Measurements of clathrate-hydrate film thickness using laser interferometry

Using a laser interferometer having a resolution of similar to 1 mu m. 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 (similar to 7 K) below the triple (liquid-R-134a/hydrate/liquid-water) equilibrium temperature, the initial thickness was similar to 10 mu m. This thickness was maintained for a few tens of hours; then it gradually increased, reaching similar to 30 mu m 100 h after the film formed. With pure water at a smaller subcooling (less than or similar to 2 K), the thickness of hydrate films was initially as large as similar to 80 mu m; it rapidly decreased to 15-20 mu m and then increased to 30-40 mu m 150 h after the film formed. With presaturated water at small subcoolings (less than or similar to 6 K), each hydrate him was a mosaic of polygonal hydrate crystal plates. Each plate was several millimeters wide and 20-170 mu m thick. No measurement could be made with presaturated water at a large subcooling (greater than or similar to 6 K) because of sword-like hydrate crystals that have grown in crowds into the water phase from the hydrate film surface. (C) 2000 Elsevier Science B.V. Ail rights reserved.