Viscosity

Question 1

Define: Viscosity

Answer 1

A measure of a fluid’s resistance to flow. The perception of thickness

Question 2

Define: Shear viscosity

Answer 2

The ratio of the applied shear stress (𝝉) to the resulting rate of shear (𝜸 ̇)

Question 3

Why is measuring viscosity important

Answer 3

• Ingredient specifications may require a certain viscosity - can affect food quality e.g sedimentation - Food safety e.g. different gels can stop bacterial growth - Health issues. something more viscous in the small bowel can change the rate at which something is absorbed - processing. moving it through pipes in a factory - Pharmaceutical application - NPD

Question 4

Define: Shear stress

Answer 4

The stress acting in a plane as the result of a force acting parrallel to the plane 𝜏 =𝐹/𝐴 𝜏 = shear stress F = force applied A = surface area It is measured in pascals (Pa)

Question 5

Define: Shear rate

Answer 5

The rate at which we are moving the material

Question 6

How can we measure viscosity

Answer 6

Parallel plate Cone and plate Concentric cylinder

Question 7

How do the different methods of measuring viscosity differ

Answer 7

A larger surface area allows for more precise measurements. In terms of measurements parallel plate < cone and plate < Concentric cylinder

Question 8

How is rotational viscosity measured

Answer 8

Rotating the measuring device in one direction and analysing the amount of force applied

Question 9

What is complex (dynamic) viscosity?

Answer 9

Measuring the viscosity through oscillating (backwards and forwards)

Complex (or Dynamic) Viscosity (η*) is denoted as: η* = G*/ ω

ω = frequency of oscillation

G* = (G’ 2 + G’’ 2) 1/2

Question 10

Define: Storage modulus

Answer 10

Also known as G’, it is the measure of the deformation energy stored in the system . Reflects the solid (elastic) part of the system

Question 11

Define: Loss modulus

Answer 11

Also known as G’’, it is a measure of the liquid (viscous) part of the sample

Question 12

Explain this graph

Answer 12

Graph shows two different newtonian fluids. In Newtonian fluids if we change the viscosity it remains proportional to the shear rate e.g. honey, oil, milk

Question 13

Explain the purple line in this graph

Answer 13

Graph shows a non-newtonian fluid which is sheer thickening. Viscosity increases as we increase the shear rate

Question 14

Explain the green line in this graph

Answer 14

Graph shows a non-newtonian fluid which is sheer thinning. Viscosity decreases as we increase the shear rate

Question 15

Define: Non-Newtonian fluid

Answer 15

A fluid thats viscosity is dependent on its shear rate

Question 16

Define: Newtonian fluid

Answer 16

A fluid thats viscosity remains constant independent of shear rate

Question 17

Explain the blue line on this graph

Answer 17

Graph shows a newtonian fluid. The viscosity remains constant irrelevant of the shear rate

Question 18

Explain what this graph is showing

Answer 18

Straight line on log:log graph suggests that the system is particulate

Question 19

Explain what this graph is showing

Answer 19

Graph shows that chocolate is sheer thinning.

Question 20

Explain what this graph is showing

Answer 20

The size of the molecule affects the viscosity and sheer thinning ability.

Question 21

When looking at a Cox-Merz graph what are we looking at information for

Answer 21

The viscoelastic properties of a fluid during oscillation

Question 22

Explain what the clear and coloured triangles on this graph show

Answer 22

Both coloured (dynamic viscosity) and clear triangles (complex viscosity) are on the same line. Therefore in a newtonian solution you get the same viscosity whether measured by oscillation or rotation

Question 23

What can affect viscosity behaviour

Answer 23

• Microstructure (rods, macromolecular, polymer solution)
• Particle size
• Concentration
• Particle properties (hardness or softness)
• Temperature

Question 24

How does a rod shaped microstructure affect viscosity behaviour

Answer 24

At rest the fibres will be randomly oriented. Apply shear rate and the fibres will slowly orient in flow direction. With increasing orientation due to increasing shear rate the individual fibre will present less resistance to the flow and viscosity will decrease. Shear thinning

Question 25

How do macromolecular structures affect viscosity behaviour

Answer 25

At rest the solutions are tangled and coiled. Under high sheer the molecules may deform, disentangle or completely unravel into fibres resulting in decreased viscosity. Shear thinning

Question 26

How do dispersions affect viscosity behaviour

Answer 26

particles can diffuse between laters in flowing suspensions. At high shear rates they can become trapped between ayers causing shear thickening or thinning.

Question 27

How does particle size affect viscosity

Answer 27

Larger particles interact with one another more frequently therefore the liquid is thicker. Starch granules swell under temperature hence why is is a good thickener.

Question 28

Define:

Yield Point

Answer 28

The yield point or yield stress is the minimum force that must be exceeded in order to break down a sample’s structure at rest, and thus make it flow.

Question 29

Why do different substances have different viscosities?

Answer 29

All liquids are composed of molecules. When put into motion, molecules and particles are forced to slide along each other. They develop a flow resistance caused by internal friction.

Larger components present in a fluid are the reason for higher viscosity values.

Question 30

Describe ideally elastic behaviour and give an example of a substance with these properties.

Answer 30

A substance that has no viscous portion and will return to its original size and shape when force is removed.

(e.g. Eiffel Tower)

Question 31

Describe ideally viscous behaviour and give an example of a substance with these properties.

Answer 31

A substance that has no elastic portion and whose size and shape are entirely dependant on the container they are in.

(e.g. Water)

Question 32

Define:

Brownian Motion

Answer 32

The random movement of microscopic particles in a fluid as a result of continuous bombardment from molecules of the surrounding medium.