This paper reports the visual observations of the formation and growth of structure-II hydrate crystals on a water droplet partially immersed in liquid cyclopentane and exposed to difluoromethane gas. Each of the experiments was performed under prescribed temperature and pressure conditions in the range from 281.7 to 297.0 K and from 0.12 to 1.10 MPa in order to investigate the effect of the driving force for the hydrate crystal growth. The experiments were conducted at 25 different temperature-pressure conditions. It was found that the behavior of the hydrate crystal growth in this three-component system can be classified into three modes, which we called "cover", "expansion" and "line", depending on the temperature and pressure. The descriptions of the three types are summarized as follows. "COVER": Hydrate crystals first formed on the water-droplet surface and then grew to form a polycrystalline layer covering the surface. After complete surface coverage, no more hydrate growth and little change in the shape of the hydrate-covered water droplet were observed. "EXPANSION": Like "cover", the first crystals were observed on the water-droplet surface. They grew not only along the surface, but also toward the gas phase, and then continued to grow for more than several tens of minutes after complete coverage. "LINE": Unlike the other two modes, hydrate crystals first formed at the three-phase interfacial line and grew along this line. The shape of the hydrate crystals eventually became like a doughnut, since the center of the water droplet collapsed when they grew.

Variation in crystal growth behavior of clathrate hydrate depending on the temperature and pressure was visually demonstrated in the krypton-gas/liquid-cyclopentane/ liquid-water system, which is expected to be a working medium of highly efficient hydrate-based heat pumps. The experiments were conducted at 28 different temperature− pressure conditions in the range from 279.0 to 299.7 K and from 0.59 to 6.44 MPa. It was found that the behavior of the hydrate crystal growth in this three-component system can be classified into three modes, which we called "cover", "expansion", and "line", depending on the temperature and pressure. The descriptions of the three types are summarized as follows. Cover: the first hydrate crystals were observed at the water droplet surface and then grew to form a polycrystalline layer covering the surface. After the coverage, no more remarkable hydrate crystal growth was observed. Expansion: the first crystals were observed on the water droplet surface and grew not only along the surface but also toward the gas phase. They continued to grow as if the water droplet expanded even after the complete coverage. Line: the first hydrate crystals were observed at the three-phase interfacial line and grew along the line. Finally, they grew to spread radially from the line.