Density-driven segregation, extensively studied in a simple rotating drum, is enriched with a wide range of underlying physics. Diverse symmetrical segregation patterns formed by mixing two types of dry mono-sized grains have been revealed due to variations in heavy and light grain densities ${\rho }_{\text{h}}$ and ${\rho }_{\text{l}}$ and rotating speeds $\omega$. We engender experimentally a nearly complete segregation, not occurring in dry conditions of the same ${\rho }_{\text{h}}$, ${\rho }_{\text{l}}$, and $\omega$, in submerged states. Furthermore, based on the experiment-validated simulations, using coupled computational fluid dynamics and the discrete element method, it is found the mixing index can be well predicted over a wide parameter space in the effective density ratio, $\mathcal{D}=({\rho }_{\text{h}}-{\rho }_{\text{f}})/({\rho }_{\text{l}}-{\rho }_{\text{f}})$ with ${\rho }_{\text{f}}$ being the fluid density. Specifically, with increasing $\mathcal{D}$ well-mixed states transit to fully segregated states with a rising number of vortices and more severe asymmetrical patterns. When the global Reynolds number ${\text{Re}}_{\text{g}}$ is enlarged, the vortex area of heavy particles shrinks for lower $\mathcal{D}$, while the area of light particles gradually saturates; meanwhile, for higher $\mathcal{D}$ a new vortex with a continuously expanded area can be encountered in the light particle zone. These results improve our understanding of segregation transitions, especially in submerged granular systems, and shed new light on various science and engineering practices.

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• Geophysical fluid dynamics
• Granular segregation
• Granular fluids