【作者】 马渊；

【导师】 彭晓峰；

【作者基本信息】 清华大学 ， 动力工程及工程热物理， 2008， 博士

【摘要】 论文以生物组织冷冻特性为对象,通过可视化实验、微CT扫描重构实验和显微PIV实验方法,探讨生物组织冷冻特性,总结影响冷冻过程及生物组织变化的因素,分析水分形态和内部结构对于生物组织冷冻特性的影响。进一步结合热科学的理论和研究方法,重点考察核化过程、冰晶界面特性、晶枝间的竞争现象以及界面附近的流动形态等基础现象,进行深入基础理论探析,全面了解和认识生物多孔介质冷冻过程中的传递现象及其机理。生物材料冷冻中冰晶产生生长及冻融过程后生物活性的维持,都与生物组织中水分形态及分布有密切联系,而生物组织内部结构影响了水分分布,并且冷冻过程中本身的变化,也会对冰晶产生及界面附近的传递现象产生影响。水分存在形态与生物组织的内部结构存在相互影响的耦合效应。剥离出水分形态和组织结构两个影响生物组织冷冻特性的因素,利用预置骨架结构进行模拟实验来简单讨论组织结构对溶液冷冻特性的影响,以盐溶液来对水分形态的影响进行模拟研究,着重实验观察探讨冰晶的生长特性。将统计力学涨落理论引入过冷溶液核化理论中,引进核化特征长度定义,该概念一方面代表了系统内产生核化点的平均分布尺度,另一方面也是对系综内核化能力的衡量。借助核化几率的推导、计算,可以对一定体积的封闭溶液系统在降温过程中的核化几率变化有更为清晰的认识,并对细胞内冰晶产生这一影响生物组织冷冻损伤的重要过程进行讨论。结合实验现象总结溶液单向冷冻过程中冰晶生长和界面形态发展特性,分析其中的控制因素和产生机理。定义冰晶界面发展过程的三个阶段,通过对界面区扰动发展、浓度分布的分析说明界面规则形态的形成和维持。总结了界面发展过程中因为内部或者外界扰动导致的晶枝间竞争现象,分析其驱动力。分析探讨界面区域微细流动对冰晶界面附近传递过程的影响,给出晶枝间区域存在的热毛细流动理论表达式,讨论了流动对于界面生长和传递过程的影响,数值计算与荧光显微PIV实验观测结果的比较显示出良好的一致性。更多还原

【Abstract】 A series of experimental observations, including visualization experiments, micro-CT scanning experiments and micro-PIV experiments, was conducted to investigate freezing/thawing characteristics of bio-tissues and materials, and particularly an emphasis was addressed on exploring and understanding the fundamental mechanisms of transport phenomena associated with liquid-solid phase transition. Water morphology and bio-tissue/material inner structure were considered to be the most important factors affecting the fundamental phenomena and transport processes. Incorporating with classical theories and methods in thermal science, characteristics of the nucleation, solidification interface and ice crystal growing competition were discussed comprehensively.Ice crystal growth and the survival of cells in tissues and/or bio-materials were strongly influenced by water morphology and distribution which were originally decided by the tissues structure. Furthermore, the coupling effects of water distribution and tissue structure would also have important influence on freezing/thawing process.For evaluating their own influences on the freezing/thawing and associated transport phenomena, a technique was introduced to separately investigate impact of water morphology and structure. Visualization experiment was conducted to observe directional freezing of solutions with solid frames accounting for the effect of structure. The focus was addressed on observing the characteristics of ice crystal growth and the effects of solution properties.Classical nucleation theory was revisited as long with fluctuation theory for analyzing the freezing nucleation in bio-tissues. A concept, nucleation character length was introduced and defined as the size of average energy space a nuclei needed. This proposed concept represents both the averaged spatial distribution of nuclei generated and the nucleation capability in an ensemble. With computation of nucleation probability, the change of nucleation probability in an enclosed solution system was clearly described, and a comprehensive discussion was conducted on intracellular ice formation which significantly influenced the tissue damage in freezing process.Characteristics of crystal growth and interfacial morphology in a directional freezing process were theoretically discussed together with experiment results. From the experimental observations the ice front development was characterized as three periods during freezing of aqueous sodium chloride solutions. The coupling effect of concentration distribution with temperature profile near the interface was explored to have critical importance in forming and maintaining the regular interfacial morphology. Particularly, the growing competition phenomena between crystals in the interfacial region were high dependent upon the characteristics of both concentration and temperature profiles. Actually any disturbance of these two factors inside or outside the interfacial region would induce non-uniform crystal growth and growing competition. The concentration-dependent liquid subcooling was found to be a main driving force of the competition phenomena.Both natural convection and thermocapillary flow in the interface zone were simulated to explore their importance in this kind of freezing/thawing. The flow can significantly alter the transfer process and influence ice crystal growth. The numerical data was compared with experiment results and quite reasonably consistent with each other.更多还原