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Food Rheology An overvie

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Title: Food Rheology An overvie


1
Food Rheology
  • An overview
  • http//sst.tees.ac.uk/external/U0000504/Notes/mscn
    otes/

2
The Scope of Rheology
  • The study of the deformation and flow of matter
  • Concerned with
  • the effect of shear stresses on materials having
    properties of fluids
  • The properties of materials lying between two
    ideal states
  • the elastic solid
  • the Newtonian liquid

3
Stress and strain
  • When a material is subject to static forces, the
    material is deformed
  • The degree of deformation depends on
  • the material properties
  • the dimensions of the object
  • In order to standardise the latter we define two
    quantities, stress and strain

4
Shear
  • Shear is the result of two forces acting out of
    line.
  • It is illustrated in the diagram opposite
  • Shear stress, t and shear strain, g are defined
    from

5
The elastic solid
  • An elastic solid is one which returns to its
    original state after being deformed.
  • The relationship between shear stress and shear
    strain is a linear one
  • For shear, the slope of the line is called the
    shear modulus, G and is defined as

6
The Newtonian Fluid
  • If a force is applied to a fluid, it shears
  • As a result a velocity gradient is set up in the
    fluid which is proportional to the shear stress
  • This constant of proportionality is called the
    viscosity

7
The Newtonian fluid (2)
  • In symbols

g-dot is the shear rate
8
Non-Newtonian flow
  • For a Newtonian fluid, viscosity is a constant
    and a graph of shear stress vs. shear rate is a
    straight line
  • Fluids whose viscosity is not constant are called
    Non-newtonian. There are three main categories
  • Shear thinning
  • Shear thickening
  • Time dependant

Properties of time independent fluids
9
Non-Newtonian models
  • Time independent non-Newtonian fluids conform to
    one of the following models
  • Bingham plastic
  • Power law
  • Herschel Bulkley

10
Apparent viscosity
  • By rearranging the power law we can define a
    property called apparent viscosity, happ

11
Time dependant viscosity
  • There are two time dependant types of fluid
  • Thixotropic, where viscosity decreases with time
  • Dilatent where viscosity increases with time

12
Visco-elasticity
  • Some solids display liquid-like properties.
  • Such solids are described as visco-elastic.
  • Two particular properties characterise
    visco-elastic solids. These are
  • Creep
  • Stress relaxation

13
Visco-elasticity (2)
  • Creep is the continuing extension of a solid when
    a constant load is applied
  • Stress relaxation is the reduction in stress
    needed to maintain constant strain

Creep curve
14
Models of visco-elastic behaviour
  • Visco elastic behaviour can be modelled by spring
    and dashpot models.
  • The spring represents the elastic properties
  • The dashpot represents the viscous properties
  • The two simplest are the Maxwell and Kelvin
    elements
  • These are shown on the next two slides

15
Dashpot and spring models
  • Maxwell element
  • Kelvin or Voigt element

This element models stress relaxation
This element models creep
16
Dashpot and spring models (2)
  • Neither the Maxwell nor the Kelvin model on their
    own fully explain visco-elastic behaviour and
    more complex models are required
  • The example on the right quite effectively models
    the behaviour of cheese.

17
Complex stress strain
  • Another method of characterising visco-elastic
    materials is to apply varying strain
  • This is usually achieved by applying an
    oscillatory shear strain to a sample
  • The equations describing the response to such a
    strain involve complex numbers, hence the term
    complex stress and strain

18
Complex stress and strain (2)
  • The response to an applied oscillatory shear is
    illustrated below

19
Complex moduli
  • The rheological properties in oscillatory shear
    are characterised by the complex moduli.
  • The storage modulus, G
  • The loss modulus, G
  • These are related by the phase angle, d
  • These are defined on the right

tm and gm are the maximum values of shear stress
and shear strain respectively
20
Complex moduli
  • The storage modulus relates to the bulk
    properties of the solid
  • The loss modulus relates to the vibrational state
    of the molecules.
  • They can be combined to give an overall complex
    modulus, G, though this property is not as
    useful as the separate moduli.

21
For more information
  • This lecture has only been an overview.
  • More information is available via the Module
    website.
  • Or go directly to
  • http//sst.tees.ac.uk/external/U0000504/Notes/mscn
    otes/
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