lec 3 rheology, thickening gels Flashcards
Why is rheology important?
- 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.
what is viscosity?
resistance to deformation and flow
Viscosity increase
thickening
where does viscosity arise from In low molar mass liquids?
friction between molecules
– Viscosity example:
molasses > triglyceride oil > water
how is viscosity described In solutions and dispersions?
In solutions and dispersions viscosity is related to the
volume occupied by large molecules and particles
– Creates disturbances in the flow of the solvent
What influences the rheological behaviour?
Rheological properties of the dispersed and continuous phase
what do the Rheological properties of a dispersion depend on?
Rheological properties of a dispersion depends on:
* Concentration of dispersed particles
* Size and size distribution of particles
* Shape and deformability of particles
what do the Rheological properties of continuous phase influence?
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)
what is Shear stress?
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.
what is strain?
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.
what is the Shear rate?
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.
what is Dynamic viscosity?
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.
what are the Different flow behaviours
- 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
give 3 Examples of approaches to viscosity measurements
- 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
what is Yield point?
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)
what is the power law?
A power law can be used to describe the flow
behaviour
τ=K⋅( γ˙) ^n
What is a gel?
- 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
Gels require formation of a 3D-network, name two ways this can be done
- 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
give an example of a Particle gel
Casein micelles in cheese curd
give an example of a Polymer gel
Pectin gel
what is Elasticity
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”
what is viscoelasticity
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.
what is the storage modulus?
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.
what is the loss modulus?
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.
what is the Complex modulus?
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
what is the phase angle
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
what is amplitude
Maximum of the oscillatory motion
what is Frequency
1 Hz corresponds to one oscillation pre second
