態。本研究將液晶盒沒入水槽內，探討向列型液晶受到斜束超音波照射產生的聲光效應。夾於兩片玻璃之間的液晶層假設為具有單一黏滯係數的牛頓流體，並以等向性固體近似。數值分析係以全域矩陣法推導液晶盒的反射係數與聲場能量分布，解釋聲導波操控不同尺寸液晶盒內向列型液晶分子排列的能力。沒水式超音波以不同入射角照射液晶盒，在特定頻率下於試片中產生特定模態的聲導波，實驗結果顯示液晶盒受到多重模態聲導波的影響，液晶分子會發生重排。數值模擬顯示液晶盒中央的液晶層聲場能量主要源於對稱聲導波，對稱聲導波扮演重排液晶分子的重要推手角色。低頻聲導波對於較薄液晶盒之液晶分子重排的影響比試片較厚者大。在目前的沒水式超音波實驗架構下，不易觀察出高頻聲導波改變液晶分子排列的效果。

Nematic liquid crystals (NLC) are optically uniaxial, birefringent materials. Besides electric and magnetic fields, the orientations could be altered from the equilibrium states by acoustic fields. This thesis investigates the acousto-optic effect of liquid crystal cells (LLC) which are immersed in water tank and insonified by obliquely incident acoustic beam. The liquid crystal material sandwiched between two glass plates is regarded as a Newtonian fluid and modeled to be an isotropic hypothetical solid with a single viscosity. Numerical analysis based on the global matrix formulation is performed to determine the reflection coefficients and acoustic energy distribution in the LLC. Experimental results show that the realignment of NLC molecules is due to multi-modal guided acoustic waves excited in LLC by changing the angle of incidence and driving frequency. Numerical simulations indicate that most acoustic energy in the middle liquid layer is contributed by the symmetric guided modes. The symmetric modes play significant role on the acoustic realignment of NLC. In addition, low frequency acoustic guided waves have stronger influence on reorientation of the NLC molecules in a thin cell than that in a thick one. Unfortunately, it is not distinguishable the effect of high frequency guided waves on acoustic realignment of the NLC molecules under the present experimental setup of immersion ultrasound.