This paper reports the visual observations of the formation and growth of clathrate hydrate crystals on the surface of a water droplet exposed to gaseous methane, ethane, or propane. The hydrate crystals formed and grew at the guest-water interface. The nucleation of the hydrate first occurred at a random point on the water droplet and then grew to form a polycrystalline layer covering the surface. We observed the individual crystals that constitute the polycrystalline hydrate layer and classified the morphology of the hydrate crystals depending on the system subcooling ∆T sub , the difference between the system temperature and the guest-hydrate-water three-phase equilibrium temperature corresponding to the system pressure. As a general trend, at ∆T sub g 3.0 K, the shape of hydrate crystals is typically swordlike or triangular, and the shape changes to a polygon at ∆T sub from 2.0 to 3.0 K, and then to larger-sized polygons with one side of the polygon typically 0.5-1.0 mm in length. It may be concluded that the crystal morphology of the hydrate crystals formed at the interface between the liquid water and methane, ethane or propane gas can be classified using ∆T sub as the common criterion. The lateral growth rate of the hydrate crystal were also measured and presented.

This paper reports visual observations of clathrate hydrate crystals growth at the interface of water/seawater and a guest substance to reveal the effect of salts on the crystal growth. Cyclopentane was used as the guest substance to form a structure II hydrate. Cyclopentane hydrate crystals grew along the interface of cyclopentane and seawater to form a polycrystalline layer covering the entire interface. We visually analyzed the individual crystals and classified the morphology of the hydrate crystals according to the subcooling (ΔT sub ) from the cyclopentane hydrate equilibrium temperature of atmospheric pressure. The results showed that the morphology of the individual cyclopentane hydrate crystals in water and seawater are qualitatively similar at similar subcooling. As a general trend, for ΔT sub <2.0 K, the shape of the hydrate crystals is typically polygons, at ΔT sub = 2.0-4.5 K, the shape changes to small polygons or slender polygons, and for ΔT sub > 4.5 K, the crystals have triangular or swordlike shape.

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.