Rheolog

Question 1

What is rheology

Answer 1

study of the flow properties of materials

Question 2

what is viscosity

Answer 2

resistance of a liquid to flow when it is subjected to stress

viscosity = shear stress/ shear rate

Question 3

What does rheological properties of material affect?

Answer 3

efficiency of mixing
flow through pipes
ease of packaging into and removal from containers
physical stability of preparation
rate of drug absorption
spreading and adherence of preparation to skin

Question 4

What are the factors that make up shear stress and shear rate?

Answer 4

Shear stress = Force / Area

Shear rate = dv/dr = velocity gradient

Question 5

What are the 2 types of liquids based on rheological properties?

Answer 5

newtonian and non-newtonian liquids

Question 6

Newtonian liquids follow the Newton’s Law of Flow, what is the equation telling us about this?

Answer 6

Shear rate is directly proportional to the shear stress

shear stress = n * shear rate

the flow curve of a newtonian liquid shows a straight line passing through the origin.

newtonian liquids have constant viscosity.

Question 7

When the shear rate is on the y axis and the shear stress is on the x axis,

what does the gradient tell us for a newtonian liquid?

Answer 7

1 / gradient = coefficient of viscosity

Question 8

what are the 3 types of non-newtonian liquids in the lecture?

Answer 8

Plastic flow (Bingham flow)
Pseudoplastic flow 
Dilatant flow

Question 9

What are the characteristics of plastic flow (bingham flow)

Answer 9

liquid behaves as an elastic solid at low shear stress, upon applying a certain shear stress Fb, then there will be an appreciable flow.

at shear stress above Fb, the liquid resembles a newtonian system.

shear thinning (i.e the more force u add, the viscosity will fall)

Question 10

What is Bingham’s equation

Answer 10

U = (F-f) / G
where U is plastic viscosity
F is shear stress
f is yield value
G is shear rate

plastic flow is associated with the presence of flocculated particles in a concentrated suspension.

Question 11

Similarities and differences of plastic and pseudoplastic flow?

Answer 11

Both have shear thinning
Both flow curves tend to linearity at higher shear stress

Pseudoplastic will flow as soon as shear stress is applied whereas plastic need at least Fb.

Question 12

What is pseudoplastic flow associated/ likened with?

Answer 12

polymers in solutions, the more stress applied, the more disentangled the polymers will be and the liquid will behave like newtonian liquid.

Question 13

Dilatant flow properties?

Answer 13

Similar to Pseudoplastic, the liquid will flow as soon as shear stress is applied.

Exhibits shear thickening

F^N = n’G
same empirical equation as depicted in pseudoplastic flow. in this case N value is <1
N is the index of dilatancy.

Question 14

what is dilatant flow associated/ likened with?

Answer 14

high concentrations of deflocculated particles i.e suspensions with high concentration (>50%) of small, deflocculated properties.

Question 15

why does dilatant flow properties pose a problem in production?

Answer 15

When producing liquids, we use mixers like turbine, propeller and paddle mixers.

At high rotation rates in propeller mixers, liquid displaying dilatant flow will exhibit strong shear thickening and the liquid may ‘freeze’, which hinders the process of manufacturing.

Question 16

What can affect the degree of structural changes in a non-newtonian liquid system when sheared?

Answer 16

Rate of shear
duration of shear
frequency of shear

Question 17

The area of hysteresis loop is formed by taking the upcurve and downcurve area loop, what does the area indicate

Answer 17

extent of structural breakdown

bigger the area, the bigger the structural breakdown of the liquid

Question 18

What are the 2 important factors of thixotropy?

Answer 18

Exhibits shear thinning

Slow recovery of apparent viscosity on standing

Question 19

Why is thixotropy good for liquid preparations?

Answer 19

Upon standing, viscosity of liquid is high. Based on stokes law, the rate of creaming will be slower.

Upon shaking, drug can be easily homogenised due to shear thinning properties, also makes it easy to pour the drug out.

Question 20

Liquids that are composed of flocculated particles or long molecules usually exhibit what property?

Answer 20

thixotropy

Question 21

Difference between kinematic viscosity vs dynamic viscosity?

Answer 21

dynamic viscosity = shear stress/ shear rate

whereas kinematic viscosity = dynamic viscosity / density

Question 22

Absolute viscosity vs apparent viscosity?

Answer 22

absolute used to measure newtonian fluids

apparent used to measure non-newtonian fluids

Question 23

Does temperature affect viscosity?

Answer 23

yes

Question 24

What type of instruments is the best for newtonian liquids.

Answer 24

Since a single point determination is adequate, we can use a one point instrument that operate at a single shear rate i.e capillary viscometer.

Question 25

What types of instruments can be used for non-newtonian liquids

Answer 25

Since shear rate not directly proportional with shear stress, we need multiple point instruments that operate over a range of shear rates to get a complete rheogram. Example: rotational viscometer

Question 26

Example of capillary viscometers?

Answer 26

U-tube viscometer | suspended level viscometer

Question 27

example of rotational viscometer?

Answer 27

concentric cylinder viscometer | cone and plate viscometer

Question 28

What can we obtain from poiseuilles law to calculate relative viscosity of 2 liquids?

Answer 28

n1/n2 = t1p1/t2p2 since kinematic viscosity: v = n/p v1/v2 = t1/t2

Question 29

What are the sizings for U-tube viscometer?

Answer 29

Size A to H

Question 30

How long should flow time be from size A to size H of U-tube viscometers?

Answer 30

At least 300s for A | at least 200s for all other sizes

Question 31

What are the sizings for Suspended-level viscometer?

Answer 31

9 sizes: 1, 1A, 2, 2A, 3, 3A, 4, 4A, 5 Size 1 should flow at least 300s other sizes at least 200s

Question 32

What are the advantages of Suspended-level viscometer over U-tube viscometer?

Answer 32

no need to fill the viscometer with a precise volume of liquid can be used for very viscous liquid (20000 to 100,000 mm^2s^-1, 10x higher than that of u-tubes)

Question 33

what are certain precautions to look out for while using capillary viscometers

Answer 33

Ensure streamline flow by avoiding vibration and maintaining a minimum flow time liquid should be free from air bubbles ensure viscometer is clean before use ensure viscometer is clamped vertically ensure accurate temperature control (in a water bath)

Question 34

The searle type of concentric cylinder viscometer is defined by the rotation of the _______ cylinder. Whereas the Couette type is defined by rotation of the _______ cylinder.

Answer 34

Inner, outer.

Question 35

What are the terms that can be varied to show the change in viscosities in the concentric cylinder viscometer?

Answer 35

angular deflection and angular velocity of the outer cylinder.

Question 36

What is the most commonly used viscometer for non-newtonian liquids

Answer 36

brookfield viscometer

Question 37

What are problems of concentric cylinder viscometers?

Answer 37

shear rate of test liquid across the gap is not constant end effects can be significant frictional heating is significant at high shear rate filling is difficult if gap is small large volume of test liquid is required if the gap is big

Question 38

Describe the cone and plate viscometer

Answer 38

consists of flat circular plate with a wide angled cone placed centrally above the angle between plate and cone should be small to avoid edge effect

Question 39

What terms can be varied to measure the viscosity of the liquid in a cone and plate viscometer?

Answer 39

``` G = torque on the cone omega = radial velocity of the plate ```

Question 40

Main advantages of the cone and plate viscometer

Answer 40

volume of test liquid needed is small filling and cleaning is easy shear rate is constant throughout the test liquid

Question 41

examples of efflux viscometers?

Answer 41

Redwood viscometer and flow cups

Question 42

function of redwood viscometer?

Answer 42

used in BP to determine the viscosity of a standard solution of tragacanth relative to that of water (as a qualitative test)

Question 43

How does Efflux viscometers work?

Answer 43

They measure the time taken for a given volume of liquid to flow through a small orifice, gives a comparative result instead of a quantitative result.