We report a concentration instability at zero Reynolds number created by hydrodynamically interacting bubbles with surfactant. This instability is driven by Marangoni stresses that force bubbles to migrate in directions perpendicular to gravity. We characterize the lateral motion of a single buoyant bubble when it is subject to a weak, low wavenumber disturbance velocity. We use this result to determine which mean flow wavevectors amplify concentration fluctuations in a dilute suspension. The suspension is linearly unstable at small horizontal wavenumbers by a mechanism similar to the concentration instabilities demonstrated in suspensions of sedimenting nonspherical or deformable particles.
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2010
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