| Summary: | When gas hydrates dissociate into gas and liquid water, many gas bubbles form in the water. The large bubbles disappear after several minutes due to their buoyancy, while a large number of small bubbles (particularly sub-micron-order bubbles known as ultra-fine bubbles (UFBs)) remain in the water for a long time. In our previous studies, we demonstrated that the existence of UFBs is a major factor promoting gas hydrate formation. We then extended our research on this issue to carbon dioxide (CO<sub>2</sub>) as it forms structure-I hydrates, similar to methane and ethane hydrates explored in previous studies; however, CO<sub>2</sub> saturated solutions present severe conditions for the survival of UFBs. The distribution measurements of CO<sub>2</sub> UFBs revealed that their average size was larger and number density was smaller than those of other hydrocarbon UFBs. Despite these conditions, the CO<sub>2</sub> hydrate formation tests confirmed that CO<sub>2</sub> UFBs played important roles in the expression of the promoting effect. The analysis showed that different UFB preparation processes resulted in different promoting effects. These findings can aid in better understanding the mechanism of the promoting (or memory) effect of gas hydrate formation.
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