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PSF > Rheology > Flashcards

Rheology Flashcards

1
Q

[sigma]

A

Shear stress

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2
Q

[gamma]

A

Shear rate

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3
Q

V

A

Velocity

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4
Q

D

A

Distance

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5
Q

n[eta]

A

Dynamic viscosity

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6
Q

F

A

Force

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7
Q

A

A

Area

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8
Q

v[nu]

A

Kinematic viscosity

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9
Q

p[rho]

A

Density

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10
Q

[Phi]

A

Volume fraction

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11
Q

M

A

Molecular weight

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12
Q

R

A

Radius

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13
Q

P

A

Pressure drop

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14
Q

Define rheology

A

The study of the deformation and flow of matter

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15
Q

Define viscosity

A

The viscosity of a liquid is its resistance to flow or move due to IMF.
Viscosity measure resistance to gradual deformation
E.g. Water has weak attractions between the molecules and low dynamic viscosity
Syrup has strong attractions between the molecules and higher dynamic viscosity

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16
Q

Areas in pharmaceutics where rheology is important - fluids

A 
Mixing
Particle size reduction 
Passage through orifices (opening on body e.g. Mouth,rectum)
Pumping
Physical stability
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17
Q

Areas in pharmaceutics where rheology is important - semisolid

A

Spreading and adherence to kin
Removal from jars or extrusion from tubes
Mixing of solids with miscible liquids
Release of drug base

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18
Q

Areas in pharmaceutics where rheology is important - solids

A

Powder flow

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19
Q

Areas in pharmaceutics where rheology is important - processing

A

Processing efficiency

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20
Q

Define shearing rate and equation

A

Difference in velocity between layers
Shearing rate = dv/dr
(Velocity/difference) unit s-1

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21
Q

Define newtons law

A

The rate of flow is directly related to the applied stress

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22
Q

Shearing stress equation

A

F/A

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23
Q

Dynamic viscosity equation

A

n = [sigma] / [gamma]

Nm-2 s or Pa s

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24
Q

Define dynamic viscosity

A

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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25
Q

Kinematic viscosity

A

Takes into account the density p of the liquid: v = n/p

Unit: m2s-1 or stoke (S) cm2s-1

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26
Q

Define colloidal dispersion

A

One phase dispersed in another immiscible phase in the form of particles. E.g. Oil in water

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27
Q

Colloidal dispersion relative viscosity equation

A

nr = n/no

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28
Q

For colloidal dispersion the specific viscosity equation

A

nsp = n/no -1 = n-no/no = nr -1

n= viscosity of dispersion (dispersed phase plus solvent)
no = viscosity of the solvent 
nsp = usually measure relative to a standard/reference liquid
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29
Q

Einstein equation

A

n = no (1+2.5 volume fraction)

n= viscosity of dispersion 
no = viscosity of solvent

Specific viscosity is only dependent on volume fraction

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30
Q

Characteristics of Newtonian flow

A

Liquids are classified according to their flow type as either Newtonian or non Newtonian.
Newtonian fluids are simple fluids - linear relationship between shear stress and rate of shear (directly proportional otherwise it’s non Newtonian), viscosity is not affected by shear rate

31
Q

Boundary layer

A

Place of less stationary fluid. E.g. Water touching inside of tube will move slower than water water towards inside of the tube

32
Q

Laminar flow

A

Flow between layer. When all materials move together

33
Q

Transitional flow

A

Laminar to turbulent

34
Q

Turbulent flow

A

Random

35
Q

Reynolds number Re

A 
Re= pvd/n
For tubular flow
If Re <2000 laminar flow
If Re >4000 turbulent flow
If 2000< Re <4000 depending on surface - if smooth laminar, if rough distributed
36
Q

What does capillary viscometer measure

A

The flow time of a set volume of liquid between 2 marks at a given temperature

37
Q

Hagen-Poiseuille equation for capillary viscometer

A

n= pi r4 tp / 8LV

r = radius of capillary
P = pressure difference between ends of tube
L = length of capillary
V = volume of the liquid 
t = flow time
38
Q

What is specific to a given viscometer

A

r L and V are specific to a given viscometer and can be combined together with pi into a constant k. P can be determined by measuring a reference liquid
Equation becomes n = kpt
Meaning if the time taken for a liquid to flow through the viscometer is greater the viscosity is higher

39
Q

How does a falling sphere viscometer (Hoppler) obtain viscosity and what law applies

A

Obtains viscosity by messing the time it takes for a cylindrical or spherical object to fall through a sample over a specific distance
Stoke law applied (the diffusivity is inversely proportional to the viscosity)

40
Q

Equation for stokes law

A

Sedimentation - stokes law
v = 2r2 (p2-p1) g / 9n

v is the velocity of sedimentation
r is radius of the spherical particle 
p2 is density of the particle 
p1 is density of the medium 
n is viscosity 
g is the acceleration of gravity
41
Q

What particles does stokes law apply to and when can viscosity be calculated

A

Spherical

Viscosity can be calculated after v is determined from the time needed to travel a certain distance

42
Q

How does a rotational viscometer obtain viscosity

A

By turned a disc, bar or cylinder immersed in a liquid. The more viscous the liquid the stronger the resistance and hence higher the torque. Viscosity can be calculated from measured rotation speed vs applied torque

42
Q

Define non Newtonian fluids

A

Complex pharmaceutical systems
Applied shear stress causes a change or interaction inside the system
Viscosity becomes non linear, it varies with shear rate

43
Q

What are the 3 types of non Newtonian fluids

A

Plastic
Pseudo-plastic
Dilatant

44
Q

Define plastic flow

A

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

45
Q

Define pseudoplastic flow

A

Substance will start to flow immediately when stress is applied hence the curve passes through the origin. Viscosity decreases as shear rate increases

46
Q

Define dilatant flow

A

Rarest of flow types. Materials increase in volume during shearing and exhibit shear thickening. Dilatant systems usually are suspensions containing high percentage of particles (>50%)

47
Q

Apparent viscosity

A

napp used because shear rate calculated from tangent to curves in graphs
napp = shear stress (Pa) / shear rate (s-1)

For Newtonian fluids napp always remains the same
For non Newtonian fluids napp invariably changes

48
Q

Shear thinning

A

Response of the system is non linear
Corresponding rate increases with stress and viscosity decreases
Pseudoplastic behaviour

49
Q

Power law

A

[sigma]n = n’ [gamma]
n’ is the viscosity coefficient
n is the index of pseudo-plasticity or power law index

Take log on both sides of above equation
n log [sigma] = log n’ + log [gamma]
log [gamma] = n log [sigma] - log n’
Linear relationship between log gamma and log sigma

50
Q

Different flow systems have different n value
n = 1
n >1
n <1

A

n = 1 Newtonian flow
n >1 pseudoplastic flow
n <1 dilatant flow

51
Q

Yield value

A

Ideal plastic is characterised by a yield value followed by ideal flow
At stresses below yield value the substance act as elastic materials
A typical folcculated suspension demonstrates plastic flow properties: it doesn’t flow when shear is very low and starts to flow when shear is increased over certain value

52
Q

Yield value

Plastic flow can be described by

A

U = stress - f / shearing rate
f = yield value
Once yield value is exceeded the plastic viscosity changes proportionally with shearing stress resembling Newtonian flow

53
Q

Causes for shear thinning

A

Changes inside a system which can lead to viscosity decreases:

  • breaking of non covalent bonds (H bonds, charge interactions, hydrophobic interactions)
  • alignment of molecules/polymers along the direction of shear
  • non linearity of response is caused by continuous random breaking and formation of bonds
54
Q

Shear thickening

A

Response of system is non linear
Corresponding rate decreases with stress resulting in an increase of viscosity
Dilatant behaviour

55
Q

Causes for shear thickening

A

Changes inside a system which can lead to a viscosity increase:

  • the spaces between particles tend to increase under shear resulting in voids
  • particles become less lubricated after the voids are increased to a certain degree hence increased friction inside the system
  • more interaction between particles due to disturbance of system
56
Q

Time dependent behaviour

A

Viscoelastic materials may not adapt immediately to new shear conditions

57
Q

Thixothrooy

A

Structure does not re form at the same speed and in the same way it is broken down
Can be seen for both shear thinning and thickening

58
Q

Examples of plastic flow in pharmacy

A

Flocculated particles in conc disperse systems e.g. Ointments pastes creams

59
Q

Pseudo-plastic flow in pharmacy examples

A

Natural synthetic and semi systemic polymers in solution e.g. Acacia, sodium alginate, methyl cellulose, hydroxylropylmethyl cellulose

60
Q

Examples of dilatant flow in pharmacy

A

40-50% w/v suspensions of starch/water

61
Q

Define flocculated particles

A

Internet

62
Q

Can viscosity be measured at any point for non Newtonian fluids

A

No it cannot, it matter where it is measured and also how

63
Q

What are capillary viscometers not capable of giving

A

Not capable of giving multiple points

64
Q

Ideally which viscometers are used

A

Rheometers as falling sphere and simple rotational viscometer can be tedious

65
Q

Rotational rheometers

A

Allow to employ a wide range of shear stress and rate
Small sample volume and good temp control
Allow continuous monitoring of structural changes in a sample over a range of stress, rate, time , temp

66
Q

What are the different geometries

A

Parallel plate
Cone and plate
Mixer
Concenteric cylinder

67
Q

Concentric cyclometer

A

For low viscosities or evaporating samples

68
Q

Cone and plate

A

For viscous samples preferred for flow curves

69
Q

Parallel plate

A

For samples containing particulates for oscillation and creep experiments

70
Q

Mixer

A

In line viscosity control during manufacturing

71
Q

Advantages of concentric cylinder

A 
Little possibility of sample evaporation or expulsion 
Can measure low shear stress which is useful for emulsions and suspensions
Easy to load with liquid dosage forms
n = T/kw
n = dynamic viscosity 
T = torque (stress)
K = constant 
w = speed of rotation
72
Q

Disadvantages of concentric cylinder

A

Possible air incorporation or breaking of structure when loading semi solids
Large volume of material needed
Shear rate across gap in not constant varies with distance from wall. Can lead to slip or plug flow

73
Q

Advantages of cone and plate

A

Cone has a very shallow angle of contact
Constant shear rate across radius
High shear rates possible
Small sample volume required
Easy to fill and clean even with highly viscous samples
Little disturbance of sample structure
Rapid temperature equilibration due to thin film

PSF (11 decks)

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