The motion of a single MH-bubble in quiescent fluid was examined experimentally. If a methane-gas bubble is placed under the condition that hydrates nucleation can initiate, typically at high pressure and low temperature, a thin methane hydrate film is formed on the bubble surface and it alters the motion of the bubble in the fluid drastically. It behaves as if solid did.To clarify the fluid dynamics of MH-bubbles, we devised the special apparatus which enabled us to observe the three dimensional behaviours of the MH-bubbles under high pressure condition. The three dimensional motions of the MH-bubbles were captured by two high-speed cameras whose resolution was 1024*1024 pixel at 500Hz. To analyze the motions of the MH-bubbles, Direct Linear Transformation Method was adopted.Equivalent diameter of the MH-bubble was altered from 3.8 to 7.8 mm, which corresponded to the range from 555 to 1155 in Reynolds number, by replacing the nozzles with different diameters.The MH- bubbles in this range exhibited the zigzag motion, while methane bubbles the spiral motion. Main findings in this research are as follows:(1) The aspect ratio, the height to width ratio, decreased linearly with the increment of the equivalent diameter of a MH-bubble. This reflects the fact that the shape of the MH-bubble shifts from spherical to ellipsoidal. (2) Strouhal number which characterizes the zigzag motion of the MH-bubbles increased with Reynolds number. (3) The drag coefficients were measured up to the Reynolds number of about 1000. The drag coefficient of a MHbubble departs from the standard drag curve when the shape deformation become notable. After that, it rise in accordance with the progress of the deformation. These phenomena also have been observed in the behaviour of a bubble with a surfactant.
