L1 Rheology of Liquids

Question 1

Name some things that rheological properties of a material could affect?
L1S4

Answer 1

1. Efficiency of mixing
2. Flow through pipes
3. Ease of packaging into and removal from containers
4. Physical stability ofpreps
5. Rate of drug absorption
6. Spreading and adherence of preparation to skin

Question 2

Formula or equation for shear stress?

Answer 2

Force / Area = Shear stress

Question 3

Formula or equation for Shear rate?

Answer 3

dv/dr = Velocity gradient = Shear rate

dv/dr is the difference in velocity between the uppermost layer and each subsequent lower layer of an object when a force is applied to the top layer

Question 4

Equation for viscosity

Answer 4

Viscosity = Shear stress/Shear rate

Question 5

State the equation that relates shear rate and shear stress for newtonian fluids, according to Newton’s Law of Flow

Answer 5

F/A = η dv/dr,

where η is the coefficient viscosity

Question 6

Describe the viscosity of a newtonian fluid

Answer 6

Constant viscosity, regardless of shear rate or shear stress applied

Question 7

For the rheogram of a newtonian liquid with Shear rate in the y-axis and Shear stress in the x-axis, what does the gradient of the line represent?

Answer 7

1/η

Question 8

Name some examples of newtonian fluids of pharmaceutical interest?

Answer 8

• Water
• Ethanol
• Chloroform
• Glyceryl trinitrate
• Olive oil
• Castor oil
• Glycerol

Question 9

Describe the rheogram (flow curve) of a newtonian fluid?

Answer 9

Regardless whether the graph is Shear rate against Shear stress or vice versa:

the rheogram is a straight line passing through the origin,

Question 10

Name the three types of flow that a non-newtonian fluid can exhibit

Answer 10

1. Plastic flow (Bingham flow)
2. Pseudoplastic flow
3. Dialatant flow

Question 11

The equation relating plastic viscosity and shear rate/stress for liquids showing plastic flow

Answer 11

U = (F-f)/G

U is plastic viscosity
F is shear stress
f is yield value
G is shear rate

Question 12

Comment on the rheological property of a fluid exhibiting plastic flow

Answer 12

• At low shear stress, material behaves as an elastic solid
• A shear stress equivalent to yield value must be exerted before appreciable flow begins
• As shear stress increases above yield-value, the material resembles Newtonian system
• Materials exhibiting plastic flow are shear thinning

Question 13

Fluids exhibiting plastic flow are ______ particles in a concentrated suspension

Answer 13

flocculated

Question 14

For a solution exhibiting pseudoplastic flow properties, what does the solution contain?

Answer 14

Polymers

E.g. aqueous dispersions of hydrolloid like tragacanth, alginates etc.

Question 15

Comment on the rheological property of a fluid exhibiting pseudo-plastic flow

Answer 15

• Shows shear-thinning properties
• The material flows as soon as shear stress is applied
• Its viscosity decreases with increasing rate of shear
• Flow curve may tend towards linearity at higher shear stress

Question 16

Distinguish between the rheograms of plastic flow and pseudoplastic flow

Answer 16

• For plastic flow, graph does not start at origin unlike pseudoplastic flow

Question 17

How do we use a rheogram to find the viscosity of a fluid exhibiting pseudoplastic flow at a specific shear rate/ shear stress?

Answer 17

At the specific shear rate/ shear stress value, draw a tangent to the curve and obtain its gradient

Question 18

Microscopically, in a solution containing polymers, when shear stress is applied, what happens to the polymers?

Answer 18

• Initially, the polymer molecules are entangled with one another when dissolved in solution
• Shear stress causes disentanglement of the polymer molecules

Question 19

Fluids with dilatant flow exhibit shear _______ properties

Answer 19

thickening

Question 20

Name an example of a fluid which exhibit dilatant flow?

Answer 20

(associated with high concentrations of deflocculated particles)
E.g. Suspensions with high concentrations (>50%) of small, deflocculated particles

Question 21

Comment on the rheological properties of a fluid exhibiting dilatant flow?

Answer 21

• Exhibits shear-thickening
• Material flow as soon as a shear stress is applied
• Its viscosity increases with increasing shear stress

Question 22

Why do liquids with dilatant flow properties pose a problem in production of liquid dosage forms?

Answer 22

It is difficult to stir the solution even with a mixer as a high shear stress applied by the mixer will only cause the liquid’s viscosity to increase tremendously, which makes the liquid difficult to stir

Question 23

For a solution exhibiting dilatant flow, explain in the microscopic level what happens when shear stress is applied

Answer 23

• Initially, deflocculated particles are close to one another. At equilibrium, inter-particle space is minimal, fully filled with liquid medium. This liquid medium lubricates the particles for flow
• When stress is applied, particles move apart, causing inter-particle space to increase. However the liquid medium (being constant volume) are unable to fill the new exapanded inter-particle space, hence less lubrication and greater resistance to flow

Question 24

When a non-Newtonian fluid is sheared, structural changes occurs in the system. What are the three properties of the shear that could determine the degree of these changes?

Answer 24

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

Question 25

Distinguish between upcurve and downcurve in a rheogram?

Answer 25

- Upcurve: flow curve obtained by increasing shear rate or shear stress - Down curve, flow curve obtained by DECREASING shear rate or shear stress Remember to quote whether apparent viscosity quoted is obtained from up or down curve

Question 26

What is a hysteresis loop, and what does its area indicates?

Answer 26

Hysteresis loop is formed by the up and downcurve. The area of hysteresis loop indicates the extent of structural breakdown/change

Question 27

Is the down curve for pseudoplastic, plastic and dilatant flow displaced to the left or the right of the upcurve?

Answer 27

1. Pseudoplastic and plastic: left | 2. Dilatant: right

Question 28

A non-superimposable upcurve and downcurve indicates that________

Answer 28

Reversible changes are not instantaneous after a shear stress/ shear rate is applied to the fluid

Question 29

What is thixotropy characterised by?

Answer 29

1. Decreased apparent viscosity upon applying shear forces (i.e. shear thinning) 2. Slow recover of apparent viscosity on standing of the system (i.e. upcurve and downcurve are not superimposable)

Question 30

Comment on the microscopic properties of a thixotropic system

Answer 30

- Composed of asymmetrical particles/macromolecules - At rest: Produce a loose 3D structure - Upon shearing: particles aligned to flow - Upon removal of shear stress: the 3D structure begins to reform - Recovery time can be reduced by applying a gentle rolling or rocking motion which helps in reformation of the initial structure/network

Question 31

One point instruments refer to instruments that ______________, which can be used to determine the viscosity of _________ fluids

Answer 31

operate at a single shear rate; Newtonian

Question 32

The basic principle of capillary viscometers

Answer 32

- Measure flow time for test liquid - Measure flow time for reference liquid - Using those time acquired, known viscosity of the ref liquid, you can calculate the viscosity of the test liquid

Question 33

What is the relationship between kinematic viscosity and dynamic viscosity?

Answer 33

Kinematic viscosity is the dynamic viscosity divided by the density of the fluid Dynamic viscosity can be obtained by taking the ratio of shear stress to shear rate

Question 34

Absolute viscosity only applies to _____ fluids, and it is the _______ viscosity of that type of fluid

Answer 34

Newtonian, dynamic

Question 35

Apparent viscosity only applies to _______ fluids

Answer 35

Non-newtonian

Question 36

What is relative viscosity?

Answer 36

The ratio of the viscosity of the solution to the viscosity of the pure solvent (loosely defined as ratio of viscosity between 2 different fluids)

Question 37

What are the advantages of using a suspended level viscometer over U-tube viscometer?

Answer 37

1. Suspended level viscometer does not need a precise volume of liquid to be added due to the side arm which helps ensure constant volume regardless of the volume of liquid added in 2. Suspended level viscometer can be used for very viscous liquids (10 times higher than U-tube viscometer)

Question 38

For capillary viscometers, a range of capillary bore sizes are available. For a very viscous liquid, should the bigger or the smaller bore sized capillary be used?

Answer 38

Bigger, as it allows a larger volume of liquid to flow hence faster liquid flow

Question 39

For a U-tube viscometer, the bore size is selected accordingly such that flow time of at least _____s for size A and _____s for all other sizes are obtained

Answer 39

- 300s for size A | - 200s for other sizes

Question 40

For a Suspended-level viscometer, the bore size is selected accordingly such that flow time of at least _____s for size 1 and _____s for all other sizes are obtained

Answer 40

- 300s for size 1 | - 200s for all other sizes

Question 41

Name some precautions when using a capillary viscometer

Answer 41

1. Ensure streamline flow: avoid vibration and maintain minimum flow time 2. Ensure liquid is free from air bubbles 3. Ensure viscometer is clean before use 4. Ensure viscometer is clamped vertically 5. Ensure accurate temperature control

Question 42

Why must the flow rate not be too fast in a capillary viscometer?

Answer 42

If too fast, flow is turbulent, not laminar, which will result in error when calculating viscosity

Question 43

"Multiple point" instruments operate at a ___________ and are required to obtain a complete rheogram for ________ fluids

Answer 43

variety of shear rates; Non-newtonian

Question 44

Name 2 examples of Rotational viscometer

Answer 44

1. Concentric cylinder viscometer | 2. Cone and plate viscometer

Question 45

General principle of operation for the rotational viscometer:

Answer 45

1. Vary spindle rotational speed (hence varying shear rate) 2. Record stress experienced by liquid. It can be read off the movement of a pointer over a scale to give dial reading 3. The dynamic viscosity of the liquid at each rotational speed = dial reading x spindle factor

Question 46

Would a large or small spindle in a rotational viscometer more appropriate for less viscous, non-newtonian liquid?

Answer 46

Large For less viscous liquid, torque is very low and difficult to detect. Hence, using a large spindle allows more stress to be exerted on the spindle hence a torque and a readable dial reading

Question 47

List some disadvantages of concentric cylinder viscometers

Answer 47

1. Shear rate of test liquid across the gap is not constant 2. End effects can be significant 3. Frictional heating may be significant at high shear rate 4. Filling is difficult if gap is too small 5. Large volume of test liquid is required if the gap is big

Question 48

Compared to concentric cylinder, what advantage(s) does cone and plate viscometer have?

Answer 48

1. Only a small volume of test liquid required 2. Filling and cleaning is easy 3. Shear rate is constant throughout test liquid as the plate and cones are very close together

Question 49

How to avoid edge effect in cone and plate viscometer?

Answer 49

Ensure that the angle between the cone and plate is small

Question 50

Principle of Efflux viscometers

Answer 50

1. The time taken for a volume of liquid to flow through a small orifice is determined 2. Efflux time can be used to obtain an arbitrary measurement of viscosity, and not the actual numeric value of viscosity 3. Longer flow time = more viscous liquid

Question 51

Name some examples of efflux viscometers

Answer 51

- Redwood Viscometer | - Flow cups