lec 3 rheology, thickening gels

Question 1

Why is rheology important?

Answer 1

• Mechanical and flow properties of a material
• Sensory properties
  – Texture, mouthfeel etc.
• Process design
  – Pipes, pumps, mixers, homogenizers etc.
• Shelf-life
  – Colloidal stability, sedimentation/creaming etc.

Question 2

what is viscosity?

Answer 2

resistance to deformation and flow

Question 3

Viscosity increase

Answer 3

thickening

Question 4

where does viscosity arise from In low molar mass liquids?

Answer 4

friction between molecules
– Viscosity example:
molasses > triglyceride oil > water

Question 5

how is viscosity described In solutions and dispersions?

Answer 5

In solutions and dispersions viscosity is related to the
volume occupied by large molecules and particles
– Creates disturbances in the flow of the solvent

Question 6

What influences the rheological behaviour?

Answer 6

Rheological properties of the dispersed and continuous phase

Question 7

what do the Rheological properties of a dispersion depend on?

Answer 7

Rheological properties of a dispersion depends on:
* Concentration of dispersed particles
* Size and size distribution of particles
* Shape and deformability of particles

Question 8

what do the Rheological properties of continuous phase influence?

Answer 8

Rheological properties of the continuous phase influence the rheology of the total system
* Flow properties of the continuous phase (Newtonian, shear thinning etc.)
* Molar mass (actually size!) of dissolved molecules
* Interaction between dissolved molecules
* Network formation (gel formation)

Question 9

what is Shear stress?

Answer 9

Shear stress (either σ or τ) = Force/Area (N/m 2 = Pa)

Shear stress is a type of mechanical stress that occurs when forces are applied parallel to the surface of an object or material, causing layers of the material to slide or deform relative to one another. It is a measure of the internal resistance of a material to sliding or shearing forces.

Question 10

what is strain?

Answer 10

Strain (either ε or γ) = ΔL/L

strain describes how much a material stretches or compresses compared to its original size or shape when subjected to external forces. It is a dimensionless quantity typically expressed as a ratio or percentage.

Question 11

what is the Shear rate?

Answer 11

Shear rate ( ̇ 𝛾) = velocity of moving plate/distance to stationary plate
(unit: s-1)

shear rate describes how quickly the material deforms or flows in response to an applied shear stress.

Question 12

what is Dynamic viscosity?

Answer 12

Dynamic viscosity: η = σ/ ̇ 𝛾

dynamic viscosity describes how “thick” or “sticky” a fluid is, with higher viscosity indicating greater resistance to flow and lower viscosity indicating easier flow.

Question 13

what are the Different flow behaviours

Answer 13

• Newtonian: low molar mass liquids (water, oil etc.)
• Shear thinning: Polymer solutions and dispersions (very common in foods)
• Shear thickening: Highly concentrated dispersions of relatively large particles (approx. > 100μm, rare in foods)
• Herschel-Bulkley (yield point): Shear-thinning but appear distinctly “solid-like” at rest or has a
  noticeable resistance to start flowing

Question 14

give 3 Examples of approaches to viscosity measurements

Answer 14

• Dip-in rotational viscometers
  – Rotation is controlled
  Shear stress and viscosity is measured
• Bostwick consistometer
  – Flow-distance
  – Give relative viscosity
• Ostwald viscometer (glass capillary)
  – Time to flow through capillary
  – Gives relative viscosity

Question 15

what is Yield point?

Answer 15

the yield point is the stress level at which a material starts to permanently change shape, exhibiting noticeable plastic deformation, such as stretching or bending, without springing back to its original form.

• A certain stress needs to build up in the material
  before it starts to flow
  – Breaking or perturbing the structure created by
  proteins, polymers, particles, droplets or bubbles
• The yield point can be low (barely noticeable
  resistance to start flowing) or high (“solid-like” at rest)

Question 16

what is the power law?

Answer 16

A power law can be used to describe the flow
behaviour

τ=K⋅( γ˙) ^n

Question 17

What is a gel?

Answer 17

• A gel is a soft, solid or solid-like material of
  two or more components, of which one is a
  liquid present in substantial quantity
• A gel is a material with a considerable yield
  point

Question 18

Gels require formation of a 3D-network, name two ways this can be done

Answer 18

• Non-covalent crosslinking –> transient –> flow
  – Example: created by physical interaction between
  polymers
  – Very common in foods
• Covalent crosslinking –> permanent –> break
  – Example: can occur in protein gels

Question 19

give an example of a Particle gel

Answer 19

Casein micelles in cheese curd

Question 20

give an example of a Polymer gel

Answer 20

Pectin gel

Question 21

what is Elasticity

Answer 21

The tendency for a body of a material to recover the
shape and dimensions when stress is removed
– Complete elasticity – No permanent deformation after stress is
removed
– Elastic materials are “homesick”

Question 22

what is viscoelasticity

Answer 22

a material’s ability to exhibit both viscous (flow-like) and elastic (solid-like) behavior simultaneously when subjected to deformation.

viscoelastic materials possess characteristics of both fluids (viscosity) and solids (elasticity), and their behavior depends on the rate and duration of applied stress.

Question 23

what is the storage modulus?

Answer 23

G’ = storage modulus, elastic modulus (Pa)

The storage modulus represents the material’s ability to store elastic energy and is associated with the in-phase response of the material to applied stress. It quantifies the stiffness or rigidity of the material.

Question 24

what is the loss modulus?

Answer 24

G’’ = loss modulus, viscous modulus (Pa)

The loss modulus represents the material’s ability to dissipate energy as heat and is associated with the out-of-phase response of the material to applied stress. It quantifies the material’s viscous or damping properties.

Question 25

what is the Complex modulus?

Answer 25

It represents the combined effects of both elastic and viscous properties of a material under dynamic loading conditions
G=G’ + iG’’
* G’ = |G|cos δ
* G’’= |G*|sin δ
* δ = phase angle

Question 26

what is the phase angle

Answer 26

δ = 0 completely elastic
material (Hookean solid)
the stress and strain responses are perfectly in phase, indicating purely elastic behavior.
δ = 90° completely viscous material (Newtonian fluid) the stress and strain responses are perfectly out of phase
0 < δ < 90° viscolelastic material, the stress and strain responses exhibit a phase shift

Question 27

what is amplitude

Answer 27

Maximum of the oscillatory motion

Question 28

what is Frequency

Answer 28

1 Hz corresponds to one oscillation pre second