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When making cacao (the non-instant type), one usually mixes the powder with a small amount of milk or water before mixing this slurry into the rest of the milk/water. Some manufacturers of slurry feeding systems for biogas plants claim the same principle for their product: One mixes the solid feedstock into a small amount of slurry from the fermenter (in the hopper of a pump like this), the mixture is then pumped into the reactor vessel and mixed into the slurry there with conventional agitators. The manufactureeres of these feeding systems claim, and we have observed this (but did not do a real experiment), that less energy is needed for agitators if a liquid feeding system is in use (compared to dumping the solid feedstocks into the vessel via an auger or similiar).

Now, the only reason this should be the case is that with the smaller volume it is easier to exert shear forces on the conglomerates of solids and to thus disperse them. But this assumes that the solids cling to each other - evident for cocoa powder, less so for the substrates our plants handle.

On the other hand, I would assume that thicker liquids are always harder to mix, as there is less shear (if all else is equal) and particles or conglomerates are somewhat protected by the liquid.

So I assume that one major factor is that the real factor here is the size of the mixing implement relative to the volume (think spoon in cup vs. spoon in pot), so the forces acting on the particle conglomerates don't depend on flow.

So my question is ...

  • is this even the general case (it's easier to mix solids into a thicker slurry)
  • if so why, is the reasoning given above correct?
  • Can one translate my reasoning above from prose into proper physics?
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1 Answer 1

If you're trying to mix anything with starch grains (cornflour is the classic example) a problem you'll run into is that concentrated suspensions are dilatant. If you add the powder slowly this may not be a problem, but if you throw it in as a lump you get a ball of dry powder with high viscosity dilatant suspension insulating it from the rest of the mix. This ball can be very difficult to break up even with high shear because it responds to the shear by becoming more viscous.

Mixing the powder into a concentrated slurry still requires care, but at least the high viscosity of the concentrated slurry makes it easier to break up any chunks of dilatant goo. Once you've got a slurry of less that say 30% solids by volume, this can be mixed into a low viscosity mixture without any special precautions.

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