Double-diffusive fingering instability of a surfactant-glycerine-water drop in water

We present an experimental setup that allows easy visualization of single fingers formed by double diffusive convection. This setup consists of a drop ( containing water and two solutes) that is injected at the bottom of a dish filled with water. The gravity-driven expansion of the drop induces a toroidal bulk ow of the surrounding water. This ow straightens and confines the fingers horizontally and quasi two-dimensionally. An extremely low diffusion coefficient of one of the two solutes (a surfactant in our case) causes unusually stable configurations. This stability is robust within a wide range of concentrations. Wavelength measurements agree well with theoretical estimations.