二元混合溶液中熱擴散致被迫瑞利散射的機制研究

Study of Thermal Diffusion Forced Rayleigh Scattering in Binary Mixtures of Solution

林承漢(Lin, Chenghan)

指導教授 ： 魏台輝

國立中正大學/理學院/物理學系/碩士(2012年)

摘要

本論文使用兩道交叉的532 nm CW雷射光為寫入光源(pump)，另使用一道CW He-Ne雷射光為偵測光源(probe)，對加入甲基紅染料的二元成分混合溶液(二硫化碳-乙醇及甲醇-乙醇)進行熱擴散致被迫瑞利散射(Thermal Diffusion Forced Rayleigh Scattering，TDFRS)實驗。當寫入光源在染料的二元混合溶液中造成熱致光柵(thermally induced grating)後，藉著量測偵測光的布拉格繞射信號強度隨時間變化的情形，我們研究探討二元混合溶液中熱致光柵的生成與擴散機制。
在理論上，本論文首先從從流體力學的Fick’s law 出發，探討質量擴散現象，並且根據Maxwell-Stefan equation推導出二元混合溶液的質量通量與其驅動力間的關係。其後，我們將得自流體力學的結果與熱動力學概念結合，並利用Onsager-Casimir relation得到濃度梯度與溫度梯度的耦合關係，並導出(熱致)折射率隨時間變化的公式。
在實驗上，我們針對混合溶劑比例不同、寫入光柵週期不同、寫入光偏振不同的實驗條件量測溶劑繞射效率隨時間變化的情形，然後根據實驗結果進行定性的討論。

關鍵字

被迫瑞利散射 ； 熱擴散 ； 質量擴散 ； 索雷特效應

並列摘要

In this thesis, we use the thermal diffusion forced Rayleigh scattering (TDFRS) technique to investigate thermal diffusion properties of two binary mixtures of solutions: carbon disulfide-ethanol and methanol-ethanol. A small fraction of methyl red dye is dissolved in both solutions as the light absorber of 532 nm light. We use two CW Nd:YVO4 laser beams at 532 nm as the pumps, also referred to as writing beams, and a CW He-Ne laser beam at 632.8 nm as the probe, namely the reading beam. The pump beams intersect in the solution and create a thermal refraction grating in both solutions which diffract the probe beam. By measuring the power of the diffracted 632.8 nm light as a function of time, we investigate thermal diffusion properties of the binary solution mixtures evolve with time.
In the theoretical aspect, we start from Fick's law of fluid mechanics and derive the relation between mass flux of the binary solution mixtures and the driving force of mass flow according to the Maxwell-Stefan equation. This helps to deal with mass diffusion. Afterwards, we combine the abovementioned mass diffusion and thermal diffusion, a concept of thermal dynamics, as the overall driving force of mass flow. Here the relation involving both mechanisms is named the Onsager-Casimir relation. From this relation, we derive the variation of (thermally induced) refractive index with time.
In the experimental aspect, we measure the diffracted signal of the binary solution mixtures under various experimental conditions: different ratio of the individual solution concentrations, different grating periods and different polarization combinations of the writing beams.

並列關鍵字

TDFRS ； thermal diffusion ； mass diffusion ； Soret effect

參考文獻

pulses, 國立中正大學物理研究所, 2008.

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