Subject: Re: Viscosity

From: William Kloek <[email-masked]>
Date: 15 May 1996 06:45:06 GMT

These dispersions will have a yield stress (stress that has to be applied 
before it starts to flow).At stresses lower then the yield stress, 
viscosity will be infinite: it is a  solid This yield stress will prevent 
the big particles from creaming or sedimenting. The creaming- or 
sedimenting velocity can be roughly approximated using a simple Stokes 
equation. Therefore you need a viscosity and as already indicated by 
another responder, there is not one viscosity. The viscosity depends on 
the shear rate (the lower viscosity at a higher shear rate). If you want 
to know the viscosity to calculate creaming behaviour, you have to 
measure the viscosity of the product without the big pieces at the 
relevant shear rate (rate= 3 * creaming velocity/ diameter particle).
As the product will form a network again when disturbed the viscosity 
will increase in time. Therefore the creaming becomes less and less 
important.

Technical: If you want to measure the viscosity of a dispersion, for 
instance using a kind of Couette device, the biggest particle should be 
10 times smaller then the split width of the measuring geometry, other 
wise the big particles are "seen", resulting in non-reproducable noizy 
measurements. Therefore remove them!