Thermodynamics and Kinetics Laboratory, Department of Mechanical and Industrial Engineering, University of Toronto , 5 King's College Road, Toronto, Canada M5S 3G8.
Langmuir. 2013 Oct 29;29(43):13239-50. doi: 10.1021/la403177r. Epub 2013 Oct 16.
During sessile droplet evaporation, studies with IR thermography and shadowgraphs have indicated temperature pulsations. We confirm those observations with microthermocouples, but microthermocouples also indicate temperature pulsations in the atmosphere of the droplet. The pressure in this atmosphere pulsated as well and was correlated with the temperature pulsations in the droplet. Also, we find that if a droplet evaporates into its own vapor, there are no temperature or pressure pulsations. The pulsations occur only if the droplet evaporates into an atmosphere with a component having a heat of solution with the droplet when it adsorbs-absorbs. None of the currently proposed mechanisms for the temperature pulsations provide an explanation for the coupling between the temperature pulsations in the droplet and the vapor-phase pressure pulsations, and for the absence of the pulsations when the system is single-component. As a mechanism for the pulsations, we propose that when a droplet is exposed to an atmosphere containing a component that has a heat of solution with the droplet, energy will be released from adsorption-absorption. This energy will cause pulsations in the evaporation flux, and these pulsations could cause the observed temperature and pressure pulsations. We examine this mechanism by showing that, if the measured temperature pulsations in a water droplet exposed to a methanol atmosphere are used as the input to a theory of evaporation kinetics (statistical rate theory), the pressure pulsations of the water vapor in the methanol atmosphere are predicted and agree with those measured with a quadrupole mass analyzer. When the inputs and outputs are reversed in the theory, we find that the temperature pulsations in the droplet are correctly predicted from the measured water vapor pulsations in the atmosphere.
在静止液滴蒸发过程中，利用红外热成像和阴影图的研究表明存在温度脉动。我们用微热电偶证实了这些观察结果，但微热电偶也表明液滴大气中的温度脉动。该大气中的压力也发生脉动，并与液滴中的温度脉动相关。此外，我们发现如果液滴蒸发到自身蒸气中，则不会出现温度或压力脉动。只有当液滴蒸发到一个具有与液滴吸附-吸收时的溶解热的组分的大气中时，才会出现脉动。目前提出的温度脉动机制都没有解释液滴中的温度脉动与蒸气相压力脉动之间的耦合关系，也没有解释在单一组分系统中不存在脉动的原因。作为脉动的一种机制，我们提出当液滴暴露于含有与液滴具有溶解热的组分的大气中时，能量将从吸附-吸收中释放出来。这部分能量将引起蒸发通量的脉动，而这些脉动可能会导致观察到的温度和压力脉动。我们通过以下方式检验该机制：如果将暴露于甲醇气氛中的水滴的测量温度脉动用作蒸发动力学理论（统计速率理论）的输入，则可以预测甲醇气氛中的水蒸气压力脉动，并与四极质量分析仪测量的结果相吻合。当理论中的输入和输出颠倒时，我们发现可以从大气中测量到的水蒸气脉动正确地预测出液滴中的温度脉动。
