The buoyancy driven motion of a lighter fluid rising spontaneously into a heavier miscible fluid in vertical or tilted tubes give rise to flows as different as diffusive turbulent mixing and counterflow without mixing (Fig. 1).

Two miscible fluids of different densities are initially in an unstable configuration, each of them occupying one half-length of a long transparent tube (d=20mm,L=4m). The lower half is filled with a solution of fluorescein in water. The upper half contains a denser transparent solution of CaCl2 salt in water. The upper half of the tube is illuminated by a vertical thin plane of laser light (532nm) containing the tube axis (Fig. 2).

The mixing pattern involves several competing effects. Buoyancy components parallel to the tube axis drive a longitudinal interpenetration of the two fluids, inducing in...

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