Freezing of micrometer-sized liquid droplets of pure water evaporatively cooled in a vacuum
Freezing of micrometer-sized liquid droplets of pure water evaporatively cooled in a vacuum
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DOI：
10.1039/c8cp05955a
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发表时间：
2018-12-07
影响因子：
3.3
通讯作者：
Terasaki, Akira
中科院分区：
文献类型：
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作者：
Ando, Kota;Arakawa, Masashi;Terasaki, Akira
据报道，真空中产生的直径尺寸范围为 49-71 m 的纯水滴的冷冻过程。通过测量冷冻曲线来表征每个尺寸的过程，其中冷冻液滴的分数作为时间的函数进行评估。研究发现，49 m 的液滴在室温下生成后，在 7.0 至 7.9 ms 的时间内冻结，其中冻结液滴的比例从 5% 增加到 95%；因此，冻结时间统计分布在 1 毫秒内。随着尺寸从49增加到71 m，冻结时间延迟了约3 ms，而冻结曲线的上升时间几乎没有变化。冷却曲线（即液滴的温度随时间变化）的数值模拟表明，当前尺寸范围内的液滴在 233 至 236 K 之间几乎相同的温度下冻结。基于模拟冷却曲线的数值模拟结合先前报道的纯水基于体积的均匀冰成核率的温度依赖性，很好地再现了实验中测量的冻结曲线。研究还发现，液滴在冷冻时会分解成一些碎片，这表明在液滴的外部区域形成了冷冻壳。
Freezing processes are reported for pure-water droplets generated in a vacuum in the size range of 49-71 m in diameter. The process is characterized for each size by measurement of a freezing curve, where the fraction of frozen droplets is evaluated as a function of time. The 49 m droplet was found to freeze at a time between 7.0 and 7.9 ms after being generated at room temperature, where the fraction of frozen droplets increased from 5% to 95%; the freezing time was thus distributed statistically within 1 ms. The freezing time was retarded by about 3 ms as the size increases from 49 to 71 m, while the rise time of the freezing curve was almost unchanged. Numerical simulation of a cooling curve, i.e., the temperature of a droplet as a function of time, revealed that the droplets in the present size range are frozen at almost the same temperature between 233 and 236 K. The freezing curves measured in the experiment were well reproduced by numerical simulation based on the simulated cooling curves combined with the temperature dependence of the volume-based homogeneous ice nucleation rates of pure water reported previously. It was also found that a droplet is disintegrated into a few fragments upon freezing, which suggests formation of a frozen shell in the outer region of a droplet.