Interfacial Nanobubbles and the Memory Effect of Natural Gas Hydrates

The memory effect in the nucleation of gas hydrates is one of the longest outstanding mysteries in the field of gas hydrates. We previously investigated the memory effect of natural gas hydrates in the presence of a solid wall using a first-generation high-pressure automated lag time apparatus (HP-ALTA MkI) and confirmed that the memory effect existed beyond statistical variation that is inherent in the nucleation of gas hydrates. The amount of the memory effect also varied depending on the sample cells used, which indicated that the presence of a solid wall was a major factor. In the present study, we used a second-generation HP-ALTA (MkII) to investigate the memory effect of the same natural gas hydrates on the surface of a quasi-free water droplet suspended in squalane. No significant increase was detected in the nucleation rate with the reduction of the superheating temperature in the quasi-free water droplet, which suggests that the presence of a solid wall is required for the memory effect. A new hypothesis based on the presence of interfacial nanobubbles and other interfacial gaseous states on solid walls is proposed to account for all known attributes of the memory effect.