Buoyant mixing of miscible fluids in tilted tubes

Buoyant mixing of two fluids in tubes is studied experimentally as a function of the tilt angle $θ$ from vertical, the density contrast and the common viscosity $μ$⁠. At high contrasts and low $θ$⁠, longitudinal mixing is macroscopically diffusive, with a diffusivity $D$ increasing strongly with $θ$ and $μ$⁠. At lower contrasts and higher $θ$⁠, a counterflow of the two fluids with little transverse mixing sets in. The transition occurs at an angle increasing with density contrast and decreasing with $μ$⁠. These results are discussed in terms of the dependence of transerse mixing on $θ$ and an analogy with the Boycott effect.