Fig. 2

Evidence for double-diffusive convection in the model experiments and simulations. a−d Dyed water (0.01 wt% methylene blue hydrate, 30.0 ml) is injected into 9.1 wt% sodium chloride solution (340.9 ml) with two different injection velocities: \(U = 0.17\,{\mathrm{m}}\,{\mathrm{s}}^{ - {\mathrm{1}}}\) (a, b) and \(U = 0.37\,{\mathrm{m}}\,{\mathrm{s}}^{ - {\mathrm{1}}}\) (c, d). a, c correspond to the dynamics during the injection and (b, d) to the mixture 30 min later. The distinguished boundaries of the layers are marked and enumerated. e The density profiles of the mixtures presented in a−d. The dashed lines refer to the density profiles 1 min after the injection, and the solid lines refer to those observed 30 min later. The black and the blue lines refer to the results at injection velocities \(U = 0.17\,{\mathrm{m}}\,{\mathrm{s}}^{ - {\mathrm{1}}}\) and \(U = 0.37\,{\mathrm{m}}\,{\mathrm{s}}^{ - {\mathrm{1}}}\), respectively. Vertical steps (solid blue line) indicate layers in the mixture, and their boundaries are enumerated according to those in d, where \(z = 0\) refers to the top of the mixture. f, g Particle streak-lines (f) and the velocity vectors (g) measured in PIV experiments for \(U = 0.37\,{\mathrm{m}}\,{\mathrm{s}}^{ - {\mathrm{1}}}\), 10 min after the injection. The vertical dashed line refers to the axis of the cylindrical domain, and the horizontal dashed line marks the bottom of the mixture. h, i Numerical results (details in Methods, Supplementary Figs 3 and 4 and Supplementary Discussion) show the density distribution (h) as well as the velocity vectors and the vorticity magnitude \(\omega\) (i), 10 minutes after the injection with \(U = 0.37\,{\mathrm{m}}\,{\mathrm{s}}^{ - {\mathrm{1}}}\). The scales for the length and the velocity magnitude are identical in g and i