低温保护剂微液滴在液氮表面冷冻过程中的结晶度及其影响因素分析

Analysis on the crystallization degree inside the cryoprotectant microdroplets during freezing process on the liquid nitrogen surface

借助非等温结晶动力学理论模型,较为系统地研究了低温保护剂微液滴在液氮表面冷冻过程中的温度和结晶度分布.研究结果表明,微液滴在经历相变温区时,呈现出表面区域结晶度最高,中间次之,中心区域最低的变化规律;且浓度越高,温度和结晶度分布越不均匀,如50%甘油微滴表面区域与中心区域温差为8K,但前者的结晶度大约是后者的41倍,而对于微水滴,两个区域的温差为1.10K,结晶度则相差6倍;微液滴体积对低浓度溶液的最终结晶度没有影响,只是延长了其冻结相变持续时间,如2和8μL20%甘油的相变时间分别约为14.48和19.82s;而对于高浓度溶液,没有明显的冻结相变温区,但呈现出体积越大,结晶度越高的变化规律,如2和8μL50%甘油的结晶度分别约为0.29×10^(-6)和0.80×10^(-6);随着低温保护剂浓度的增大,其相变温度会明显降低(水为273K,20%甘油为250K,50%甘油为225K,Vs55为158K),其最终结晶度也会显著降低,如20%和50%甘油溶液的最终结晶度分别为1和5.61×10^(-7),而Vs55仅有5.70×10^(-16).

Based on the non-isothermal crystallization kinetic model,the temperature distribution and the degree of crystallization inside cryoprotectant（CPA） microdroplets are theoretically investigated when they are frozen on the liquid nitrogen surface.The results show that the ice crystallization degree in surface region is the largest one during the phase transition temperature zone,while the middle region is intermediate,and the center region is the lowest.Also,the higher the concentration of CPA,the more uneven both the temperature and the crystallization inside the droplets.The maximum temperature difference between the surface and the center region of 50%glycerol with a volume of 4 μL is about 8 K,but the crystallization degree of the former is about 41 times larger than that of the latter.For water case,the temperature difference is only 1.1 K,and the degree of ice crystallization on the surface is roughly 6 times higher than that in the center.The volume of microdroplets seems to have few effects on the final crystallization degree for lower concentration solutions,but prolongs the freezing phase transition duration（i.e.14.48 s for 2 μL and 19.82 s for 8（xL for 20%glycerol case）.For the higher concentration solutions,the freezing phase change temperature zone is not obvious,but the final crystallization degree will increase significantly as the volume of microdroplets rising（i.e.0.29×10^（-6） for 2 μL,0.80×10^（-6） for 8（xL for 50%glycerol case）.As CPA agent concentration increases,the phase transition temperature obviously decreases（i.e.273.15 K for water,250 K for 20%glycerol,225 K for 50%glycerol,158 K for Vs55） and the final crystallization degree also has the same trends（i.e.1 for both water and 20%glycerol,5.61×10^（-7） for 50%glycerol,and 5.7×10^（-16） for Vs55）.