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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
Which model is used to show viscoelastic solid behaviour and what is it?
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.
26
What does a graph look like illustrating plastic deformation and indicate yield point.
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.
27
What is plastic deformation?
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.
28
Sketch a graph indicating a brittle material and indicate the fracture point.
Y axis = stress X axis = strain Elastic line which suddenly drops directly down towards x axis, this point is the fracture point.
29
What is a brittle material and give an example?
Strong material as little stress for high strain. They fracture/rupture suddenly with little/no plastic deformation e.g. hydrocoloid gel/glass
30
What is the fracture point?
The point where the strain is no longer proportional to the stress and the material fails.
31
What is elastic/Young’s modulus?
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.
32
How do you calculate Young’s modulus and what are its units?
E = o(sigma)/e-(epsilon) = stress/strain on graph Units = pa or N/m^2
33
What is the equation to calculate normal stress?
Sigma = force (N) / Area (m^2) Units = pascals
34
How do you calculate normal strain?
Epsilon = difference in length (m)/ initial length (m)
35
How do you calculate shear stress?
Theta = force (N) /Area (m^2) Units = pascals
36
How do you calculate shear strain?
Gamma (Y) = difference in length (m) / initial length (m)
37
Draw a graph to indicate a yield point and give a definition.
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
What is the elastic limit?
The max limit until a material deforms
39
What is the proportional limit?
The point where proportionality ends (stress and strain are no longer proportional)
40
Indicate the elastic and proportion limits on a graph
Often the same point - where the line stops being elastic
41
What is the modulus of deformability?
A material that only has a small elastic region. | It is a measure of stress to corresponding strain (within specific limits)
42
How many psi is 1 pascal?
0.000145
43
What does hooked law state and what is the equation?
Deformation is proportional to applied stress O(sigma) = E e-(epsilon)
44
What is newtons law and how is it calculated?
T =ny(gamma) A Newtonian fluid has constant viscosity
45
What is viscosity and how is it calculated?
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
What is shear thinning?
The viscosity of a fluid decreases with increased shear (stirring) - most liquids are like this
47
Indicate shear thinning on a graph
``` Y = viscosity (n) X= shear rate (y) ``` Graph starts high on left and moves down - looks like a ski slope
48
What is the equation for shear thinning nd what are the conditions for n?
T = ky(gamma)^n(viscosity) N<1
49
What is shear thickening?
Where increased shear = increased viscosity (occurs due to clumping. E.g. cornstarch
50
Draw a graph to represent shear thickening
Y axis = viscosity X axis = shear rate Starts mid way up y axis then dips slightly then rises like a wave
51
What is the equation for shear thickening and what are the conditions for n?
T = k y(gamma) ^n (viscosity) N>1
52
What 2 models represent plastic behaviour?
Bingham model | Herschel-Buckley model
53
What is the Bingham model?
When there is a yield stress followed by a nearly Newtonian flow above this stress.
54
What is the equation for the Bingham model?
T (theta) = To +n ^y
55
What is the Herschel-Buckley model?
When a material has a yield stress and once this is surpassed there is non-Newtonian (plastic) behaviour.
56
What does the Herschel Buckley model look like on a graph?
Y axis = theta X axis = gamma Yield stress followed by curve (plastic model)
57
What is the equation for the Herschel-Buckley model?
T(theta) = To K y ^n
58
What is a pseudo - plastic?
A material where viscosity decreases with increased shear. Shear thinning
59
What is a Newtonian fluid?
A liquid with a constant viscosity that is not effected by shear
60
What is a dilatant?
A fluid where viscosity increases with increased shear. Shear thickening
61
What does theoretic mean?
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
What is a plastic?
A material that permanently deforms after a sufficient applied stress
63
What is a thixotropic fluid? Give an example
Fluids that will lose structure during shear and rebuild during standing E.g. ketchup
64
What does hyestereosis mean?
A change in viscosity due to shearing
65
Indicate Newtonian fluid behaviour on a graph
Y axis = viscosity (n) X axis = shear rate Horizontal line
66
What is the equation used to represent a Newtonian fluid?
T(theta) = n(viscosity)^y + o
67
How do you calculate the viscosity of a line on a graph?
Y axis / x axis
68
What describes fluids which don’t obey newtons law?
Fluids where behaviour is effected by the shear rate Shear stress and rate are no longer linear.
69
What is stokes law?
Defines the drag force that exists between a sphere moving through a fluid at a constant viscosity
70
What is debrorahs number?
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
What is dynamic/shear viscosity?
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
What is kinematic viscosity?
Measures the resistance of the fluid subject under the weight of gravity e.g. honey
73
How do you calculate kinematic viscosity?
V = n(dynamic viscosity) / p (density) Unit = m^2 s^-1
74
What is relative viscosity?
The viscosity of diluted polymer solutions increases with the molar mass.
75
What is the equation for relative viscosity?
Viscosity of solution/ viscosity of solvent
76
Why does shear thinning occur?
Because particles e.g protein is being broken down
77
Why is rheology studied in food science?
Look at how things flow Will it pour/ spread How will food react with gravity Is it stable to support environmental stresses
78
What 2 instruments are used to measure Newtonian fluids?
Flow cups | Capillary viscometer s
79
What 2 instruments are used to measure kinematic viscosity?
Flow cups | Ostwald viscometer
80
What instrument is used to measure on-Newtonian fluids?
Rotational rheometers
81
What is used to measure solids and semi solids and how?
Texture analyser - compression tests - extensional tests - forward extrusion Mimicking mouths biting and chewing action

The Physics Of Cooking (2 decks)

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