Glossary

Viscosity and rheological behavior

Viscosity is a measure of the internal resistance of a liquid or gas that opposes the flow. The viscosity depends on various factors, such as the temperature, the composition or the shear or flow rate in a process and can vary greatly, from very low (as in water) to very high (as in toothpaste or cake batter). 

The change in viscosity as a function of the shear stress of a substance is called rheology. The viscosity can increase (shear-thickening or dilatant) or decrease (shear-thinning or structurally viscous) with increasing flow velocity (shear rate).  A fluid with constant viscosity with varying shear stress or flow velocity is called a Newtonian or ideal viscous fluid. In addition, the viscosity can change depending on the temporal shear stress. In this case, we speak of thixotropic (time-dependent shear-thinning) or rheopectic (time-dependent shear-thickening) behavior. A typical example of a thixotropic product is ketchup. If ketchup is not stirred for a long time, the fluidity is very low, i.e. the viscosity is very high. On the other hand, if you shake the bottle and move (shear) the ketchup, the viscosity decreases. Only after prolonged storage without movement does the viscosity increase again.

The rheological behavior of dispersions has a decisive influence on various processes:

• Mixing processes
• Wet grinding processes
• Classifying