just some experiment with Houdini particles. It took about 8 hours to render with 4 GPUs (Redshift)
Skull (Houdini particles)
wip_00
wip_01
wip_02
Particles do not interact with each-other like sand, they just move through the particles that cover the surface. To make it look better, I made particles to move from the surface right after they were generated.