Rheology of liquids

Question 1

Types of liquids

Answer 1

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)

Question 2

What is the degree of change dependent on when Non-Newtonian liquid is sheared?

Answer 2

1) Rate of shear
2) Duration of shear
3) Frequency of shear

• Structural change reversible/irreversible

Question 3

What is the hysteresis loop formed by?

Answer 3

Upcurve & downcurve

–> Indicates extent of structural breakdown

Question 4

What is thixotrophy flow?

Answer 4

• 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

Question 5

Method to measure flow properties of Newtonian liquids

Answer 5

Capillary viscometers (Eg. U-tube viscometer)

Question 6

Method to measure flow properties of Non-Newtonian liquids

Answer 6

Rotational viscometers (Eg. Concentric cylinder viscometer)