Freezing/Thawing Behavior Of Bio-materials And Fundamentals Of Associated Transport Phenomena

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.