Experimental investigation of gravitational instabilities at the particle suspension-fluid interface

Gravitational instabilities occurring at the interface between a suspension of granular particles and a clear fluid are studied experimentally using a sealed Hele–Shaw cell. Special attention is paid to the effects of particle Reynolds number (Re) and the initial particle packing on the growth of the interfacial perturbation. Glass beads are immersed in the glycerin–water mixture and are placed at the bottom of the cell initially. The mass content of the glycerin in fluid mixtures is varied to obtain the desired fluid viscosities and thus the Re value. The cell is then inverted to place the suspension above the fluid to trigger the gravitational instabilities. When the particle packing is dense, the distinct interface separating the fluid transforms progressively into an asymmetrical cusp-shaped structure. In contrast, when the packing is loose, the interface between the fluid and suspension takes on a symmetrical sinusoidal form initially and evolves into a mushroom-like pattern reminiscent of the canonical Rayleigh–Taylor instability. The growth rate of this symmetrical perturbation observed for loose packing surpasses that of its asymmetrical counterpart at the equivalent Re. Both types of perturbations exhibit larger growth rates with increasing Re. Linear stability analysis adapted to the loose packing configuration predicts a growth rate that is comparable to the observed rate during the initial growth of the symmetrical perturbation and suggests a similar dependence on Re.