Crystal growth of clathrate hydrates formed at the interface of liquid water and gaseous methane, ethane, or propane: Variations in crystal morphology

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 ΔT_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 ΔT_sub ≥ 3.0 K, the shape of hydrate crystals is typically swordlike or triangular, and the shape changes to a polygon at ΔT_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 ΔT_sub as the common criterion. The lateral growth rate of the hydrate crystal were also measured and presented.