That's how I see the rheology of granular materials - they're just like fluids, only really different. We saw a few weeks back that under enough stress, theoretically all granular media can act like Bingham fluids with a yield stress and terminal velocity for falling objects, which is just like a fluid. Similarly, ATA Scientific has a list of simple results from the rheology of particles which is worth a look. Consider the first two examples from their list
"1. Decrease particle size and viscosity will increase.Nice simple explanations which I always like. Again, look at the rest of the list (but maybe after you finish my post).
In a constant volume fraction, the number of particles will increase when particle size decreases. As a result of this, the number of interactions between particles increases as well, leading to an overall increase in viscosity (the resistance to force that causes flow). The effect is more common at low shear rates, as a particle-particle interaction is a weak force.
2. Increase particle size and viscosity will decrease
Keeping the previous point in mind, it stands to reason that if you were to increase the particle size, this would lead to a lower amount of particle-particle interactions, resulting in a decrease in viscosity. As before, this effect is most common at low shear rates."
The one trend I noticed in the list, and that you even see in the example I gave, is that particle rheology examples always seem to have far more restrictions and assumptions than fluid mechanic examples do. "...under enough stress...", "...in a constant volume fraction..." etc. If you ignore those prescripts, then you start finding strong divergences from fluid mechanics.
And boy are there plenty of examples of divergences, whether it's the Brazil nut effect, bridging, rat holing and a whole host of other examples, particle rheology is just like fluid mechanics, only really different.