To investigate experimentally how ultra-fine bubbles (UFBs) may promote hydrate formation, we examined the formation of propane (C 3 H 8 ) hydrate from UFB-infused water solution using two preparation methods. In one method, we used C 3 H 8 -hydrate dissociated water, and in the other, C 3 H 8 -UFB-included water prepared with a generator. In both solutions, the initial conditions had a UFB number density of up to 10 9 mL −1 . This number density decreased by only about a half when stored at room temperature for 2 days, indicating that enough amount of UFBs were stably present at least during the formation experiments. Compared to the case without UFBs, the nucleation probabilities within 50 h were ∼1.3 times higher with the UFBs, and the induction times, the time period required for the bulk hydrate formation, were significantly shortened. These results confirmed that UFB-containing water promotes C 3 H 8 -hydrate formation. Combined with the UFB-stability experiments, we conclude that a high number density of UFBs in water contributes to the hydrate promoting effect. Also, consistent with previous research, the present study on C 3 H 8 hydrates showed that the promoting effect would occur even in water that had not experienced any hydrate structures. Applying these findings to the debate over the promoting (or "memory") effect of gas hydrates, we argue that the gas dissolution hypothesis is the more likely explanation for the effect.

Formation and dissociation processes of gas hydrates have been studied for managing the flow of natural gas and for developing new natural gas resources. Recently, such studies have also examined the unique properties of gas hydrates for use in industry, such as gas and energy storages. However, a large degree of supercooling (or supersaturation) and a long induction time are required to form gas hydrates even when their components (water and guest gas) are in hydrate-forming conditions. To overcome these obstacles, various formation-promoting technologies have been studied. In this mini review, we summarize the technologies for promoting the nucleation process in gas hydrate formation. We then focus on recent studies on the use of ultrafine bubbles (UFBs) to exploit the memory effect, which reduces the induction time. We add new data to clarify our experimental results on the formation of single-gas hydrate under an agitated environment and show that UFBs play an important role in promoting gas hydrate formation.