Abstract:

Droplets evaporation is a common phenomenon. It's necessary to keep studying on the evaporation mechanism of multi component droplets, which can promote the development of related industry and technologies, such as pesticide powder spraying, inkjet printing, industrial coating and rapid cooling technology. Based on silicon and PTFE films, the evaporation of water-ethanol mixed liquid drops are studied by using 10%, 20% and 30% concentrations of ethanol and pure water. The CCD camera is used to record the change of the appearance of sessile droplets. Then evaporation characteristics of mixed droplets are obtained by analyzing the contact angle and contact radius. It can be seen from the experiment that the evaporation time shortens obviously after adding ethanol. The evaporation of mixed droplets on silicon wafers consists of three evaporation stages as similar as pure water. But the evaporation time of the both first and second stages reduced. And there is a sudden increase in angle at second stage in the experiment of 10% and 20%. On the PTFE film, the pure water droplet has only constant contact radius and mix evaporation stage. When the ethanol content increased to 20%, the first stage turns to be the constant contact angle evaporation mode. As the concentration of ethanol increases, the rate of mix stage is mainly raised. Besides, from the angle of rate and capacity of evaporation, it was found that two liquids may evaporate at the same time. But when the concentration of ethanol is high, the ethanol evaporates faster than pure water. For droplets on the silicon, pure ethanol evaporates in the first stage when the concentration reaches 30%. On the surface of PTFE, the concentration threshold of the ethanol is 20%. Therefore, the concentration of ethanol which can change the order of evaporation varies with the kind of surface. In summary, the addition of ethanol will affect the droplet evaporation mode and the total duration.

Key words: mixing, evaporation, mass transfer, ethanol

摘要：

选用单晶硅片和聚四氟乙烯（PTFE）膜作为基底，采用四种浓度的乙醇水溶液，研究水-乙醇二元混合液滴在固体表面上的蒸发规律。实验结果表明，添加乙醇缩短了液滴的蒸发时间，减弱了液滴的钉扎效应，使得固着液滴更容易发生滑移。并且，在同一蒸发阶段可能同时存在着两种液体的蒸发，但当乙醇浓度较高时，乙醇先于水蒸发。对于硅片表面，当乙醇浓度到达30%时，第一阶段为纯乙醇蒸发；而对于PTFE表面，该乙醇浓度阈值为20%，即固体表面的亲疏水性会影响混合液滴中两组分的蒸发次序。

关键词: 混合, 蒸发, 传质, 乙醇