Wednesday, July 13, 2011

Just the same, only really different

One of the baseball players in town here, Justin Morneau is from Canada, a country normally associated with producing hockey players, not baseball players. He has radio commercial in which he makes fun of that stereotype and himself by saying he thinks baseball is just like hockey, only really different.

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.
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."
Nice simple explanations which I always like. Again, look at the rest of the list (but maybe after you finish my post).

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...", " 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.

1 comment:

Eric F. Brown said...

The particle size effect is similar in emulsions. The smaller the droplet size, in general, the higher the viscosity.