In this work, the evaporation of a water-ethanol mixture (80%-20%) in a 2D Hele-Shaw cell is examined with digital holographic interferometry. This shows that, as ethanol is preferentially evaporated, a boundary layer rich in water is formed near the interface. Now, as this layer is heavier than the underlying mixture, dense water plumes tumble down, like in the classical Rayleigh-Taylor instability. This creates extra mixing and replenishes the ethanol in the upper layer, which in turn enhances the evaporation rate.