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The Physics Of Cooking > Rheology > Flashcards

Rheology Flashcards

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1
Q

Define rheology

A

The study of flow.

Viscosity measurements, characterisations of flow behaviour and determination of material structure.

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2
Q

What is an ideal solid?

A

One that has infinite viscosity and therefore wont deform regardless of the stress applied.

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3
Q

What is an ideal fluid?

A

No viscosity and thus offers no resistance to strain.

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4
Q

How do you calculate pascals?

A

N/m^2

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5
Q

A small area and a big force will lead to a higher or lower stress?

A

Increased stress

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6
Q

What are the 2 types of force?

A

Normal and shear

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7
Q

What is normal stress?

A

When a once is applied at a 90 degree angle with the surface/ perpendicular to the surface.

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8
Q

What is shear stress?

A

When a force is applied at a 180 degree angle with the surface/ parallel to the surface.

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9
Q

Compression and extension are 2 types of which stress?

A

Normal

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10
Q

How do you define force?

A

Mass (kg) x acceleration (m/s^2)

in newtons

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11
Q

In what units do we measure force?

A

Newtons

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12
Q

How do you define strain?

A

Relative deformation

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13
Q

What is the equivalence of newtons?

A

Kg m/s^2

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14
Q

In what unit do we measure stress?

A

Pa

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15
Q

Is stress an internal or external force?

A

Internal

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16
Q

Is force an internal or external?

A

External

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17
Q

What are the units for strain and why?

A

No unis as they cancel out

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18
Q

Which two extremes do all real bodies fall between in rheology?

A

Perfectly elastic and perfectly liquid

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19
Q

Materials which have properties of both an ideal liquid and and ideal solid are know as what?

A

Viscoelastic

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20
Q

Do elastic materials store or dissipate energy?

A

Store

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21
Q

Do viscous materials store or dissipate energy?

A

Dissipate

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22
Q

What is a dash pot used to measure?

A

Fluid behaviour, they are assumed to deform continuously (flow) as long as there is a force to deform them

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23
Q

What is a spring used to measure?

A

The elastic solid behaviours, there is a limit to how much a spring can deform.

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24
Q

Which model is known to be a viscoelastic fluid model and what is it?

A

Maxwell model.
Spring in a dash pot.
Force applied causes both the spring and the dashpot to deform but the spring will only deform a little while the dashpot will keep deforming as long as force is maintained.

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25
Q

Which model is used to show viscoelastic solid behaviour and what is it?

A

Kelvin-Vogt model.

Dashpot and spring in parallel.
Dashpot cannot undergo continuous deformation due to the response of the spring to the applied loads - will only deform to a certain point.

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26
Q

What does a graph look like illustrating plastic deformation and indicate yield point.

A

Stress on y axis
Strain on axis

Straight diagonal going up from 0 which then curves creating n shape
Yield point is where the straight bit ends.

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27
Q

What is plastic deformation?

A

When a material enters the plastic region, it does not regain original shape once stress is removed

The yield point is the point at which the material start to deform plastically.

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28
Q

Sketch a graph indicating a brittle material and indicate the fracture point.

A

Y axis = stress
X axis = strain

Elastic line which suddenly drops directly down towards x axis, this point is the fracture point.

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29
Q

What is a brittle material and give an example?

A

Strong material as little stress for high strain. They fracture/rupture suddenly with little/no plastic deformation e.g. hydrocoloid gel/glass

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30
Q

What is the fracture point?

A

The point where the strain is no longer proportional to the stress and the material fails.

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31
Q

What is elastic/Young’s modulus?

A

They are both the same.

The measure of rigidity of a material. The value of stress that can be applied per area without leading to permanent deformation.

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32
Q

How do you calculate Young’s modulus and what are its units?

A

E = o(sigma)/e-(epsilon)

= stress/strain on graph

Units = pa or N/m^2

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33
Q

What is the equation to calculate normal stress?

A

Sigma = force (N) / Area (m^2)

Units = pascals

34
Q

How do you calculate normal strain?

A

Epsilon = difference in length (m)/ initial length (m)

35
Q

How do you calculate shear stress?

A

Theta = force (N) /Area (m^2)

Units = pascals

36
Q

How do you calculate shear strain?

A

Gamma (Y) = difference in length (m) / initial length (m)

37
Q

Draw a graph to indicate a yield point and give a definition.

A

Y axis = stress
X axis = strain

Plastic model, yield point is where it stops behaving elastically.

Def: point at which a material starts to deform plastically (will no longer return to its original shape)

38
Q

What is the elastic limit?

A

The max limit until a material deforms

39
Q

What is the proportional limit?

A

The point where proportionality ends (stress and strain are no longer proportional)

40
Q

Indicate the elastic and proportion limits on a graph

A

Often the same point - where the line stops being elastic

41
Q

What is the modulus of deformability?

A

A material that only has a small elastic region.

It is a measure of stress to corresponding strain (within specific limits)

42
Q

How many psi is 1 pascal?

A

0.000145

43
Q

What does hooked law state and what is the equation?

A

Deformation is proportional to applied stress

O(sigma) = E e-(epsilon)

44
Q

What is newtons law and how is it calculated?

A

T =ny(gamma)

A Newtonian fluid has constant viscosity

45
Q

What is viscosity and how is it calculated?

A

n = T(theta)/y (gamma)= shear stress / shear rate

The higher the viscosity the thicker/less fluid the liquid so it is a measure of fluidity

46
Q

What is shear thinning?

A

The viscosity of a fluid decreases with increased shear (stirring) - most liquids are like this

47
Q

Indicate shear thinning on a graph

A 
Y = viscosity (n)
X= shear rate (y)

Graph starts high on left and moves down - looks like a ski slope

48
Q

What is the equation for shear thinning nd what are the conditions for n?

A

T = ky(gamma)^n(viscosity)

N<1

49
Q

What is shear thickening?

A

Where increased shear = increased viscosity (occurs due to clumping.

E.g. cornstarch

50
Q

Draw a graph to represent shear thickening

A

Y axis = viscosity
X axis = shear rate

Starts mid way up y axis then dips slightly then rises like a wave

51
Q

What is the equation for shear thickening and what are the conditions for n?

A

T = k y(gamma) ^n (viscosity)

N>1

52
Q

What 2 models represent plastic behaviour?

A

Bingham model

Herschel-Buckley model

53
Q

What is the Bingham model?

A

When there is a yield stress followed by a nearly Newtonian flow above this stress.

54
Q

What is the equation for the Bingham model?

A

T (theta) = To +n ^y

55
Q

What is the Herschel-Buckley model?

A

When a material has a yield stress and once this is surpassed there is non-Newtonian (plastic) behaviour.

56
Q

What does the Herschel Buckley model look like on a graph?

A

Y axis = theta
X axis = gamma

Yield stress followed by curve (plastic model)

57
Q

What is the equation for the Herschel-Buckley model?

A

T(theta) = To K y ^n

58
Q

What is a pseudo - plastic?

A

A material where viscosity decreases with increased shear.

Shear thinning

59
Q

What is a Newtonian fluid?

A

A liquid with a constant viscosity that is not effected by shear

60
Q

What is a dilatant?

A

A fluid where viscosity increases with increased shear.

Shear thickening

61
Q

What does theoretic mean?

A

The rare property of some non-Newtonian fluids to show a ice dependent increase in viscosity.
The longer the fluid undergoes shearing force, the higher its viscosity.

62
Q

What is a plastic?

A

A material that permanently deforms after a sufficient applied stress

63
Q

What is a thixotropic fluid? Give an example

A

Fluids that will lose structure during shear and rebuild during standing

E.g. ketchup

64
Q

What does hyestereosis mean?

A

A change in viscosity due to shearing

65
Q

Indicate Newtonian fluid behaviour on a graph

A

Y axis = viscosity (n)
X axis = shear rate

Horizontal line

66
Q

What is the equation used to represent a Newtonian fluid?

A

T(theta) = n(viscosity)^y + o

67
Q

How do you calculate the viscosity of a line on a graph?

A

Y axis / x axis

68
Q

What describes fluids which don’t obey newtons law?

A

Fluids where behaviour is effected by the shear rate

Shear stress and rate are no longer linear.

69
Q

What is stokes law?

A

Defines the drag force that exists between a sphere moving through a fluid at a constant viscosity

70
Q

What is debrorahs number?

A

De = t/T

t = time it takes a material to adjust to the stress
T = time scale fo the response/observation time 
De>1 = solid like
De = 1 - viscoelastic
De<1 = liquid like
71
Q

What is dynamic/shear viscosity?

A

The measure of a fluid resistance to flow when an external force is applied.

Measure of the internal friction of a moving fluid

The thickness of a flui

72
Q

What is kinematic viscosity?

A

Measures the resistance of the fluid subject under the weight of gravity e.g. honey

73
Q

How do you calculate kinematic viscosity?

A

V = n(dynamic viscosity) / p (density)

Unit = m^2 s^-1

74
Q

What is relative viscosity?

A

The viscosity of diluted polymer solutions increases with the molar mass.

75
Q

What is the equation for relative viscosity?

A

Viscosity of solution/ viscosity of solvent

76
Q

Why does shear thinning occur?

A

Because particles e.g protein is being broken down

77
Q

Why is rheology studied in food science?

A

Look at how things flow
Will it pour/ spread
How will food react with gravity
Is it stable to support environmental stresses

78
Q

What 2 instruments are used to measure Newtonian fluids?

A

Flow cups

Capillary viscometer s

79
Q

What 2 instruments are used to measure kinematic viscosity?

A

Flow cups

Ostwald viscometer

80
Q

What instrument is used to measure on-Newtonian fluids?

A

Rotational rheometers

81
Q

What is used to measure solids and semi solids and how?

A

Texture analyser

  • compression tests
  • extensional tests
  • forward extrusion
    Mimicking mouths biting and chewing action

The Physics Of Cooking (2 decks)

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