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Form & Tech II > L1 Rheology of Liquids > Flashcards

L1 Rheology of Liquids Flashcards

1
Q

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

A
  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
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2
Q

Formula or equation for shear stress?

A

Force / Area = Shear stress

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3
Q

Formula or equation for Shear rate?

A

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

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4
Q

Equation for viscosity

A

Viscosity = Shear stress/Shear rate

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5
Q

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

A

F/A = η dv/dr,

where η is the coefficient viscosity

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6
Q

Describe the viscosity of a newtonian fluid

A

Constant viscosity, regardless of shear rate or shear stress applied

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7
Q

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?

A

1/η

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8
Q

Name some examples of newtonian fluids of pharmaceutical interest?

A
  • Water
  • Ethanol
  • Chloroform
  • Glyceryl trinitrate
  • Olive oil
  • Castor oil
  • Glycerol
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9
Q

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

A

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

the rheogram is a straight line passing through the origin,

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10
Q

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

A
  1. Plastic flow (Bingham flow)
  2. Pseudoplastic flow
  3. Dialatant flow
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11
Q

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

A

U = (F-f)/G

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

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12
Q

Comment on the rheological property of a fluid exhibiting plastic flow

A
  • 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
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13
Q

Fluids exhibiting plastic flow are ______ particles in a concentrated suspension

A

flocculated

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14
Q

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

A

Polymers

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

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15
Q

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

A
  • 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
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16
Q

Distinguish between the rheograms of plastic flow and pseudoplastic flow

A
  • For plastic flow, graph does not start at origin unlike pseudoplastic flow
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17
Q

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

A

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

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18
Q

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

A
  • Initially, the polymer molecules are entangled with one another when dissolved in solution
  • Shear stress causes disentanglement of the polymer molecules
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19
Q

Fluids with dilatant flow exhibit shear _______ properties

A

thickening

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20
Q

Name an example of a fluid which exhibit dilatant flow?

A

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

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21
Q

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

A
  • Exhibits shear-thickening
  • Material flow as soon as a shear stress is applied
  • Its viscosity increases with increasing shear stress
22
Q

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

A

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

23
Q

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

A
  • 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
24
Q

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?

A
  1. Rate of shear
  2. Duration of shear
  3. Frequency of shear
25
Q

Distinguish between upcurve and downcurve in a rheogram?

A
  • 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

26
Q

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

A

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

27
Q

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

A
  1. Pseudoplastic and plastic: left

2. Dilatant: right

28
Q

A non-superimposable upcurve and downcurve indicates that________

A

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

29
Q

What is thixotropy characterised by?

A
  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)
30
Q

Comment on the microscopic properties of a thixotropic system

A
  • 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
31
Q

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

A

operate at a single shear rate; Newtonian

32
Q

The basic principle of capillary viscometers

A
  • 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
33
Q

What is the relationship between kinematic viscosity and dynamic viscosity?

A

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

34
Q

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

A

Newtonian, dynamic

35
Q

Apparent viscosity only applies to _______ fluids

A

Non-newtonian

36
Q

What is relative viscosity?

A

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)

37
Q

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

A
  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)
38
Q

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?

A

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

39
Q

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

A
  • 300s for size A

- 200s for other sizes

40
Q

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

A
  • 300s for size 1

- 200s for all other sizes

41
Q

Name some precautions when using a capillary viscometer

A
  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
42
Q

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

A

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

43
Q

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

A

variety of shear rates; Non-newtonian

44
Q

Name 2 examples of Rotational viscometer

A
  1. Concentric cylinder viscometer

2. Cone and plate viscometer

45
Q

General principle of operation for the rotational viscometer:

A
  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
46
Q

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

A

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

47
Q

List some disadvantages of concentric cylinder viscometers

A
  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
48
Q

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

A
  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
49
Q

How to avoid edge effect in cone and plate viscometer?

A

Ensure that the angle between the cone and plate is small

50
Q

Principle of Efflux viscometers

A
  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
51
Q

Name some examples of efflux viscometers

A
  • Redwood Viscometer

- Flow cups

Form & Tech II (6 decks)

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