Effect of Micro- and Nanobubbles on the Crystallization of THF Hydrate Based on the Observation by Atomic Force Microscopy

Abstract

The air micro- and nanobubbles on a silicon wafer surface, generated by ethanol–water exchange method in THF solution, are found with anomalous small contact angles on the gas side due to the pinning effect. As the pinning effect is only with the limited region of a bubble and varies with bubble size, the difference in contact angle between the microbubbles and nanobubbles is recognized. With a high-resolution atomic force microscopy, in situ direct observations of THF hydrate nucleation are performed in the presence of air micro- and nanobubbles in solution. On the basis of the observations, the sizes of the hydrate crystallites along the bubble edge are much larger than those in nonbubble regions, which can be explained by the lower nucleation barrier at the contact line region as to the classical nucleation theory. The growth of hydrate crystals at the bubble contact line experiences the competition for THF molecules, probably through Oswald ripening process, resulting in the spaced distribution of THF hydrate crystallites along the bubble edge.