Rheology Flashcards
What is rheology
Rheology: rheo (to flow) + logos (science)
– The study of the changes in form, shape and flow of matter
What is viscosity
the resistance to flow
eg. water is easier to stir than honey - therefore lower viscosity
How is the viscosity of Newtonian systems measured
Capillary methods (Ostwald, Ubbelohde, Cannon-Fenske, Houillon)
Viscosity is measured by one value (ex. water = 1 cP, honey = 10000 cP)
How are Non-Newtonian systems measured
– Plastic flow (Bingham bodies)
– Pseudoplasic flow
- Dilatant flow
What is Newtonian Flow
viscosity is a constant value - does not change regardless of shear rate
What is the formula for viscosity
viscosity = shear stress/shear rate page 53 (for units)
What is a rheogram
A rheogram is a graph of flow properties
Viscosity is the reciprocal of the slope:
as the slope ↑ η ↑
a steeper slope indicates a thicker liquid
What are examples of Newtonian systems
water, alcohol, glycerol, propylene glycol, mineral oil, true solutions
What is a non-newtonian system
viscosity is not a constant value (changing shear rate changes the viscosity)
What are the types of Non-Newtonian systems
- Plastic flow (Bingham flow)
- Pseudoplastic flow
- Dilatant flow
What is plastic flow
materials do not flow unless yield value is exceeded
- once yield value is exceeded, viscosity decreases (shear thinning) and flows readily
- examples: flocculated suspensions, ointments, creams, tomato ketchup
What is pseudoplastic flow
- no yield value (begins flow as soon as shear rate starts)
- viscosity decreases with increased shear rate (shear thinning)
Examples: polymers in solution, dispersions of natural and synthetic gums, tragacanth, alginates, gelatin, methylcellulose
What is Dilatant flow
- viscosity increases with increasing shear rate (shear thickening)
- components clump after mixing
What is thixotropy
Reversible, time-dependent decrease in apparent viscosity ⇒ gel—sol—gel
- gel = higher viscosity state, sol = lower viscosity
- For these materials → viscosity is dependent on the length of time shear stress is applied
- structure does not reform immediately
how is viscosity determined
newtonian fluids - one point determination (viscosity is constant)
non-newtonian fluids - require a complete rheogram (determination at more than one shear rate)
