Granular dynamics of density profiles in a suspension interface.

The temporal evolution of the density profiles at the interface of a Rayleigh-Taylor instability in a sedimenting suspension is experimentally investigated. It is found that the sand-glycerin density gradients within the interface change with time, and that the evolution of the gradients differs significantly, depending on the location: The density profiles become steeper in the regions where the suspension flows downwards, whereas the profiles become flatter in the regions where the fluid flows upwards. This observation shows that there is a motion of the sand grains relative to the carrier fluid and hence reveals the prevailing granular dynamics in the suspension interface. It shows a behavior of the suspension which is different from the behavior of a homogeneous Newtonian one-component fluid whose density profiles do not change in time. Another interesting result is that the slopes of the profiles differ already at the very beginning (t=0) of the evolution of the instability, indicating that the suspension might have self-structured prior to instability onset.