Rheology of liquids Flashcards
Types of liquids
1) Newtonian liquids (Constant velocity)
- Shear stress = n x Shear rate
- Shear rate directly proportional to shear stress
2) Non-newtonian liquids
- Consists of a solid component
- More than 1 type of deviation
Types:
i) Plastic flow (Bingham flow)
- Behaves as elastic solid at low shear stress
- At shear stress ABOVE yield value, liquid resembles Newtonian system
- -> Liquids exhibit shear thinning
- -> Apparent viscosity dependent on shear rate
[U = (F-f)/ G], where U = Plastic viscosity
(Eg. Flocculated particles in a concentrated suspension)
ii) Pseudoplastic flow (limited applications)
- Flow as soon as shear stress is applied
- -> Viscosity decrease with increasing shear rate
- -> Shear-thinning
- -> Flow curve tends towards linearity with higher shear stress
- -> Apparent viscosity dependent on shear rate
logG = NlogF - logn’
(Eg. polymers in solution)
iii) Dilatant flow
- Flow as soon as shear stress is applied
- -> Viscosity increase with increasing shear stress
- -> Shear-thickening
- -> Pose problem in production ):
F^N = n’G
(Associated with high conc of deflocculated particles)
What is the degree of change dependent on when Non-Newtonian liquid is sheared?
1) Rate of shear
2) Duration of shear
3) Frequency of shear
- Structural change reversible/irreversible
What is the hysteresis loop formed by?
Upcurve & downcurve
–> Indicates extent of structural breakdown
What is thixotrophy flow?
- Shear thinning
- Slow recovery of apparent viscosity upon standing
- -> On shearing, the structure breaks down, particles gradually b/c aligned & flow properly
- -> When shearing stops, particles reform
- -> Recovery time can be shortened by gentle rolling/rocking motion which will help in reformation
Eg. Flocculated particles/ long molecules
Method to measure flow properties of Newtonian liquids
Capillary viscometers (Eg. U-tube viscometer)
Method to measure flow properties of Non-Newtonian liquids
Rotational viscometers (Eg. Concentric cylinder viscometer)