Rheology 2

Question 1

What are three types of Non-Newtonian fluids?

Answer 1

Plastic

Pseudoplastic

Dilatant

Question 2

Describe the graph of Newtonian flow?

Answer 2

It is a straight line, when you plot shear stress over the rate of shear.

Question 3

Describe the graph for plastic non-Newtonian fluids?

Answer 3

The graph does not increase straight away. It begins to increase when the shear stress is a certain value

Question 4

Describe the graph for Pseudoplastic non-Newtonian fluids?

Answer 4

The graph will start to increase from the original, so the system starts to flow from the beginning. But it’s an under curve compared to dilatant flow.

Question 5

Describe the graph for Dilatant non-Newtonian fluids?

Answer 5

It is the same as Pseudoplastic flow but over curve.

Question 6

Explain Pseudoplastic behaviour?

Answer 6

When you increase the shear stress, the shear rate increases. So the system is getting thinner as viscosity gets lower. And causes a faster flow This is causes a shear thinning system.

Question 7

What is the n value, for different flow systems?

Answer 7

N = 1, Newtonian flow N > 1, pseudoplastic flow N < 1, dilatant flow

Question 8

Explain what could cause shear-thinning?

Answer 8

Changes can occur inside a system and result in viscosity decrease. This might be because non-covalent bonds such as H-bond, charge interactions, hydrophobic interactions etc, were broken. Or Polymer molecules aligned along the direction of shear. This causes the friction between them to reduce so viscosity decreases.

Question 9

What indicates a shear thinning system?

Answer 9

A faster flow time As it increases in flow rate, so decrease in viscosity.

Question 10

*What is an equation for Plastic flow?

Answer 10

The viscosity calculated from the section of the straight line is the plastic viscosity.

Question 11

Explain Plastic flow?

Answer 11

Once the yield value is increased, the plastic viscosity changes proportionally with shearing stress (a straight line).

Question 12

Explain when it is and isn’t possible to calculate viscosity using the plastic flow equation?

Answer 12

If stress is smaller than f, then the top would give a negative value, so the system won’t move. This is the same thing for if stress is equal to f, so it would give you a value of 0. But if stress is larger than f, then the top would be a positive number and the system would move. So you could calculate the viscosity. There needs to be a higher value than yield value

Question 13

What do Dilatant systems normally have?

Answer 13

A high percentage of particles

Question 14

Explain how shear thickening occurs?

Answer 14

When subject to shear: Spaces between particles tend to increase under shear resulting in voids. Particles become less lubricated after the voids are increased, which increases friction inside the system More interaction between particles due to disturbance of system This leads to an increase in viscosity

Question 15

Where might shear thickening usually happen?

Answer 15

In dilatant systems because it has a high percentage of particles

Question 16

Why might the volume of a shear thickening system increase after subjected to shear?

Answer 16

Because the concentrated particles may create voids when sheared

Question 17

Explain Thixotropy - time-dependent behaviour?

Answer 17

As you increase shear stress and shear rate, the internal structure changes. So when you decrease shear rate, the internal structure cannot restore the previous configuration. So it forms a loop called the Hysteresis loop.

Question 18

Explain Thixotropy - time-dependent behaviour?

Answer 18

As you increase shear stress and shear rate, the internal structure changes. So when you decrease shear rate, the internal structure cannot restore the previous configuration. So it forms a loop called the Hysteresis loop. The larger the area, the stronger the behaviour

Question 19

Why is the Hysteresis loop formed?

Answer 19

Because a different route is taken when there is a decrease.

Question 20

Explain the advantages and disadvantages of a high viscosity for drug suspensions?

Answer 20

A high viscosity means you can leave it on the shelves because the particles won’t settle too fast; good stability. However if too high, it will be harder for the patient to take the suspension out. It won’t flow nicely.

Question 21

Explain the system made in drug suspension?

Answer 21

So when the patient shakes the suspension, a certain amount of shear is applied. The system becomes thinned and the viscosity is reduced. When they stop shaking, the viscosity does not go back straight away. Leaving a time window of a low viscosity for the patient to use the suspension. So when shear is reduced, the internal structure does not re-form at the same speed and in the same way it was broken down. This is seen for both shear-thinning and shear-thickening.