Rheology

Question 1

Rheology

Answer 1

Is the study of flow properties of material

Question 2

Viscosity

Answer 2

The resistance of a fluid to flow when it is subjected to stress

Question 3

It is important to understand rheology properties of material because they affect

Answer 3

It is important to understand the rheological properties of materials because they affect the following:

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

Formula

Shear stress
Shear rate

Viscosity

Answer 4

Stress = F/A
Rate = dv/dr 
dv= difference of velocity between two planes of liquid
dr= distance between two planes of liquid 

Viscosity = stress / rate

Question 5

Rate unit

Answer 5

S^-1

Question 6

Stress unit

Answer 6

Nm^-2

Question 7

Newtonian fluid

Properties and formula

Answer 7

F/A = n dv/dr

n = coefficient of viscosity
Shear rate is directly proportional to shear stress

The flow curve / rheogram
Shows a straight line passing thru origin

Newtonian fluid have Constant viscosity

Question 8

Rheogram axis

Answer 8

Stress (y) against rate (x)
n= Gradient = viscosity

Rate (y) against stress (x)
n= 1 / gradient

n = dynamic viscosity

Question 9

Example of Newtonian fluid

Answer 9

Water
Organic solvent
Oils
True solution 
DILUTE suspension and DILUTE emulsion

Question 10

Non Newtonian fluid types and meaning

Answer 10

Do not follow Newtonian flow

1) plastic
2) pseudoplastic
3) dilatant

Question 11

Bingham flow and associated particle

Answer 11

Plastic flow

 The material 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 material resembles a Newtonian system
 Materials exhibiting plastic flow are shear thinning.

Formula
U= (F-f) / G

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

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

Question 12

Pseudoplastic and associated particles

Answer 12

Pseudoplastic materials show shear-thinning properties

 The material will flow as soon as a shear stress is applied.
 Its viscosity decreases with increasing rate of shear.
 It is often noted that the flow curve tends towards linearity at higher shear stress
 The viscosity of a pseudoplastic material is best represented by the entire flow curve.

F^N = n’G

F= shear stress
N = index of pseudoplasticity (N>1)
n' = viscosity coefficient 
G= shear rate 

Log G = NlogF - log n’

 Pseudoplastic flow is associated with polymers in solution.

E.g. Aq dispersion of hyrdrocolloid such as tragacanth, alginates, methylcellulose and synthetic materials such as PVP (polyvinylpyrrolidone)

Question 13

Dilatant material and associated particles

Answer 13

 Dilatant materials show shear-thickening properties
 The material will flow as soon as a shear stress is applied
 Its viscosity increases with increasing shear stress

Dilatant flow is associated with high concentration of deflocculated particles
Eg suspension with high concentration (>50%) of small deflocculated particles

Dilatant flow properties pose a problem in production.

F^N = n' G
N = <1

Question 14

Explain shear thickening

Answer 14

Under zero shear the particles are closely pack and interparticulate voids are at a minimum, which the vehicle is sufficient to fill and at low Shear rate such as those created during pouring.

AS flow rate increases the particles become displaced from the uniform distribution and the clumps that are produced results in the creation of a larger void into which the vehicle drains, so that the resistance to flow is increase and viscosity rises.

The effect is progressive with increased shear rate until eventually the material may appear paste-like as flow ceases.

Removal of shear stress results in the reestablishment of the fluid nature

Question 15

When a non Newtonian fluid is sheared, structural changes will occur in the system, resulting in changes in viscosity of the fluid.

The degree of change is dependent on

Answer 15

 When a non-Newtonian fluid is sheared, structural changes will occur in the system, resulting in changes in viscosity of the fluid.
 The degree of changes is dependent on :
1) Rate of shear
2) Duration of shear
3) Frequency of shear

 The structural changes may be reversible or irreversible

Question 16

Why thixotropy occurs

Answer 16

When non Newtonian material is subjected to shear, structural changes occur and the viscosity changes.

Once the shear stress is removed, even if the breakdown of structure is reversible, it may not return to its original condition instantly (rheological ground state).

Thus When material subjected to increasing shear rate and then decreased to zero, the down curve will be displaced wrt the up curve and a hysteresis loop will be included.

The area of the loop may be used as an index of the degree of breakdown. (Extent of break down)

Question 17

Up curve and down curve

Answer 17

Up curve is obtain from flow curve by increasing shear rate or stress

Down curve is from decreasing rate or stress

Question 18

Characteristics of thixotropy

Answer 18

Shear thinning ( increase stress = viscosity decrease)

Slow recovery of the apparent viscosity on standing of the system. ( up and down curve is not superimposable)

Question 19

Why thixotropy occur at molecular level

Answer 19

 Thixotropic systems are usually composed of asymmetrical particles or macromolecules that are capsule interacting by numerous secondary bond.

At rest, they produce a loose three-dimensional structure, so that the material is gel-like when unsheared.

On shearing, they become aligned and flow as the energy imparted during shearing disrupts the secondary bond thus viscosity decreased (sol-gel transformation)

Upon removal of shear stress, the system starts to reform (not immediate)

The recovery time may be reduced by applying a gentle rolling or rocking motion which helps in the reformation of the network (rheopexy) as this increases the rate of collision of particles allowing bond formation.

 Thixotropy is a desirable property in pharmaceutical suspensions and emulsions

Question 20

Dynamic viscosity

Units

Answer 20

Dyne. cm^-2. S

SI = Pa.S

Alternative: poise (P)
Centipoise cP
1cP = 1 mPa. S

Question 21

Kinematic visosity unit

Answer 21

Cm^2. S^-1

SI: m^2. S^-1

Alternative: stoke (St)
Centistoke (cSt)
1cSt = 1mm^2. S^-1

Question 22

Absolute viscosity (applies to Newtonian fluid only)?

Answer 22

Dyne. Cm^-2. S

SI: Pa. S

Alternative: poise
Centipoise

Question 23

Apparent viscosity (non Newtonian fluid)

Answer 23

Dyne. Cm^-2. S

SI: Pa. S

Alternative: poise
Centipoise

Question 24

Determination of rheological properties and the equipment required

Answer 24

 Proper choice of instrumental method is essential for meaningful assessment of rheological properties of fluids

Newtonian fluids
 Shear rate is directly proportional with shear stress
 A single point determination using a certain shear rate or stress is theoretically adequate.
”one point” instruments that operate at a single shear rate can be used.

E.g. Capillary viscometers.

Non-Newtonian fluids
 Shear rate is not directly proportional with shear stress
 “Multiple point” instruments that operate at a variety of shear rates are required to obtain a
complete rheogram
E.g. rotational viscometers

Question 25

Types of capillaries viscometer

Answer 25

U tube viscometer

Suspended level viscometer

Question 26

Types of rotational viscometer

Answer 26

Concentric cylinder viscometer

Cone and plate viscometer

Question 27

Types of efflux viscometer

Answer 27

Redwood viscometer

Flow cups

Question 28

Capillary viscometer eqn

Answer 28

n1/n2 = t1p1/ t2p2

P = density 
T = time 
n = dynamic viscosity 

V = n / p

V = kinematic viscosity.

V1/v2= t1/t2

Question 29

Capillary bore size for Ostwald viscometer

And requirement

Answer 29

U tube

Size A-H
H = biggest therefore faster flow time.

The appropriate size should be selected such that a flow time of at least 300s for Size A and at least 200s for all the other sizes obtained.

Question 30

Ubbelohde viscometer

Answer 30

Suspended level
No need to fill viscometer with a precise volume.

9sizes (1, 1A, 2 , 2A, 3, 3A, 4, 4A, 5)
Increasing bore size.

The appropriate one should be selected such that a flow time of at least 300 s for size 1 and at least 200 s for all the other sizes is obtained

Question 31

Precautions for capillary viscometer

Answer 31

Precautions for capillary viscometers
 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
(Increase temp = viscosity drop)

Question 32

Concentric cylinder Viscometer

Answer 32

 The viscometer consists of 2 coaxial cylinders
 The inner cylinder, which is suspended freely by a torsion wire, is immersed in the test liquid contained in the outer cylinder
 Either the inner cylinder (Searle type) or outer cylinder (Couette type) is rotated

Question 33

Couette type principle

Answer 33

Couette Type
Rotation of “outer cylinder” produces movement of the test liquid and a torque is transmitted to the inner cylinder.
The resultant stress on the inner cylinder is indicated by angular deflection of the pointer which moves past a scale. This corresponds to the viscosity.

n = (kø) / w
Ø = angular deflection 
W= shear rate

Question 34

Problems of concentric cylinder viscometer

Answer 34

Problems of concentric cylinder viscometers
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 35

Cone and plate viscometer

Answer 35

Cone and plate viscometer
 This viscometer consists of a flat circular plate with a wide angle cone
placed centrally above.
 The tip of the cone just touches the plate and the test liquid is loaded into the included gap.

n= KG/ ♎️

G = torque = stress 
♎️ = radial velocity of the plate = rate

Question 36

Advantages of cone and plate viscometer

Answer 36

 In the cone and plate viscometer, the angle should be small to avoid edge effect

Main advantages of the cone and plate viscometer:
 Volume of test liquid needed is small (~100ul)
Filling and cleaning is easy
Shear rate is constant throughout the test liquid (because of the small volume )

Question 37

Efflux viscometer principle

Answer 37

EFFLUX VISCOMETERS
 The time taken for a given volume of liquid to flow through a small orifice is determined
 The efflux time is an arbitrary measurement of viscosity
 E.g. Redwood viscometer, Flow cups
 Redwood viscometer is used in BP to determine the viscosity of a standard solution of tragacanth relative to that of water