低温保护剂微液滴在液氮表面的Leidenfrost效应及分析

Analysis on the Leidenfrost effect of cryoprotectant microdroplets on the liquid nitrogen surface

借助红外摄像和图像分析技术,并结合非等温结晶动力学模型理论分析,系统研究了微量低温保护剂溶液在液氮表面的Leidenfrost效应及其影响因素.研究结果表明:微水滴的相变温度最高(约273K),最大结晶度变化率达到3×10^(-2),最大运动速度值为100~130mm/s,但其Leidenfrost时间最短;随着低温保护剂浓度的增大,其相变温度降低(20%和50%甘油分别约为250和225K),结晶变化率显著减小(20%和50%甘油分别约为1.4×10^(-3)和7×10^(-7)),其最大运动速度值也会降低(20%和50%甘油分别为90~120和85~105mm/s),且Leidenfrost时间会随着浓度增大而增长;Vs55的相变温度最低(约158K),结晶度变化率最小(约2.8×10^(-16)),最大运动速度值降低到75~95mm/s,且其Leidenfrost时间最长(约是水的2~3倍);微液滴体积越大,其Leidengfrost时间越长,但对低浓度溶液(水和20%甘油)最大运动速度的影响不大,而对于高浓度溶液(50%甘油和Vs55)则是呈现出"体积越大,速度越快"的变化趋势.这些结论对于微液滴玻璃化保存生物样本方案的优化设计具有重要的指导意义.

The Leidenfrost effects of cryoprotectant （CPA） micro droplets on the liquid nitrogen surface are investigated with combination of the infrared photography technology and the theoretical model of non-isothermal crystallization kinetics. The results show that the phase change temperature of water droplet is the highest one （about 273 K）, and both the degree of ice crystallization （about 3 × 10^-2） and the maximum speed of droplets （100-130 mm/s） are also the largest ones, respectively. But the Leidenfrost time of water droplet is the shortest one. As the concentration of CPA rising, the phase change temperature obviously shifts to the lower temperature, i.e. 250 K for 20% glycerol solution and 225 K for 50% glycerol solution, and the degree of ice crystallization significantly deceases, i.e. 1.4x 10-2 for 20% glycerol solution and 7 × 10^-7 for 50% glycerol solution, which makes the speed of CPA droplets slow down 90-120 mm/s for 20% glycerol solution and 85-105 mm/s for 50% glycerol solution, but prolong the Leidenfrost time with increasing concentration of CPA. In the case of Vs55, both the phase change temperature （about 158 K） and the degree of ice crystallization （about 2.8×10^-16） are the lowest ones, respectively. The maximum speed of Vs55 droplets is about 75-95 mm/s, and the Leidenfrost time of it is the longest one （about 2-3 times more than water case）. Generally, larger droplets can make the Leidengfrost time longer, and there is a trend that larger droplets let them run faster for high concentration cases, i.e. 50% glycerol solution and Vs55. However, there is no such tendency for both pure water and 20% glycerol solution. These results can be expected to guide the optimal design for vitrification of biomaterials sealed with micro droplets in the future work.