Surface free energy analysis for stable supercooling of sodium thiosulfate pentahydrate with microcosmic-visualized methods

A novel method of microcosmic-visualizing was employed in investigating the supercooling stability of a phase change material (PCM). The supercooling behavior of the PCM under circumstances of different surface free energy was studied both dynamically and statically, and a theory analysis of how the surface free energy of the substrate affects the stable supercooling of PCM was carried out. The results showed that, the PCM droplets phase changed after a 20-cm free fall on substrate with larger surface free energy, while remained supercooled on substrate with lower surface free energy. The PCM in PTFE container initiated crystallization at −8.99 °C, withstood over 400% on hours at liquid state compared with the sample in glass container, while a large supercooling degree of over 57 °C was achieved. It indicated that containers made of materials with lower surface free energy could be considerably beneficial in stabilizing supercooling of PCMs, which could give a guide to the practical application on long-term energy storage of PCMs. What is more, the surface free energy theory also works on the air-liquid interface. The oil-covered PCM sample withstood a temperature of 5 °C for 10 months without crystalized in a refrigerator, showing great potential on supercooling stabilization.