Ray Asit K, Devarakonda Venkat, Gao Zhiqiang
Chemical and Materials Engineering, University of Kentucky, Lexington, KY 40506, USA.
Faraday Discuss. 2008;137:85-98; discussion 99-113. doi: 10.1039/b702122d.
An elastic or a Raman scattering intensity versus size parameter spectrum from a droplet shows a series of resonances. Each resonance contains a unique relation between the size and the refractive index of the scattering droplet, and the resonances from homogeneous droplets behave significantly differently than the resonances from layered droplets. These characteristics can be used to analyze observed resonances, and to determine the size and the refractive index of a homogeneous droplet or the inner and outer radii and the core and shell refractive indices of a layered droplet. We show that many microdroplet processes can be studied by applying resonance-based light scattering techniques to single droplets suspended in an electrodynamic balance and to highly monodisperse droplets in linear arrays. This paper focuses on deciphering internal composition distributions in microdroplets that develop due to fast physical or chemical processes. Experiments were conducted on linear streams of droplets that were generated by a modified vibrating orifice aerosol generator from a solution of non-volatile dibutyl phthalate dissolved in volatile freon. The residence time of the droplets in the gas phase was altered by varying the distance between the droplet generator and a laser beam that illuminates the droplets. The variation of the frequency of the droplet generator causes the droplet size to change in a prescribed manner, and thus, the scattering intensity as a function of the frequency shows a series of resonances due to the variation of the size. We have analyzed the resonance peak frequencies to obtain the size and the composition distribution inside the droplets as functions of time. During evaporation, the resonances of non-uniform composition droplets shift differently from those of uniform composition droplets. Specifically, lower order resonances from non-uniform droplets shift significantly more than higher order resonances. This is the basis for the determination of the size along with the composition distribution from the observed resonances. The experimental data show that various resonances observed in the Raman scattering spectra shift differently with time, as predicted by the theory. The size and composition distribution results obtained from the analysis of resonances show behavior that is expected from a droplet evaporation model.
液滴的弹性或拉曼散射强度与尺寸参数谱显示出一系列共振。每个共振都包含散射液滴的尺寸与折射率之间的独特关系，并且均匀液滴的共振与分层液滴的共振表现出显著不同。这些特性可用于分析观察到的共振，并确定均匀液滴的尺寸和折射率，或分层液滴的内半径和外半径以及核和壳的折射率。我们表明，通过将基于共振的光散射技术应用于悬浮在电动天平中的单个液滴以及线性阵列中的高度单分散液滴，可以研究许多微滴过程。本文重点在于解读因快速物理或化学过程而形成的微滴内部成分分布。实验是在由改进的振动孔口气溶胶发生器从溶解在挥发性氟利昂中的非挥发性邻苯二甲酸二丁酯溶液产生的液滴线性流上进行的。通过改变液滴发生器与照射液滴的激光束之间的距离，改变了液滴在气相中的停留时间。液滴发生器频率的变化使液滴尺寸以规定方式改变，因此，散射强度作为频率的函数由于尺寸变化而显示出一系列共振。我们分析了共振峰频率，以获得液滴内部尺寸和成分分布随时间的函数关系。在蒸发过程中，成分不均匀液滴的共振与成分均匀液滴的共振移动方式不同。具体而言，不均匀液滴的低阶共振移动比高阶共振显著得多。这是根据观察到的共振确定尺寸以及成分分布的基础。实验数据表明，拉曼散射光谱中观察到的各种共振随时间的移动方式不同，这与理论预测一致。从共振分析获得的尺寸和成分分布结果显示出与液滴蒸发模型预期相符的行为。
