Viscoelasticity: Introduction to Rheology

Question 1

What is rheology?

Answer 1

The science of measuring and modelling flow

Question 2

What are the 4 types of fluids (based on their flow behaviour)?

Answer 2

1. Newtonian
2. Bingham
3. Shear thickening
4. Shear thinning

Question 3

How to Newtonian fluids perform under deformation and shear?

Answer 3

shear stress is proportional to the viscosity and shear rate (viscosity is independent of shear rate)

• as viscosity increases, more shear stress is required to get the same shear rate

Question 4

What is the Weissenburg effect?

Answer 4

Non-Newtonian fluids will climb up a rod spinning in the fluid (due to cohesive forces)

Question 5

What is the extrudate swell?

Answer 5

As polymer exits through a nozzle, it expands because highly entangled chains want to return to original shape (elastic deformation reversed)

Question 6

What is shear thickening behaviour?

Answer 6

• under shear, particles agglomerate and resist flow (causing the viscosity to increase with shear rate)

Question 7

What is shear thinning behaviour?

Answer 7

• as shear rate increases, viscosity decreases (breaks entanglement and aligns the chains, making it easier for them to flow)

Question 8

How does the Power Law Model predict shear thinning behaviour

Answer 8

• a piecewise function that predicts the constant viscosity and a linear line through the logarithms of viscosity and shear rate

Question 9

How does the Carreau Model predict shear thinning behaviour?

Answer 9

It uses a 3 parameter model to predict behaviour

• the 1/lambda predicts when it changes from a constant viscosity to a slope

Question 10

What happens to the Carreau Model as n decreases?

Answer 10

The slope becomes steeper

Question 11

What happens to the Carreau Model as lambda increases?

Answer 11

The slope starts earlier

Question 12

What happens to the shear thinning viscosity at very high shear rates?

Answer 12

Levels out to a viscosity, inf

– occurs when all chains are completely aligned (this is not predicted by the Carreau model)

Question 13

How does the Carreau-Yasuda model predict shear thinning behaviour?

Answer 13

It predicts all 3 regions of viscosity change: the initial constant viscosity, the slope as viscosity changes, and the final plateau at viscosity,inf

Question 14

What happens to the Carreu-Yasuda model as viscosity,inf increases?

Answer 14

The final plateau ends higher

Question 15

What happens to the Carreau-Yasuda model as alpha increases?

Answer 15

the abrupt transition to the sloped region increases (less gently curve)

Question 16

How does molecular weight impact viscosity?

Answer 16

As molecular weight increases, the transition to changing viscosity takes longer, but converges to the same slope

Question 17

How does temperature impact viscosity?

Answer 17

Only for amorphous and semi-crystalline (no fixed cross-links), the viscosity changes with temperature