中文摘要：

本文通过单光束梯度力光学镊子一拉曼光谱系统，对硫酸镁单液滴随着相对湿度变化的反应进行了探究。当硫酸镁单液滴被光镊捕获之后，通过相对湿度的梯度变化探究了捕获液滴的蒸发动力学变化。发生在与耳语回音模相称波长的受激拉曼散射可以用来准确地确定液滴半径，因此，可以通过腔增强拉曼散射得到在不同湿度下处于平衡的液滴半径信息。本研究通过光镊对硫酸镁单液滴的实时监测，阐述了在某个相对湿度范围内该液滴的粒径变化的过程和结果，在此之前的硫酸镁吸湿性研究中没有先例。研究结果表明在相对湿度逐渐降低至40％的过程中硫酸镁液滴半径的变化速率逐渐变小。而当相对湿度低于40％时，液滴半径的减小会被严重抑制。这种现象表示在高浓度条件下硫酸镁液滴中水的蒸发扩散速率会降低。另一方面，在蒸发过程后的相对湿度上升过程中，硫酸镁液滴尺寸的增加明显缓慢于湿度增长速度。这一现象显示液滴的尺寸变化是不可逆的。说明胶态的形成导致了传质受阻，从而阻碍了液滴中水分子的交换。

英文摘要：

This paper is aimed to study the response of a single MgSO4 droplet to the varia- tion of relative humidity through the combination of single beam gradient force optical tweezers and Raman spectroscopy. The evaporation dynamic of a single MgSO4 droplet which is trapped by a single beam gradient force optical tweezers is investigated by varying the relative humidity in a stepwise manner. The measurements of the variation in equilibri- um wet droplet size with relative humidity （RH） are made using cavity enhanced Raman scattering（CERS）, where the stimulated Raman signals appear at wavelength commensurate to the Whispering Gallery Modes （WGMs） are applied to determine the size of droplet with high precision. This study demonstrates the real time monitoring of MgSO4 droplet size over a RH range that had not previously been explored with aerosol optical tweezers. The results show that the MgSO2 droplet radius variation gradually becomes smaller as the RH=44% and is greatly depressed when RH lower than 40%, indicating that water evaporation rate of captured MgSO4 droplet is limited at high concentration. On the other hand, the deliquescence of shrinked MgSO4 droplet is not equivalent with the dehumidification process. The re-growth of MgSO4 droplet size is observed to be significantly slower in response to the increase of RH. It is suggested that mass transfer effect caused by the gel formation has contributed to the retardation of water mass transfer.