Rheology of liquids Flashcards

1
Q

Types of liquids

A

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)

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2
Q

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

A

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

  • Structural change reversible/irreversible
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3
Q

What is the hysteresis loop formed by?

A

Upcurve & downcurve

–> Indicates extent of structural breakdown

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4
Q

What is thixotrophy flow?

A
  • 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

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5
Q

Method to measure flow properties of Newtonian liquids

A

Capillary viscometers (Eg. U-tube viscometer)

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6
Q

Method to measure flow properties of Non-Newtonian liquids

A

Rotational viscometers (Eg. Concentric cylinder viscometer)

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