Dynamics of a Single Air Bubble Rising in a Thin Gap Filled with Magnetic Fluids

Abstract

The behavior of a single air bubble rising in magnetic fluids without magnetic fields is visualized using a mini-gap between two parallel vertical glass plates. Water-based Fe3O4 magnetic fluids with particle volume concentration of 6.33 % and density 1261.96 kgm 3 are filled in these gaps and a single air bubble is produced through the orifice at the bottom of the gap. The thicknesses of the gaps are 1 and 2 mm, respectively. Diameters of the orifices are 0.64 and 1.02 mm for 1 mm gap, and 0.64, 1.02 and 1.6 mm for 2 mm gap. In addition, four working fluids including pure water, 25% tetramethylammonium hydroxide (TMAH) aque- ous solution and mass concentration 30% and 50% sucrose solution also have been tested. The results show that the shape of the bubble in magnetic fluids initially keeps oblate ellipse like that in the water, but after the middle of the journey, it becomes as an oblate elliptical cap like that in the surfactant aqueous solution. A reasonable explanation is obtained considering the diffusion of the surfactants to the interface of the bubble. Rise velocity of the bubble in magnetic fluids increases with the width of the gap, and the rise motion of the bubble become instable in the 2 mm gap, even breakup of a bubble generated by 0.64 mm orifice occurs.