Rheology of Liquids Part I

Question 1

What is the definition of rheology, and the definition of viscosity?

Answer 1

Rheology: study of flow properties of materials
Viscosity: Resistance of a liquid to flow when it is subjected to stress

Question 2

How to calculate viscosity in general?

Answer 2

Shear stress/ Shear rate,

where shear stress = F/A and shear rate = velocity gradient

Question 3

What are some properties of Newtonian Liquids?

Answer 3

• rheological properties follow Newton’s Law of Flow.
• viscosity is constant
• denoted by n = shear stress/shear rate
• shear rate is directly proportional to shear stress and shows a straight line passing through the origin.

Question 4

What are some examples of Newtonian liquids?

Answer 4

• water, organic solvents, oils and true solutions
• some suspensions and emulsions
• chloroform has lowest dynamic viscosity, glycerol has the highest.

Question 5

How would you characterise Non-Newtonian liquids graphically?

Answer 5

• any form of graphical deviation that is not a straight line.

Question 6

What are the different types of flow?

Answer 6

• Plastic flow (bingham flow)
• Pseudoplastic flow
• Dilatant flow

Question 7

How would you describe plastic flow?

Answer 7

• Graphically, it starts off as a slope that is not at the origin, and progresses upwards in a straight line.
• Liquid behaves as an elastic solid at low shear stress.
• A certain shear stress equivalent to the yield value must be exerted before appreciable flow begins.
• At shear stress above yield value, the liquid resembles a Newtonian system.
• Liquids exhibiting plastic flow are shear thinning.
• Apparent viscosity dependent on the shear rate.

Question 8

What is the equation used to express plastic flow/

Answer 8

Bingham’s equation,

U = (F-f)/G

Question 9

What is plastic flow associated with?

Answer 9

The presence of flocculated particles in a concentrated suspension.

Question 10

How would you describe pseudoplastic flow?

Answer 10

• graphically: gently sloping curve
• the liquid will flow as soon as a shear stress is applied
• viscosity decreases with increasing shear rate
• flow curve tends towards linearity at higher shear stress
• liquids exhibiting pseudoplastic flow are shear-thinning.
• apparent viscosity is dependent on the shear rate.

Question 11

What is the equation that is used to express pseudoplastic flow?

Answer 11

F^N = n’G

Question 12

What is pseudoplastic flow associated with?

Answer 12

• polymers in solution

- aqueous dispersions of hydrocolloids

Question 13

How would you describe dilatant flow?

Answer 13

• curving upwards, increasing at a decreasing rate
• liquid will flow as soon as a shear stress is applied
• viscosity increases with increasing shear stress
• liquids exhibit dilatant flow are shear-thickening.
• apparently viscosity dependent on the shear rate

Question 14

What is dilatant flow associated with?

Answer 14

High concentrations (>50%) of small, deflocculated particles

Question 15

What are the degree of changes for non-Newtonian liquid dependent on?

Answer 15

• rate of shear
• duration of shear
• frequency of shear
• structural changes may be reversible or irreversible.

Question 16

What does a non-Newtonian liquid consist of?

Answer 16

• a solid component, which may be flocculated particles, deflocculated particles or long molecules.

Question 17

What does the degree of structural changes that occur as a result of the shearing of non-Newtonian liquids depend on?

Answer 17

• rate of shear
• duration of shear
• frequency of shear

Question 18

Structural changes made to non-Newtonian liquids are irreversible (T/F)?

Answer 18

false, they can be reversible as well

Question 19

What does the upcurve and downcurve mean, and why is there a gap between them?

Answer 19

upcurve - flow curve obtained from increasing shear rate
downcurve - flow curve obtained from decreasing shear rate
Both curves are non-superimposable on one another, therefore the gap between them.

Question 20

What do the upcurves and downcurves form, and what does the area between them denote?

Answer 20

both curves join to form a hysteresis loop, area between them denotes the extent of structural breakdown.

Question 21

What is thixotropy characterised by?

Answer 21

• shear thinning (decrease upon apparent viscosity once shearing forces are applied)
• upcurve and downcurve are non-superimposable (slow recovery of apparent viscosity)

Question 22

How to reduce recovery of apparent viscosity on the upcurve and the downcurve?

Answer 22

• by applying a gentle rocking or rolling motion, which helps in the reformation of the structure.