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Jihong Han's lectures > Rheology > Flashcards

Rheology Flashcards

1
Q

What is rheology?

A

Is the study of deformation and flow of matter

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2
Q

What is the viscosity of a liquid defined as?

A

Its resistance to flow or move due to intermolecular van der vaals forces

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3
Q

What is viscosity?

A

Is the interaction between molecules and particles within a system

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4
Q

What is shearing rate?

A

Is the rate of change of velocity at which one layer of fluid passes over an adjacent layer

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5
Q

What is the equation and units for shearing rate?

A

Shearing rate = dv/dr s-1

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6
Q

What does Newton’s law suggest?

A

The rate of flow is directly related to the applied stress

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7
Q

Define dynamic viscosity?

A

Is the measure of the internal resistance or friction involved in the relative motion of one layer of molecules with respect to the next resulting from an applied force

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8
Q

What does kinematic viscosity take into account?

A

It takes into account the density of the liquid

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9
Q

What is the units for kinematic viscosity?

A

m2s-1

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10
Q

What is colloidal dispersion?

A

One phase dispersed in another immiscible phase in the form of particles

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11
Q

For a colloidal dispersion the relative viscosity is defined as what?

A

Viscosity of the dispersion/ Viscosity of the solvent

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12
Q

What is the volume fraction as described by Einstein equation?

A

Volume of the dispersed phase versus the total volume between 0 and 1

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13
Q

Specific viscosity is only dependent on volume fraction. statement TRUE OR FALSE?

A

TRUE

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14
Q

Liquids are classified according to their flow types as either Newtonian or Non-Newtonian. TRUE OR FALSE?

A

TRUE

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15
Q

Newtonian fluids are not simple fluids. TRUE OR FALSE?

A

FALSE

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16
Q

In Newtonian fluids what is not affected by shear rate?

A

viscosity

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17
Q

In Newtonian fluids there is a linear relationship between shear rate and shear stress. TRUE OR FALSE?

A

TRUE

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18
Q

What is boundary layer?

A

a layer of more or less stationary fluid (such as water or air) immediately surrounding an immersed moving object

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19
Q

What is Laminar flow?

A

occurs when a fluid flows in parallel layers, with no disruption between the layer

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20
Q

What is transitional flow?

A

The process of a laminar flow becoming turbulent

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21
Q

What is turbulent flow?

A

Flow in which the fluid undergoes irregular fluctuations and fluid moves in smooth paths or layers

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22
Q

For tubular flow, if Reynolds number is less than 2000, what flow does this equate to?

A

Laminar flow

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23
Q

For tubular flow, if Reynolds number is greater than 4000, what flow does this equate to?

A

Turbulent flow

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24
Q

If Reynolds number is greater than 2000 and less than 4000 on a smooth surface what flow does this equate to?

25
Q

What is measured in a capillary viscometer?

A

The flow time of a set volume of liquid between two marks at a given temperature.

26
Q

If the time taken for a liquid to flow through the viscometer is greater, the viscosity is higher. TRUE OR FALSE?

27
Q

How does a falling sphere viscometer obtain viscosity?

A

By measuring the time it takes for a cylindrical or spherical object to fall through a sample over a specific distance

28
Q

What does Stoke’s law apply?

A

That diffusivity is inversely proportional to the viscosity

29
Q

Stoke’s law applies to spherical particles. TRUE OR FALSE?

30
Q

How does a rotational viscometer obtain viscosity?

A

By turning a disc, bar or cylinder immersed in a liquid

31
Q

The more viscous the liquid, the stronger the resistance and hence higher the torque. TRUE OR FALSE?

32
Q

Viscosity can be calculated from measured rotational speed vs applied force. TRUE OR FALSE?

33
Q

In non Newtonian fluids viscosity varies with shear rate. TRUE OR FALSE?

34
Q

List the three types of Non-Newtonian fluids

A
  • Plastic
  • Pseudoplastic
  • Dilatant
35
Q

What occurs in plastic (Non-Newtonian fluid)?

A

The substance will not start to flow until a stress corresponding to the lower yield value is applied

36
Q

What occurs in Pseudoplastic (Non-Newtonian fluid)?

A

The substance will start to flow immediately when stress is applied

37
Q

What occurs in dilatant (Non-Newtonian fluid)?

A

Materials increase in volume during shearing and exhibit shear thickening

38
Q

Which of the Non-Newtonian fluid is the rarest?

39
Q

Dilatant systems usually are suspensions containing high percentage of particles. TRUE OR FALSE?

40
Q

For Newtonian fluids apparent viscosity never remains the same TRUE OR FALSE?

41
Q

For non-newtonian fluids apparent viscosity invariably changes. TRUE OR FALSE?

42
Q

What occurs during shear thinning?

A

The corresponding rate increases with stress, therefore, viscosity decreases

43
Q

In shear thinning does viscosity increase or decrease?

44
Q

In shear thinning, is the response of the system linear or non-linear?

A

Non-linear

45
Q

Within the three types of non-newtonian fluids, which one does shear thinning represent?

A

Pseudoplastic behaviour

46
Q

Ideal plastic is characterised by a yield value followed by ideal (Newtonian flow). TRUE OR FALSE?

47
Q

At substances below the yield value, the substance acts as elastic materials. TRUE OR FALSE?

48
Q

What are the causes for shear thinning?

A
  • Breaking of non-covalent bonds
  • Alignment of molecules/polymers along the direction of shear
  • The non-linearity of response is caused by continuous random breaking and formation of bonds
49
Q

In shear thickening, is the response of the system linear or non-linear?

A

non-linear

50
Q

What occurs during shear thickening?

A

The corresponding rate decrease with stress resulting in an increase of viscosity

51
Q

Within the three types of non-newtonian fluids, which one does shear thickening represent?

A

Dilatant behaviour

52
Q

What are the causes of shear thickening?

A
  • The spaces between particles tend to increase under shear resulting in voids
  • Particles become less lubricated after the voids are increased, hence increase of friction inside the system
  • More interaction between particles due to disturbance of system
53
Q

What occurs in THIXOTROPY?

A

The structure does not reform at the same speed and in the same way it is broken down

54
Q

Thixotropy can be seen for both shear thinning and thickening. TRUE OR FALSE?

55
Q

Give pharmacy examples for Plastic, Pseudoplastic and dilatant flow.

A
  • Plastic- Flocculated particles in concentrated disperse systems e.g ointments, pastes and creams
  • Pseudoplastic- Natural synthetic & semi synthetic polymers in solution e.g Acacia, sodium alginate
  • Dilatant- 40-50% suspensions of starch/water
56
Q

What does rotational rheometers allow us to measure?

A

A wide range of shear stress and rate

57
Q

Give the advantages of using concentric cylinder

A
  • Little possibility of sample evaporation or expulsion
  • Can measure low shear stress (useful for emulsions and suspensions)
  • Easy to load with liquid dosage forms
58
Q

Give the disadvantages of using concentric cylinder

A
  • Possible air incorporation or breaking of structure when loading semi solids
  • Large volume of material needed
  • Shear rate across gap is not constant; varies with distance from wall
59
Q

What are the advantages of Cone & Plate?

A
  • Constant shear rate across radius
  • High shear rates possible
  • Small sample volume required
  • Easy to fill & clean, even with highly viscous samples
  • Little disturbance of sample structure
  • Rapid temperature equilibration due to thin film

Jihong Han's lectures (11 decks)

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