Rheology Flashcards
Rheology
Deformation and flow of matter
Viscosity
Resistance to flow
How are rheological properties measured
Rheometer
A force is applied to a material and the rate of flow is measured
Shear rate equation
Velocity of top layer (m s-1) / height of cube (m)
Shear stress equation
Applied force (N) / area of top layer (m2)
Viscosity ratio
Shear stress over shear rate
Newtonian
Viscosity of a material is independent of the applied shear stress
Non Newtonian
Viscosity of a material changes with the applied shear stress
Why is flocculating agents added
To prevent slow settling to form loosely bound aggregates (flocs) to ensure uniform doses
Dilatant
Viscosity increases with applied stress
Plastic and pseudo plastic
Viscosity reduces with applied stress
Shear thinning
Gradient gets shallower with increasing stress
Plastic of Bingham flow
Minimum shear stress must be applied before a material will flow, below this the material is elastic
Pseudoplastic flow
Material flows as soon as shear stress applied
Gums, paste, creams
Shear thickening
Gradient gets steeper with increasing stress
Shear rates
Low shear rates= particles are separated, liquid acts as a lubricant
At high shear rates= particles clump together, liquid no longer a lubricant and viscosity increases
Time dependent pseudoplastic flow (thixotrophy)
Different shear rates before and after shear stress is applied and removed
Greater after removal
Time dependent dilatant flow (rheopexy)
Different shear rates before and after shear stress is applied and removed
Less after removal
