Abstract
The buoyancy-driven path instabilities of an air bubble rising in a Hele–Shaw cell filled with pure water and aqueous poly( N -isopropyl acrylamide) (PNIPAM) solutions are examined as a function of the Eotvos number ( Eo ) by measuring changes in the bubble shape, the trajectory, and the peripheral length of a bubble ( L ) as well as the visualization of the wake structures behind the bubble. The instabilities can be categorized into four regimes: (1) an oblate ellipse bubble accompanied with a vibrational trajectory, no changes in L , and a Karman vortex street for the wake structure, (2) a comma-shaped bubble with a vibrational motion of both path and L , together with a vortex street different from a Karman one, and (3) changes in the bubble shape, such as a comma-shaped bubble and a jellyfish one with a vibration of both path and L accompanied with a vortex street different from a Karman one, and (4) changes in the bubble shape, such as a bird flapping its wings bubble and a jellyfish one with a strai...
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