Experimental Investigation of Large Sessile Droplet Evaporation with Pinned Triple Line
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摘要: 利用实践十号返回式科学实验卫星蒸发对流箱，开展了三相线处于钉扎状态且接触半径大于毛细长度的无水乙醇大滴在加热PTFE表面蒸发的地基科学实验。实验发现，液滴体积随时间线性递减，但钉扎大液滴蒸发过程中没有出现恒定接触角（CCA）阶段。与小液滴蒸发的恒定接触半径（CCR）阶段相同，大液滴的平均蒸发速率也与初始体积无关，表明受重力影响明显的大液滴蒸发主要发生在三相线附近区域。瞬时蒸发速率经历了迅速上升和之后的长时间内保持稳定两个阶段。与数值模拟结果对比分析发现，准静态扩散模型低估了三相线处于钉扎状态的大液滴瞬时蒸发速率，而同时考虑蒸气扩散与空气中自然对流经验模型的准确性取决于实验所用工质。Abstract: The in-depth study on the evaporation of large sessile liquid droplets has important scientific significance and engineering application value. Using the evaporation-convection box in SJ-10 scientific experimental satellite, the scientific matching experiments of large sessile ethanol droplets evaporation on heated PTFE are conducted on the ground, in which the triple line of the liquid droplet is pinned and the contact radius is greater than capillary length. In present experiments, it is found that the volume of the large sessile droplet with pinned triple decreases linearly with time, but the CCA mode during evaporation is not observed. The average evaporation rate is also focused on, and it is found to be independent on initial volume of the large sessile pinned droplet with the same contact radius, which is similar in the tendency for small evaporating liquid drop, indicating that evaporation occurs most violently near the triple line. The instant evaporation rate is observed to increase firstly and then keep stable until the end of the lifetime. Comparisons between the present experimental results and the results predicted by the empirical models are also performed. It is deduced that the diffusion-limited evaporation model is appropriate for the small droplet, but underestimates evaporation rate of the large sessile pinned liquid droplet, while the accuracy of an empirical model considering both diffusion and natural convection depends on the experimental working medium.
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图 3 体积
$ V $ 、接触半径$ R $ 、接触角$ \theta $ 随时间的演化过程（$ {V}_{0}=95.5\;\text{µ}\mathrm{L} $ ）Figure 3. Droplet volume, contact radius and contact angle evolution when
$ {V}_{0}=95.5\;\text{µ}\mathrm{L} $ 
图 4 三相线钉扎状态下不同体积（V0）的液滴蒸干时间tf
Figure 4. Dependence of
$ {t}_{\mathrm{f}} $ on$ {V}_{0} $ when triple line is pinned (the straight line represents a linear fit)
图 5 瞬时蒸发速率的实验与理论结果对比
Figure 5. Comparison of experiment and model prediction of instant evaporation rate evolution
表 1 无水乙醇物性参数（
$ {T}_{\mathrm{a}}=20.0 $ ℃，$ {p}_{\mathrm{a}}=0.101325\;\mathrm{M}\mathrm{P}\mathrm{a} $ ）Table 1. Physical properties of ethanol (
$ {T}_{\mathrm{a}}=20.0 $ ℃,$ {p}_{\mathrm{a}}=0.101325\;\mathrm{M}\mathrm{P}\mathrm{a} $ )Liquid Density
$ \rho $/($ \mathrm{k}\mathrm{g}\cdot {\mathrm{m}}^{-3} $)Dynamic viscosity
$ \mu $/($ \mathrm{P}\mathrm{a}\cdot \mathrm{s} $)Diffusion coefficient
$ D $/($ {\mathrm{m}\mathrm{m}}^{2}\cdot {\mathrm{s}}^{-1} $)Capillary length
$ {L}_{\mathrm{c}} $/$ \mathrm{m}\mathrm{m} $Saturated vapor pressure
$ {p}_{\mathrm{s}\mathrm{a}\mathrm{t}} $/$ \mathrm{P}\mathrm{a} $Ethanol 780 1.3$ \times {10}^{-3} $ 13 1.7 5812 
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