Numerical, experimental, and theoretical investigation of bubble aggregation and deformation in magnetic fluids.

Deformation and aggregation of bubbles in magnetic fluid (ferrofluid) can be observed at high resolution by x-ray phase-contrast imaging. Images of gas bubbles in water-based ferrofluid (EMG-607/707) reveal that bubbles with diameters of a few hundreds of microns deform only slightly in applied fields up to 0.2 T, becoming prolate along the field direction. Also, neighboring bubbles readily attract one another along the field direction, forming linear chains of two or more bubbles. Comparison of experimentally measured bubble trajectories with direct numerical simulations and theoretical predictions shows that aggregation of bubbles under an externally applied field is driven by the attractive magnetophoretic force resulting from the induced fields of the bubbles. Direct numerical simulations were performed with a volume-of-fluid code that incorporates a multiple-color function scheme, to suppress numerical bubble merger, as well as Maxwell stresses as an interfacial force.