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