Measurements of clathrate-hydrate film thickness using laser interferometry

Using a laser interferometer having a resolution of approximately 1 μm, we directly measured the thicknesses of clathrate-hydrate films at the planar interface between a liquid hydrofluorocarbon, R-134a (CH₂FCF₃), and liquid water. The interferometer was mounted on a specially designed, pressure- and temperature-controlled cell filled with both liquids. After a hydrate film was formed with either pure water having no prior contact with R-134a or water presaturated with R-134a, we made a series of intermittent measurements until the film was aged up to 150 h. For hydrate films formed with pure water held at a large subcooling (approximately 7 K) below the triple (liquid-R-134a/hydrate/liquid-water) equilibrium temperature, the initial thickness was approximately 10 μm. This thickness was maintained for a few tens of hours; then it gradually increased, reaching approximately 30 μm 100 h after the film formed. With pure water at a smaller subcooling (≲2 K), the thickness of hydrate films was initially as large as approximately 80 μm; it rapidly decreased to 15-20 μm and then increased to 30-40 μm 150 h after the film formed. With presaturated water at small subcoolings (≲2 K), each hydrate film was a mosaic of polygonal hydrate crystal plates. Each plate was several millimeters wide and 20-170 μm thick. No measurement could be made with presaturated water at a large subcooling (≳6 K) because of sword-like hydrate crystals that have grown in crowds into the water phase from the hydrate film surface.