The evolution of a plume formed by ascending micro-bubbles is investigated by using quantitative visualization to understand the elementary processes that contribute to the inverse energy cascade as small scale flow grows to a large convective flow process. To make the visualization manageable, the plume is confined between two close parallel plates so that the plume-driven convection is restricted to two-dimensions. The results indicate that the bubble plume self-organizes, with a negative

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... usion coefficient of buoyancy distribution. A mechanism for the enlargement of the plume is given in terms of two-dimensional flow interaction with the liquid phase. It is confirmed that the kinetic energy is concentrated in the low wavenumber flow from the energy spectra of the liquid phase.