Unstable invasion of sedimenting granular suspensions

TL;DR

This paper studies how granular suspensions in a fluid develop instabilities like fingering and mobility inversion during radial injection, influenced by density differences, granular concentration, and flow rates.

Contribution

It reveals the mechanisms behind instability development in granular suspensions, highlighting the roles of settling time, volume fraction gradients, and Korteweg surface tension.

Findings

Fingering patterns depend on density mismatch and granular concentration.

Instability onset is governed by the granular settling time scale.

Surface tension effects influence the number of fingers formed.

Abstract

We investigate the development of mobility inversion and fingering when a granular suspension is injected radially between horizontal parallel plates of a cell filled with a miscible fluid. While the suspension spreads uniformly when the suspension and the displaced fluid densities are exactly matched, even a small density difference is found to result in a dense granular front which develops fingers with angular spacing that increase with granular volume fraction and decrease with injection rate. We show that the time scale over which the instability develops is given by the volume fraction dependent settling time scale of the grains in the cell. We then show that the mobility inversion and the non-equilibrium Korteweg surface tension due to granular volume fraction gradients determine the number of fingers at the onset of the instability in these miscible suspensions.