Coalescence of electrolytically-generated, 50- to 600– μm-diameter gas bubbles was observed using an optical technique that employs a linear photodiode array to detect interface movement with a resolution of 10−6s. When two bubbles coalesce, the surface energy that is released causes interface velocities of 2 to 4 m/s; these are followed by large-amplitude damped oblate–prolate oscillations. Within the viscosity range studied, the oscillation period is insensitive to the viscosity and damping is insensitive to surface tension, in agreement with a scaling analysis based on a damped harmonic oscillator. Surface waves are superimposed on the motion. Finite-element solutions of the free-surface, nonlinear Navier-Stokes equations are consistent with the experiments.
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