Magnetic buoyancy force acting on bubbles in nonconducting and diamagnetic fluids under microgravity

Abstract

The magnetic buoyancy force acting on a bubble in a one-dimensional magnetic field can be represented as F=(χG−χL)∫H(dH/dx)dVolB, where χG and χL are the volume magnetic susceptibilities of the gas and liquid, respectively, and H is the magnetic field strength. Since |χL|≫|χG| and most liquids are diamagnetic, this expression indicates that the magnetic buoyancy forces act in the direction of increasing magnetic field strength. Because the magnetic buoyancy force in a diamagnetic fluid is small, the motion of bubbles under normal gravity is difficult to study, but microgravity offers the possibility of detailed observations. Using a compact permanent magnet under microgravity conditions, N2 bubbles in pure water (0.01 dyne s/cm2) and in a 69:31 glycerol/water mixture (0.21 dyne s/cm2) were found to move in the direction of increasing H, and to be held stationary at the point of maximum H. The motion of the bubbles was also simulated with a theoretical model and was found to agree with measurements made under microgravity conditions. These results indicate that magnetic buoyancy can be used to control bubble motion. Since most fluids are diamagnetic, magnetic buoyancy can be used to control bubbles in many fluidic devices used in space applications.